Specfrochimico Ada. Vol. 4OA. No. 7. pp. 643-650. 1984 058468539/84 $3.00 + 0.00

Pnnted in Great Britam 0 1984 Pergamon Press Ltd.

Heavy atom and complexation effects on micelle stabilized room temperature phosphorescence of anthracene, acridine and phenazine

RODNEY WOODS and L. J. CLINE LOVE

Department of Chemistry, Seton Hall University, South Orange, NJ 07079, U.S.A.

(Received 30 Nowmber 1983)

Abstract-Micelle stabilized room temperature phosphorescence is reported for two pyridinic nitrogen molecules, phenazine and acridine and the carbocyclic analogue, anthracene. The formation of a complex between silver and the pyridinic nitrogen is proposed, through which silver enhances phosphorescence by both the external heavy atom effect and donor-acceptor complexation. Thallium forms a weaker complex and is much less effective in inducing phosphorescence in the pyridinic heterocycles. Apparent prototropic equilibria for acridine in micellar solution reflect substantial differences between bulk phase pH and apparent pH in the region of the micellar assembly where the acridine resides. Fluorescence quantum yields and wavelength maxima are reported for anthracene, acridine and phenazine in sodium dodecyl sulfate micellar solution.

INTRODUCTION

Room temperature phosphorescence characteristics of molecules solubilized in fluid micellar solution, using external and/or internal heavy atom effect enhance- ments have been reported for many carbocyclics [ 141 and for one series of pyrollic nitrogen heterocyc- lies [7]. This study reports the first micelle-perturbed luminescence data of selected pyridinic heterocycles of the linear three-fused ring class, which are precursors of several important biologically active molecules. The effect of nitrogen intercalation in the bridging ring position is examined by comparison of the phos- phorescence characteristics of the series anthracene (the parent carbocyclic), acridine (in which a single pyridinic nitrogen is inserted in the 9 bridge position) and phenazine (where two pyridinic nitrogens replace the bridging carbons at the 9, 10 positions).

External heavy atoms such as silver(I) or thallium(I) or internal heavy atoms such as bromine have been necessary to promote sufficient intersystem crossing and phosphorescence decay via enhanced spin-orbit coupling to observe micelle stabilized room tempera- ture phosphorescence (MS-RTP) in fluid solution. In the present study two different external heavy atoms gave quite different results. Thallium(I) does not induce strong MS-RTP from any of the pyridinic nitrogen heterocycles, although it is effective with the carbocyclic analogues. Silver(I) does induce strong phosphorescence from the pyridinic nitrogen hetero- cycles but promotes weaker phosphorescence for the carbocyclic analogue than does thallium(I). BOUTILIER and WINEFORDNER [8] examined silver(I) and thallium(I) as external heavy atoms at 77 K and discussed the possibility of charge transfer complex formation between nza compounds or carbocyclics with silver(I). They found thallium(I) was ineffective as a heