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Dalton Transactions

PERSPECTIVE

Received 00th January 20xx,Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

www.rsc.org/

An unexpected journey from highly tunable phosphorescence to novel photochemistry of 1,2,3-triazole-based complexes Paul A. Scattergood*a and Paul I.P. Elliott*a

The photophysical properties of transition metal complexes have long attracted interest in the literature with significant research activity during the past two to three decades due to the potential exploitation of these materials in solar energy conversion, light-emitting technology, luminescence biological imaging and photodynamic therapeutic applications to name but a few. Since the advent of the facile preparation of 1,2,3-triazole-based compounds through copper(I)-catalysed cycloaddition, ligands based on this heterocycle have received widespread attention in coordination chemistry. Inevitably, their ability to be used as pyridine-like analogues has resulted in significant attention on the photophysical properties of their resultant complexes. There are, however, two sides to this tale; on the one hand, routes to 1,2,3-triazoles have enabled the realisation of highly tunable and efficient phosphors and photosensitisers. On the other hand, 1,2,3-triazole-based complexes have allowed highly novel photochemical processes to be explored offering insights to hitherto unappreciated excited state dynamics. This Perspective review covers the developments of photophysically active triazole-based complexes over the last decade, highlighting some of the key discoveries from our own laboratory as well as seminal contributions from other groups who are active in the area. We also identify possible new avenues for investigation and exploitation stemming from the work so far.

IntroductionComplexes of kinetically inert transition metal ions such as rhenium(I), ruthenium(II), iridium(III) etc, have attracted enormous interest due to their attractive photophysical properties.1 Indeed, the well-known complex [Ru(bpy)3]2+ (bpy = 2,2’-bipyridyl) is perhaps the most studied and utilised complex in the literature. The photophysical properties of this class of compound have seen their application in light-emitting devices,2 as sensitisers for dye solar cells3 and photocatalysts4 as well as in luminescence biological imaging.5-8

Key to the successful exploitation of photoactive complexes

in these applications is the modification of their optoelectronic properties through chemical synthesis and ligand design. By far and away the most common ligand coordination motif used in these systems is that of chelating oligopyridines such as bpy and tpy (tpy = 2,2’:6’,2”-terpyridyl), although ligands based on

other heterocycles are also well-known. Through intelligently designing ligands it is possible to modify the energies of the frontier orbitals of these complexes and thus tune the electronic absorption and emission properties, electrochemical characteristics as well as their photochemical reactivity.

Photophysical properties of d6 metal complexes

Using [Ru(bpy)3]2+ as an archetypal example it is first helpful to introduce the basic electronic structure and resultant photophysical processes that occur within these complexes. The highest occupied molecular orbitals (HOMO) of [Ru(bpy)3]2+-like complexes are dominated by metal d-orbital character (Figure 1). Beneath this are molecular orbitals that are d-orbital dominated dL in character. In the case of cyclometalated complexes the HOMO typically also has contributions from the -system of the metalated aryl rings. In contrast, the lowest unoccupied molecular orbital (LUMO) has predominantly ligand-based * character. Above the LUMO lie dL* orbitals that are similarly dominated by ligand-based * character. At much higher energies are the antibonding d* orbitals associated with the metal-ligand -bonding interactions.

Photophysical processes occurring upon photoexcitation are often depicted using a Jablonski diagram familiar from

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1

a. Department of Chemistry, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK.

* Corresponding authors: [email protected]; [email protected]† Footnotes relating to the title and/or authors should appear here. Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x



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undergraduate lectures. However, these simple diagrams depicting the energies of various electronic states can often be misleading as to the true energetics underlying photophysical and photochemical phenomena as they infer a largely isostructural set of ground and excited states. It can be far more intuitively useful to describe these states using a potential energy surface diagram such as that in Figure 1 (right) in which the reaction coordinate can be related to metal-ligand bond lengths and which therefore takes into account large structural deformations following photoexcitation.

The lowest energy features in the visible region of the optical absorption spectra of d6 metal complexes are of singlet metal-to-ligand charge transfer (1MLCT) character (process a in Figure 1 right). Due to the strong spin-orbit coupling imparted by the metal centre the initial photoexcited singlet state undergoes rapid intersystem crossing (ISC) to the 3MLCT state (process b). In the case of [Ru(bpy)3]2+ ISC occurs on a sub-picosecond timescale with a quantum efficiency near to unity.9

It is then radiative relaxation to the ground state (1GS) from the 3MLCT state that is responsible for the characteristic phosphorescence exhibited by these complexes (process c). For cyclometalated complexes in which the d-orbital dominated HOMO also has aryl-* character these emissive states will also have additional 3ILCT and/or 3LLCT character depending on whether the excited electron resides on the cyclometalated or an ancillary ligand respectively. 3MLCT states can also undergo non-radiative deactivation to the ground state in addition to further intermolecular energy or electron transfer deactivation processes.

Figure 1. Simplified qualitative molecular orbital energy level diagram (left) and schematic potential energy surface diagram describing key photophysical processes (right) for [Ru(bpy)3]2+ as an archetypal example complex.

At higher energies lie triplet metal-centred (3MC) states associated with population of the d* ligand-field antibonding orbitals. If the energy barrier on the triplet potential energy surface (3PES) to their population from 3MLCT states is low enough these 3MC states can be readily populated (process d) and result in rapid excited state deactivation and even isomerisation reactions or ligand ejection photochemistry.

The successful manipulation of these various excited states and their exploitation in photophysical applications is reliant upon the ligands coordinated to the metal in these complexes. Efficient routes to ligand synthesis enabling diversity in structure and facile electronic and steric tuning are therefore essential. In this regard, 1,2,3-triazole-based ligands have

attracted significant attention over the past decade. Indeed, as will be outlined in this Perspective, the resultant d6 metal complexes of these ligands serve to excellently illustrate the tuning of the photophysical and photochemical properties described above.

Formation and properties of 1,2,3-triazole-based ligands

In recent years the exploitation of the copper(I)-catalysed 1,3-dipolar cycloaddition of alkynes and azides (CuAAC)10 to form 1,4-disubstituted-1,2,3-triazoles has become popular as a route for ligand design for transition metal complexes.11 A ruthenium(II)-catalysed version of this reaction (RuAAC) has also received attention and has been utilised in ligand design.12 The CuAAC route benefits from the wide commercial availability of terminal alkyne starting materials and/or facile preparative routes to them, the ease of access to a wide variety of azide precursors, the mild and high yielding reaction conditions and relative ease of workup and product purification.

Figure 2. Formation of 1,2,3-triazoles through copper(I) catalysed alkyne/azide cycloaddition (CuAAC) and common coordination modes of 1,2,3-triazole to metal centres.

The 1,2,3-triazole moiety exhibits broad versatility as a ligand due to the availability of several different coordination modes. Most obvious is coordination to a metal centre through the lone pairs of the N(2) position and, more commonly, the more basic N(3) position (Figure 2a and b respectively). As an N-donor ligand these triazoles have been shown to be poor -acceptors compared to pyridine donors12 stemming largely from the much higher energy of their * orbitals, thereby reducing metal-ligand orbital overlap. It is also possible for the C(5)-H position to undergo metalation to yield an anionic C-donor triazolide ligand (c).13 The protonation or alkylation of triazolide ligands or through deprotonation of 1,3,4-trisubstituted-1,2,3-triazolium salts and subsequent coordination enables access to strongly donating14 mesoionic 1,2,3-triazolylidene ‘abnormal’ carbene-based complexes (d).15

Estimation of Tolman parameters for these ligands suggest that the ‘abnormal’ carbenes are stronger donors compared to what can now be considered traditional imidazol-2-ylidene ligands but not as strong donors as their ‘abnormal’ imidazol-4/5-ylidene cousins.16, 17

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Figure 3. Structures of common pyridine- and triazole-based ligands and associated nomenclature that are featured in this contribution.

A number of recent reviews have appeared in the literature detailing the coordination and organometallic chemistry of complexes containing triazole-based ligands.11, 18,

19 Along with the rise in use of 1,2,3-triazole-based ligands in coordination chemistry, the photophysical properties of their resultant complexes has also become an area of inevitable and increasing interest. With bpy- and tpy-based ligands being ubiquitous in this area, triazole-based analogues of these archetypal ligand architectures have been studied and triazole-containing analogues of well-known cyclometalated ligand systems have also been reported.

Our own unexpected journey towards the beginning of the decade into triazole-based coordination chemistry began with the realisation of the relative dearth of literature in this area. This Perspective review will focus on the effects of triazole-based ligands on the photophysical properties of their d6 metal complexes. In particular we will review the work of key protagonists in this area over the last few years on the development of highly tunable and efficient luminescent triazole-based complexes. We will also cover our work which, on the other hand, shows how triazole-based complexes can also yield surprising and highly novel photochemical ligand ejection and rearrangement photochemistry.

Photophysical properties of 1,2,3-triazole-based complexesComplexes with neutral N-donor 1,2,3-triazole-based ligands

Our first foray into the photophysics of 1,2,3-triazole-based complexes involved the synthesis and characterisation of a series of rhenium(I) complexes bearing a monodentate triazole ligand (1a-d, Figure 4).20 These complexes are emissive (1a max

543 nm, = 482 ns in aerated dichloromethane) and show similar photophysical properties to the monodentate pyridine (py) analogue [Re(bpy)(CO)3(py)]+ (Table 1). This is indicative of the N-donor ligand in this position having a limited influence on the photophysical properties of the complex.

The influence of the triazole moiety is far more apparent when incorporated into the chelating N^N ligand in complexes of this type where it has a direct impact on the energy of the

LUMO and hence the energies of the singlet and triplet MLCT states. Obata et al. first reported the 4-(pyrid-2-yl)-1,2,3-triazole (pytz) complex 2 which exhibits a significantly blue-shifted emission maximum (538 nm) compared to that of [Re(bpy)(CO)3Cl].21 Here, the electron-rich triazole moiety leads to destabilisation of the pytz pyridine-based LUMO relative to that of bpy thus leading to higher energy 1MLCT and 3MLCT states. When the -donor axial chloride ligand is replaced by a neutral pyridine donor (3) the emission maximum is further blue-shifted to the blue-green region of the spectrum (496 nm) through stabilisation of the HOMO and results in a ten-fold increase in the measured quantum yield.22 Crowley, Gordon and co-workers have shown that inclusion of the triphenylamine (TPA) substituent in the 5-position of the pytz pyridyl ring in 4 results in the lowering of the energy of emission.23 This arises due to a switching of the nature of the emissive state from 3MLCT to 3ILCT character. This is indicated by time-resolved infra-red data in which the transient carbonyl stretching modes are observed to undergo a slight negative shift (as opposed to positive change in the cases of 3MLCT states24, 25) upon photoexcitation due to the increased electron density at the metal deriving from the TPA to pytz-* charge-transfer transition.

Figure 4. Structures of rhenium(I) and iridium(III) complexes with neutral N-donor triazole-based ligands.





Table 1. Summarised photophysical data for complexes 1-29 (wavelengths of 3MLCT emission maxima and photoluminescence quantum yields)

Complex em / nm ( / %) Complex em / nm ( / %)

[Re(bpy)(CO)3Cl] 633 (0.27)a 14 575 (-)d

[Re(bpy)(CO)3(py)]+ 549 (-)b 15 563 (-)d

1a 543 (-)b 16 Not observed2 538 (0.33)a 17 565, 610 (-)e

3 496 (3.10)a 18 544, 585 (-)e

4 573 (3.70)b 19 Not observed5 582 (0.56)c 20 476, 508 (0.82)b

6 564 (0.17)a 21 454, 483 (0.28)b

7 617 (0.06)a 22 532, 568 (4.30)b

8 477, 507 (21.0)b

23 724 (0.50)a

9 452, 483 (22.0)b

24 713 (0.60)a

10 478, 506 (2.10)a

25 444, 474 (-)e

11 455, 484 (2.20)a

27 583 (-)d

12 600, 635 (9.0)a 28 Not observed13 Not reported 29 595 (9.30)a

a acetonitrile, b dichloromethane, c 98/2 H2O/DMSO, d 77 K, n-butyronitrile, e 77 K, 4/1 EtOH/MeOH

Elaboration of the ligand architecture through the quinolyl-analogue 5 leads to a red-shift in emission compared to 2 largely through stabilisation of the ligand-centred LUMO.26 Further, the ‘inverse’ 1-(pyrid-2-yl)- and 1-(quinol-2-yl)-1,2,3-triazole ligands in complexes 6 and 7 result in red-shifted emission spectra relative to their ‘regular’ triazole-based analogues 2 and 5 respectively (Table 1). Work by the group of Crowley has shown that the complexes bearing the regular pytz ligand framework are more sensitive to donor solvent effects and have faster rates of non-radiative relaxation when compared to their inverted pytz counterparts.27

Figure 5. Emission spectra of complexes 8 and 39 in acetonitrile.

The replacement of bpy by pytz in cationic biscyclometalated iridium(III) complexes has a dramatic effect on the photophysical properties and the nature and localisation of the emissive state. The archetypal complex

[Ir(ppy)2(bpy)]+ exhibits a broad, featureless emission band characteristic of a 3MLCT/3LLCT state in which the excited electron is localised on the bpy ligand. In contrast, De Cola and co-workers reported that complexes of the form [Ir(Arpy)2(pytz)]+ (ArpyH = 2-phenylpyridine (8), 2-(2,4-difluorophenyl)pyridine (9)) exhibit an emission band at higher energy with vibronic progressions indicative of a change to 3MLCT/3ILCT state character (Figure 5). 28-30 Thus, whilst the LUMO is still localised on the ancillary pytz ligand its destabilisation relative to that of the bpy analogue results in a relocalisation of the unpaired electron in the T1 excited state which now resides on the cyclometalated ppy pyridine rings rather than on the ancillary ligand.

Similar results have been reported by Donnelly and co-workers in their investigation of more flexible triazole-based ancillary ligands featuring a methylene spacer between the pyridine and triazole rings (10 to 12).31 Here, coordination of the triazole ring is forced to occur at the N(2)-position and the resulting luminescence spectra are very similar to those of their pytz analogues indicating that emission derives from the same 3MLCT/3ILCT state.31

Figure 6. Structures of rhenium(I), ruthenium(II), osmium(II) and iridium(III) complexes with neutral N-donor triazole-based ligands.

Whilst complexes of the 5d6 metal ions rhenium(I) and iridium(III) bearing pytz-like ligands are observed to be emissive with comparable quantum yields to their bpy analogues, the same cannot be said for those complexes of the 4d6 metal ruthenium(II). The reduced ligand field splitting for ruthenium(II) compared to heavier 5d6 metal ions results in lower energy 3MC states of greater thermal accessibility from 3MLCT states. The phenyl-substituted homoleptic pytz complex 13 (Figure 6) exhibits significantly blue-shifted 1MLCT absorption bands compared to those of [Ru(bpy)3]2+ and the complex is observed to be non-emissive.32 Indeed, the





presence of triazole-containing N-donor ligands in ruthenium(II) complexes is, in most cases, observed to lead to quenching of emission intensity regardless of whether or not the excited state is localised on the triazole-containing ligand. The series of complexes 14 to 16 is observed to yield progressively blue-shifted 1MLCT absorption bands and similarly shifted and quenched emission bands with increasing pytz content within the ligand set.33 Whilst centred on the dimethyl-bpy (dmbpy) ligands, the LUMO of 14 and 15 is destabilised due to the reduced -acceptor character of the pytz ligands leading to increased electron density at the metal and hence increased dmbpy-based back-bonding. As a result, the 3MLCT state also becomes destabilised making the 3MC state more thermally accessible through incorporation of triazole donors into the ligand set. We have obtained similar results for the symmetrical 4,4’-bitriazolyl (btz) series of complexes 17 to 19.34 Interestingly, DFT results suggest that for the homoleptic complex [Ru(btz)3]2+ (19) the btz ligand-centred LUMO is so high in energy (by a further 1 eV compared to bpy) that the ordering of the 3MLCT and 3MC states are reversed such that the lowest energy singlet and triplet excited states have MC character.

Due to the intrinsically larger ligand-field splitting and hence higher energy 3MC states for 5d metals their btz complexes can exhibit photoluminescence. A series of biscyclometalated arylpyridine-based iridium(III) complexes bearing a btz ancillary ligand (20 to 22) have been reported which are emissive with reasonable quantum yields.35, 36 Our group has recently reported the heteroleptic osmium(II) complexes 23 and 24 incorporating ancillary btz ligands.37 In contrast to their ruthenium(II) analogues 17 and 18 respectively these osmium(II) complexes were observed to be emissive in the deep-red/near-IR region of the spectrum from bpy-centred 3MLCT states with comparable quantum yields to that measured for [Os(bpy)3]2+ (Figure 7). Based on these results, light-emitting electrochemical cell (LEC) test devices were prepared which resulted in electrochemiluminescence from the incorporated complexes. Unfortunately the devices showed poor stability, most likely stemming from restructuring of the active layer due to the propensity for 23 and 24 to form polycrystalline rather than amorphous films. The homoleptic complex [Os(btz)3]2+ (25) is, however, not emissive in solution at room temperature but becomes brightly emissive at 77 K in a rigid solvent glass (max = 444, 474 nm).37 The deactivation of emissive states by the incorporation of the btz ligand is also demonstrated in its rhenium(I) complex [Re(btz)(CO)3Cl] (26) which is non-emissive.38 The data for these latter two complexes shows that despite the large ligand-field splitting for these metal ions, if the 3MLCT state is localised upon the triazole rings of a ligand and hence will be of high energy then significant quenching of emission will result.

Figure 7. UV-visible absorption spectra (top left) and room temperature emission spectra (top right) in acetonitrile solutions and emission spectra at 77 K in 4:1 ethanol/methanol glass (bottom left) for complexes [Os(bpy)3]2+ and 23 to 25. Composite photograph of room temperature solution emission from [Os(bpy) 3]2+, 23 and 24 and 77 K emission from 25. Adapted from reference 37.

Tridentate 2,6-bis(1,2,3-triazol-4-yl)pyridine (btzpy) ligands have also been investigated as analogues of well-known tpy ligand systems.39 Complexes of the type [Ru(tpy)2]2+ are known to be essentially non-emissive in solution at room temperature. Here the quenching stems from the deviation from an ideal octahedral coordination environment about the metal centre due to the geometric constraints of the ligand. Unsurprisingly, and with reference to the electronic properties of triazole-containing ligands described above, the complexes [Ru(tpy)(btzpy)]2+ (27) and [Ru(btzpy)2]2+ (28) are non-emissive in room temperature solutions.40 The 1MLCT bands in their UV-visible absorption spectra are observed to undergo a similar blue-shift upon increasing triazole content to that observed in the series of pytz and btz complexes 14 to 19 described earlier.





Figure 8. Electronic absorption and photoluminescence spectra for an aqueous solution of the chloride salt of 29 (top); confocal microscopy images of emission from 29 (green) after incubation with HeLa and U2OS cells and overlaid with the mitochondrial stain Mitoview 633 (red) (bottom). Adapted from reference 41.

Our group was able to show that the osmium(II) analogue of 28, [Os(btzpy)2]2+ (29) is emissive at 595 nm due to the higher energy of the 3MC state for the complex.41 The complex was further investigated as a potential luminescence imaging probe for confocal microscopy. It was shown to be readily taken up by HeLa (cervical) and U2OS (osteosarcoma) cancer cell lines and displayed a high degree of localisation at the mitochondria (Figure 8).

Complexes with cyclometalated 1,2,3-triazole-based ligands

Schubert and co-workers first reported the preparation and photophysical properties of cyclometalated complexes bearing aryltriazole-based ligands.28 Through reaction of IrCl3 with 4-phenyl-1,2,3-triazole and subsequent addition of an ancillary ligand the heteroleptic ptz complexes 30 to 33 were realised (Figure 9). The bpy-containing complex 30 is observed to exhibit blue-shifted emission (Table 2) relative to the analogous complex [Ir(ppy)2(bpy)]+ (max 580 nm) which was attributed to destabilisation of the bpy centred LUMO for the complex. It is likely that the reduced -acceptor character of the triazole rings of the ptz ligand in 30 compared to the pyridine rings in its ppy analogue lead to increased electron density at the metal and increased -back bonding to bpy leading to the higher energy LUMO.

Figure 9. Structures of iridium(III) cyclometalated aryltriazole complexes.

De Cola later reported the analogous picolinate and 1,2,4-triazolate complexes 34 and 35 based on 4-(2,4-difluoro)phenyl-1,2,3-triazole (dfptz) derived ligands.42 Here, the fluorine substituents on the aryl rings stabilise the HOMO of the complexes relative to their non-fluorinated analogues. This leads to a significant blue-shift in emission spectrum for

the pinacolate complex 34 (max 498 nm) relative to 33 (max

527 nm). The emission from 34 is also strongly solvatochromic (max 475 nm in toluene to 534 nm in dimethylformamide) indicative of the large 3LLCT contribution to the emissive state whereas the triazolate (35) shows much greater 3ILCT character.

The groups of Zysman-Colman43 and De Cola44 have prepared and investigated a number of cationic biscylometalated complexes containing ptz or dfptz ligands with a bpy-based ancillary ligand. As with the blue-shift in emission for 34 relative to that for 33 the emission spectra of complexes 36 to 38 exhibit bands at higher energy than those for the non-fluorinated complexes 39 and 40. Emission can be further shifted towards the blue through the incorporation of -donating dimethylamino substituents into the 4- and 4’-positions of the bpy ligands thereby leading to destabilisation of the LUMO. Complexes 37 and 39 to 41 have also been shown to exhibit electrochemiluminescence.45

Table 2. Summarised photophysical data for complexes 30-76 (wavelengths of 3MLCT emission maxima and photoluminescence quantum yields)

Complex em / nm ( / %) Complex em / nm ( / %)

30 560 (45)a 54 Not reported31 495 (29)a 55 Not reported32 435 (2.0)a 56 Not observed33 527 (11)a 57 768 (-)f

34 498 (1.2)a 58 725 (-)f

35 440, 462 (5.0)a 59 723 (-)f

36 514 (73)b 60 Not reported37 497 (81)b 61 Not reported38 498 (80)b 62 500 (0.2)d

39 580 (25)b 63 464, 485 (0.3)d

40 575 (35)b 64 492 (0.7)d

41 495 (47)b 65 493 (0.3)d

42 512 (77)b 66 505 (1.5)d

43 485 (0.2)b 67 730 (0.2)b

44 452 (0.05)b 68 650 (0.82)b

45 487 (0.3)b 69 648 (-)b

46 461 (0.03)b 70 643 (11)e

47 495 (2.7)b 71 688 (7.5)e

48 Not observed 72 691 (2.5)e

49 480, 510 (76)a 73 694 (1.7)e

50 465, 489 (50)a 74 Not reported51 392, 418 (-)c 75 Not reported52 Not observed 76 600 (0.03)b

53 Not reporteda dichloromethane, b acetonitrile, c 77 K, butyronitrile, d tetrahydrofuran, e 4/1 EtOH/MeOH, f 4/1 DMF/MeOH





Interestingly De Cola and co-workers were able to prepare complex 42, which is isomeric with 37 through careful control of reaction conditions during synthesis.44 Whilst biscyclometalated iridium(III) complexes are ordinarily formed with a trans arrangement of the N-donors of the C^N ligands, this complex with a cis arrangement exhibits a slightly red-shifted emission spectrum relative to that of its more conventional trans isomer. When incorporated into LEC devices this alternative isomer was seen to yield greater luminance.

The 1,2,3-triazole moiety can of course be incorporated into both the ancillary and cyclometalated ligands of heteroleptic biscyclometalated complexes. The aryltriazole-based pytz complexes 43 to 46 display unstructured emission bands indicative of 3MLCT/3LLCT-based emission. Here, the replacement of dfppy in 9 with dfptz in 44 leads to a dramatic destabilisation of the unoccupied orbitals centred on the arlytriazole ligands compared to those dominated by the dfppy pyridine rings. Thus, the localisation of the excited electron in 44 switches to the pyridine ring of the ancillary pytz ligand instead of the cyclometalated ring as for 9.

Zysman-Colman reported the preparation of the btz complexes 47 and 48 each containing four triazole rings.35 The non-fluorinated complex 47 is only weakly emissive whilst the fluorinated complex 48 is not emissive in room temperature solutions. Here the 3MLCT/3LLCT state is forced to localise on the ancillary btz ligand due to the high energy of the unoccupied orbitals of the dfptz ligands and is consequently of high enough energy to facilitate deactivation through 3MC state population as for 19, 25 and 26.Triscyclometalated dfptz-based complexes have also been reported. The heteroleptic complexes 49 and 50 which contain a third cyclometalated ppy or dfppy ligand are intensely emissive.42 Here the emissive state is dominated by the arylpyridine ligand. The homoleptic complex 51 is non-emissive at room temperature but shows blue emission (max 392, 418 nm) at 77 K. The high energy of the triazole-centred LUMO leads to a heavily destabilised 3MLCT/3ILCT state and facile 3MC state population. For this same reason temperature dependent quenching of emission has been observed previously for pyrazole-based analogues of 51.46 These results, along with the observation detailed above for 47 and 48, again highlight the importance of avoiding localisation of the excited state on the triazole rings of the complex if efficient luminescence is desired.

A range of cycloruthenated complexes have also been reported (Figure 10). The ptz ligand in 52 results in a destabilised HOMO compared to analogous pytz-based complexes such as 14 due to the strongly -donating anionic metalated phenyl ring.47 This results in the optical absorption of the spectrum extending beyond 600 nm and into the orange/red region of the spectrum. A dinuclear analogue, 53, has been reported which contains both RuN6 and RuN5C coordination domains.48 Due to the redox asymmetry of the complex, one electron oxidation results in the observation of metal-to-metal charge transfer bands in the near-infrared at around 1300 nm.

Figure 10. Structures of ruthenium(II) cyclometalated aryltriazole complexes.

Complexes containing the bis(triazolyl)benzene (btzb) ligand framework have also been investigated. Again, due to the mixing of metalated aryl- character in the d-orbital-dominated HOMO, complexes 54 to 56 display more negative oxidation potentials than typical tpy complexes such as [Ru(tpy)2]2+.49 Schubert and co-workers have also explored complexes bearing btzb-type ligands including 57 to 59 which include carboxylate subsitituents on the terpyridyl ligand.50-53 The resultant stabilisation of the terpyridyl-centred LUMO results in the optical absorption profile extending beyond 700 nm with extinction coefficients of up to 2 x 104 dm3 mol-1 cm-1

in the visible region. DSSC devices utilising 57 achieved high power conversion efficiencies of up to 4.5 % compared to 5.2 % for cells using the benchmark “black dye” complex.

Dipyridylbenzene complexes analogous to the btzb complexes described had been shown by Sauvage54, 55 to undergo Ag+ mediated reductive C-C coupling to yield dinuclear complexes. In a similar manner 54 undergoes reductive coupling to form 60.56 An analogous complex incorporating a pyrene-based linker (61) has also been reported. Upon one-electron oxidation both 60 and 61 exhibit intervalence charge transfer bands in the near-infrared region of the spectrum between 1000 and 2700 nm. Upon removal of a second electron these IVCT bands are observed to disappear.

Complexes with C-donor triazolide and triazolylidene ligands

In addition to coordination through the N-atoms of the 1,2,3-triazole, coordination can also occur at the unsubstituted carbon atom of the ring either as an anionic triazolide57 or as a mesoionic ‘abnormal’ triazolylidene.15 Swager and co-workers reported the series of bicyclometalated iridium(III) complexes 62 to 66 (Figure 11).13 Here, copper(I) triazolides that are ordinarily formed as intermediates in CuAAC reactions, were generated and used as transmetalating agents resulting in the formation of neutral iridium(III) pyridyltriazolide complexes. The complexes displayed structured bands in their emission





spectra indicating phosphorescence from 3MLCT/3ILCT-based states. These triazolide complexes were found not to be stable, however, and could be readily converted into their cationic N^N-pytz analogues through heating in glycerol solutions or through UV photolysis.

Schubert, Berlinguette, Dietzek and co-workers subsequently reported the bis-tridentate triazolide complex 67.12 Through application of RuAAC rather than CuAAC chemistry they were able to isolate the triazolylbipyridine ligand precursor with 1,5-disubstitution regiochemistry at the triazole ring. Chelate coordination at the bpy domain of the ligand thereby precludes coordination to either of the N-atoms of the triazole ring but instead forces cyclometalation to form the triazolide. The complex was found to be emissive in solution at 730 nm and underwent formation of the corresponding triazolylidene complex 68 upon methylation of the triazole N(3)-position. Emission from 68 is significantly blue shifted (650 nm) when compared to 67 (730 nm) stemming from the far greater -donor character of the anionic triazolide ligand which results in mixing with and destabilisation of the ruthenium-centred HOMO. Interestingly, an analogue of 68 could be reversibly formed from 67 by protonation/deprotonation. Whilst the complex [Ru(tpy)3]2+ is essentially non-emissive in room temperature solutions due to its ligand-constraining deviation from an ideal octahedral coordination environment, the increased emission from 67 and 68 stems from the far stronger -donor character of the triazolide and triazolylidene donors respectively. This destabilises the 3MC states in these complexes making their population from 3MLCT states less efficient.

Figure 11. Structures of iridium(III), ruthenium(II) and iron(II/III) triazolide and triazolylidene complexes

Albrecht has reported the bidentate ruthenium(II) triazolylidene complex 69.58 The complex exhibits a slightly destabilised HOMO compared to that of [Ru(bpy)3]2+ due to the slight anionic charge of the triazolylidene C-donor atom and results in a slight red-shift in the emission spectrum. Shubert and Berlinguette reported the series of bis(triazolylidene)

complexes 70 to 73.59 As a consequence of the two strong carbene donors in the complexes 73 was found to exhibit an extremely long-lived 3MLCT state ( = 7.9 s), amongst the longest recorded for a bis(tridentate) ruthenium(II) complex.

One of the greatest challenges in modern transition metal photophysics has been the stabilisation of the excited states of iron(II) analogues of ruthenium(II) complexes. Typically, complexes such as [Fe(bpy)3]2+ undergo an extremely rapid excited state deactivation cascade on the fs to ps timescale involving population of both 3MC and 5MC states.60, 61 It is therefore of no surprise that coordination of strongly donating triazolylidene ligands to iron(II) has been investigated as a strategy for destabilising these 3MC and 5MC states in order to elongate photoexcited MLCT state lifetimes.62 Matsubara, Sundstrom, Warnark and co-workers have recently reported complexes 7463 and 75.64 By incorporating strongly donating bi(triazolylidene) ligands to destabilise metal-centred states and an electron-accepting bpy ligand on which to localise the MLCT state, an excited state lifetime of 13 ps was achieved for 75. Interestingly, in attempting to obtain a homoleptic analogue of 75 containing three bi(triazolylidene) ligands, the iron(III) d5 complex 76 was isolated.65 The complex undergoes reversed charge-transfer directionality to yield a 2LMCT state (LMCT = ligand-to-metal charge-transfer) which exhibits extraordinarily long-lived emission for an iron-based complex ( ~ 100 ps). This is somewhat surprising given that relaxation to the ground state in this complex is a spin-allowed process.

Photochemistry of 1,2,3-triazole-based complexesWhilst there are many examples of efficient

phosphorescent complexes of the 5d-metals bearing 1,2,3-triazole-based N-donor ligands, those of the 4d-metals such as ruthenium(II) have notably quenched emission or are non-emissive.33, 34 This is primarily due to the smaller ligand field splitting with comparatively stabilised 3MC states offering more efficient routes to excited state deactivation to the ground state. However, thermal population of 3MC states from photoexcited 3MLCT states offers possible access to photochemical reactivity through ligand ejection.

A common strategy for effecting photochemical ligand ejection in chelate complexes relies on the inclusion of substituents on the ligand in positions adjacent to the coordinating N-atoms to provide steric congestion.66, 67 This has the result of weakening the metal-ligand bonds, thereby stabilising 3MC states and making them more readily populated after photoexcitation. For 1,2,3-triazole-based complexes the unsubstituted N(2)-position adjacent to the coordinated N(3)-atom of the triazole ring means that such steric promotion is absent. However, as has been noted earlier the inclusion of triazole-based ligands in heteroleptic complexes can yield a destabilisation of the LUMO, regardless of whether it is localised on the triazole-containing ligand. Consequently, the 3MLCT state will be destabilised and brought into closer energetic proximity to photoreactive 3MC states. Thus, triazole-induced electronic destabilisation of 3MLCT states may therefore promote 3MC state population and resultant





photochemistry similarly to that promoted sterically through 3MC state stabilisation.

Indeed, we have shown that the complex [Ru(bpy)2(btz)]2+

(17) readily undergoes photochemical ligand ejection of the btz ligand in acetonitrile solutions to yield the cis-bis(solvento) complex [Ru(bpy)2(NCMe)2]2+ (77), reaching completion in a matter of minutes (Scheme 1 top).68 The tetrazaphenanthrene (TAP) analogue has also been reported and also undergoes photolysis when followed by UV-visible absorption spectroscopy.69 Crowley and co-workers have shown similar behaviour for the ‘inverted’ pytz complex 78 which again yields 77.27 In this case, Crowley reported NMR evidence of a ligand-loss intermediate bearing a monodentate pyridyltriazole ligand. Intriguingly, the data was suggestive of the monodentate ligand being coordinated through what would be anticipated to be the weaker triazole N(2) donor rather than through the pyridine N-atom.

With a higher energy LUMO and 3MLCT state due to the presence of a second btz ligand, the complex [Ru(bpy)(btz)2]2+

(18) might be expected to be similarly, if not more readily, photochemically reactive than 17. Indeed, the complex does undergo photochemical ejection of one of the btz ligands but its photochemical behaviour is far more fascinating. Ultimately, photochemical ejection of btz does yield a bis(solvento) complex as would be expected. However, we observed that this proceeds with an unusual concomitant rearrangement of the remaining chelate ligands such that they become coplanar, thereby yielding trans-[Ru(bpy)(btz)(NCMe)2]2+ (82, Scheme 1 bottom).70

Scheme 1. Photochemical reactivity of ruthenium(II) bitriazolyl complexes.

Of further significance is that the formation of 82 from 18 is observed to occur via a ligand-loss intermediate (80) containing a 1-btz ligand coordinated trans to a solvent ligand with the same planar [Ru(bpy)(2-btz)] fragment. Due to the trans arrangement of the monodentate btz ligand and the coordinated solvent this intermediate is remarkably stable; when left in the dark at 40 C the photogenerated intermediate trans-[Ru(bpy)(2-btz)(1-btz)(NCMe)]2+ (80) is observed to revert to the starting material 18 with a half-life in excess of 12 hours. This stability enabled 80 to be isolated and

crystallographically characterised and thus this novel species to be unambiguously identified (Figure 12 left).71

Figure 12. Molecular structures of the photoproduct complex cations trans-[Ru(bpy)(2-btz)(1-btz)(NCMe)]2+ (80) (left) and trans-[Os(btz)2(pyridine)2]2+ (84) (right) (ellipsoids at 50 % probability, hydrogen atoms, co-crystallised solvent and counterions removed for clarity). Adapted from references 71 and 72.

Computational DFT data suggested that the T1 states of both 17 and 18 are of bpy-centred 3MLCT character,34 however, we noted that the S1 and T1 states of the homoleptic btz complex 19 were both 3MC in nature. Our group went on to show that the pentatriazole complex [Ru(pytz)(btz)2]2+ (79) undergoes similar photochemical reactivity to that displayed by 18 (Scheme 1 bottom) producing photoproduct 83 via the intermediate 81 with photochemical consumption of the complex under optically dilute conditions being complete within 5 minutes (Figure 13 top).73 This complex can be viewed as bridging the gap between 18 and 19 between which the nature of the lowest energy triplet state switches. Intriguingly, computational studies indicated that whilst the S1 state is of 1MLCT character and the T1 state is of 3MC character, the singlet and triplet MLCT and MC states are extremely close in energy and in the context of the errors associated with these methods can be considered to be essentially degenerate at the ground state geometry. Thus, the Frank-Condon excited state of 79 is extremely close to, and perhaps even at, the crossing point on the triplet potential energy surface between the minima for the 3MLCT and 3MC states.

Whilst complexes of iridium(III) and osmium(II) are generally considered to be inert to photosubstitution due to the high energies and inaccessibility of their 3MC states, the incorporation of the btz ligand has enabled unmasking of latent photochemical reactivity in complexes of these ‘photoinert’ metals. Zysman-Colman and co-workers noted that the complex [Ir(dfptz)2(btz)]+ (48) was not photochemically stable in acetonitrile and ejected btz to yield a solvated photoproduct.35 With triazole donors in the cyclometalated ligand in addition to the btz ligand, the LUMO, and hence 3MLCT state of the complex is expected to be very high in energy. Evidently, in this case, this is enough to enable thermal population of the 3MC states that result in the observed photodecomposition.





Figure 13. UV-visible absorption spectra recorded during the photolysis of [Ru(pytz)(btz)2]2+ (79) (top) and [Os(btz)3]2+ (25) (bottom) in acetonitrile solutions. Adapted from references 73 and 72.

In our studies of osmium(II) btz complexes we noted that the emissive heteroleptic complexes [Os(bpy)2(btz)]2+ (23) and [Os(bpy)(btz)2]2+ (24) were photoinert to ligand loss under the conditions whereby their ruthenium(II) analogues 17 and 18 readily undergo ligand ejection.37 This is unsurprising as the osmium-based 3MC states are expected to be much higher in energy than for ruthenium, thus their population from the bpy-centred 3MLCT state would not be expected to occur.74 However, in a similar fashion to the iridium(III) btz complex 48 the homoleptic complex [Os(btz)3]2+ (25) will have a high energy LUMO and thus a high energy 3MLCT state due to the necessary localisation of this state on the triazole rings of the ligands, therefore similarly promoting photochemical reactivity. Indeed, in pyridine (py) solutions we observed that 25 undergoes rapid photochemical ligand ejection of btz in 2 minutes and yields the complex trans-[Os(btz)2(py)2]2+ (84) (Scheme 2, top route) which was crystallographically characterised (Figure 12 right). 72 Here, photochemical ligand ejection is accompanied by a similar rearrangement of the remaining chelate ligands which become coplanar as observed for 18 and 79. In acetonitrile solutions, photochemical reactivity is observed to bifurcate along two pathways; in the same manner as in pyridine, the complex is observed to form the ligand ejection photoproduct trans-[Os(btz)2(NCMe)2]2+

(trans-86). In analogous fashion to the reactivity of the ruthenium(II) complexes 18 and 79 this is observed to proceed through the ligand-loss intermediate trans-[Os(2-btz)2(1-btz)(NCMe)]2+ (trans-85). However, a second photoproduct cis-[Os(btz)2(NCMe)2]2+ (cis-86) is also observed to form. During the course of the reaction as followed by 1H NMR spectroscopy an additional intermediate is observed to grow in and decay. With six triazole proton resonances for the complex indicating that each triazole ring of the three btz ligands is magnetically

unique, we were therefore able to identify the cis ligand loss intermediate cis-[Os(2-btz)2(1-btz)(NCMe)]2+ (cis-85).

Scheme 2. Photochemical reactivity pathways for [Os(btz)3]2+ (25) in pyridine (top) and acetonitrile (bottom left and bottom right) solutions.

The extraordinary observations surrounding the photochemical ligand ejection and rearrangement chemistry have highlighted the involvement of novel processes occurring on the triplet potential energy surface of these complexes and the existence of highly original excited state species. Indeed, the results obtained for complexes 18, 79 and 25 prompted us to conduct a recent theoretical investigation in what has been a highly productive collaboration with Isabelle Dixon, Fabienne Alary and Jean-Louise Heully at the Université Paul Sabatier, Toulouse, France.75

As a starting point we optimised the geometries of a series of 3MC states for the ruthenium(II) complexes 18 and 79. These classical Jahn-Teller type distorted geometries involve population of a dz

2-like d* orbital by the excited electron and elongation of two mutually trans Ru-N bonds.76, 77 These in themselves fail to account for the observed coplanarisation in the photoproducts for these complexes. However, attempted relaxed surface scan exploration of the 3PES for 18 in an attempt to navigate from its T1 3MCLT state to these classical 3MC states resulted in the serendipitous discovery of a highly original and hitherto unrecognised class of ‘flattened’ 3MC state, 3MC(F) (where the “F” denotes its flattened structure, Figure 14). An analogous 3MC(F) state was then located for 79. These new 3MC(F) states involve the alternative population of a dx

2-y

2-like d* orbital. This results in elongation of both Ru-N bonds for the same ligand (which ultimately undergoes





dechelation and ejection from the complex) with concomitant widening of the N-Ru-N angle between the two Ru-N bonds trans to the departing ligand from 128 to 136. Thus, the Ru(N^N)(btz) fragment (N^N = bpy or pytz) can be seen to be mid-way to the planar arrangement observed in initial photoproducts 80 and 81.

Figure 14. Top: Optimised geometries of novel flattened (left) and pentacoordinate (right) 3MC states of [Ru(bpy)(btz)2]2+ (18) annotated with elongated Ru-N distances (Å) and N-Ru-N angle for the [Ru(bpy)(btz)] fragment. Bottom: Schematic potential energy surface diagram for the ligand dechelation photoreactivity of 18. The F and P in parentheses denote flattened and pentacoordinate character respectively in the geometries shown. Adapted from reference 75.

When calculations were carried out to optimise the minimum energy crossing points (1,3MECPs) for these 3MC(F) minima, subsequent singlet state optimisations led to reformation of the ground states for 18 and 79 indicating that their 3MC(F) state species are not in themselves the primary route to the observed photoproducts. However, they were clearly identified as potential intermediate 3MC states to further possible minima on the 3PES on this route.

We were then able to locate further 3MC minima on the 3PES for 18 and 79 in which the coplanarisation of the Ru(N^N)(btz) fragment was far more advanced, with the ruthenium(II) centre being pseudo-pentacoordinate square pyramidal in geometry (3MC(P) as shown in Figure 14). Here, one of the Ru-N distances to the departing ligand was elongated (~2.35 Å) whilst the second could be said to be fully de-coordinated (>3.4 Å). Optimisation of the 1,3MECPs for these states (1,3MECP(P)) followed by singlet state geometry relaxation thus resulted in formally pentacoordinate distorted square pyramidal 16-electron species (1GS(P)). When formed in acetonitrile solutions (as in experimental studies) these unsaturated species will be rapidly trapped by solvent to yield the solvento photoproducts 80 and 81. ‘Nudged-elastic band’ calculations showed that the energy barrier to 3MLCT depopulation for 18 via this route to be extremely small at 0.03 eV clearly underlining the importance of these new states to the photoreactivity of the complex. In an elegant review of ligand photoejection chemistry from ruthenium(II) complexes Turro and her co-authors had postulated that 80 was the

product of ground state isomerisation of an initially formed cis isomer of this intermediate. However, through these theoretical investigations we have been able to show that ligand rearrangement and dechelation proceeds through geometry evolution processes that occur wholly in the excited state on the 3PES of the complex.

Thus, triazole-based coordination chemistry has enabled the observation of not only highly novel photochemical reactivity but also allowed us to gain significant insights into original excited states and their dynamics which may have broader impact and relevance in transition metal photochemistry. Presumably, similar flattened and pseudo-pentacoordinate 3MC excited states are involved in the photochemistry of the osmium(II) complex 25, but here bifurcation must occur on the 3PES between the trans pathway to yield trans-85 and trans-86 and that leading to the cis-photoproducts cis-85 and cis-86.

Conclusions and OutlookThe use of 1,2,3-triazole derived ligands in coordination chemistry, especially in the area of photophysics and photochemistry, has rapidly expanded over the last decade. Versatile CuAAC reaction protocols have allowed access to readily tunable ligand systems yielding highly efficient phosphorescent materials as well as potent sensitisers for light-harvesting applications. However, the incorporation of 1,2,3-triazole-based ligands can result in electronic promotion of unusual photochemical ligand release and has enabled the illumination of as yet unexplored regions of the triplet excited state potential energy surface of tris-bidentate complexes.

The contributions to the literature highlighted here allow us to draw some important conclusions surrounding the use of 1,2,3-triazole-based ligands for photoactive compounds;

The inclusion of 1,2,3-triazole N-donor ligands in complexes of 4d metals, in particular ruthenium(II), generally leads to quenching of emission quantum yield through indirectly promoting 3MC state population.

The use of these ligands as an ancillary ligand for 5d metal ions such as rhenium(I), osmium(II) and iridium(III) enables brightly luminescent complexes to be realised.

Predominant localisation of the photoexcited 3MLCT state on the triazole ring of a triazole-containing ligand should, however, be avoided if luminescence is desired. The high energy of the unoccupied orbitals centred on the triazole moiety facilitates 3MLCT state depopulation via 3MC states.

The electronic properties of the triazole moiety, in particular with regard to the btz ligand, enables access to novel photoreactive excited states resulting in ligand ejection and unusual ligand rearrangement reactions.

Whilst there have been notable examples of the exploitation of triazole-based ligands in photophysics and photochemistry of transition metal complexes, significant





potential opportunities for wider exploration still exist. For example, the ability to generate brightly luminescent chromophores offers new avenues for continued development in light-emitting technologies. This, combined with the ease with which the triazole moiety can be used in bioconjugation reactions also presents wide scope for the development of luminescent biological cellular imaging agents and multimodal theranostic platforms. The photochemical ligand ejection reactions observed in these complexes may also enable novel systems to be developed for light-initiated DNA-binding anticancer activity. Further, this novel photochemistry may also allow routes to be explored towards light-driven supramolecular self-assembled materials.

Complexes based on 1,2,3-triazole-containing ligands have yielded many intriguing results and the future is still bright. The journey is not over and many new lands remain to be explored.

Conflicts of interestThere are no conflicts of interest to declare.

Author informationPaul A. ScattergoodPaul is a synthetic inorganic chemist with a background and strong interest in the photophysical study of transition metal complexes. He obtained his MChem from the University of Sheffield (2010), completing an undergraduate research project with Dr Nathan Patmore before undertaking doctoral studies (PhD 2014) with Prof Julia Weinstein on photo-induced electron transfer (PET) in Pt(II) complexes and the excited state IR quantum controlled switching of PET outcomes. He is currently carrying out postdoctoral research in the Elliott group at the University of Huddersfield working on the photochemical properties of triazole coordination complexes.

Paul I.P. Elliott

Paul Elliott is Associate Dean of the School of Applied Sciences at the University of Huddersfield. He obtained his MChem undergraduate degree at the University of York in 2001 before moving to the University of Sheffield to undertake doctoral studies for his PhD (2005) on iridium-catalysed methanol carbonylation under the supervision of Dr Tony Hanyes. After completing his PhD he returned to York to take up a postdoctoral research position working with Prof Simon Duckett where he carried out work on para-hydrogen induced

NMR signal enhancement and made the very first experimental observations of the SABRE phenomenon. He moved to the University of Huddersfield in 2007 to take up his academic position, developing his independent research program on the photophysics and photochemistry of the late transition metal complexes.

AcknowledgementsThe authors thank the University of Huddersfield for supporting this work.

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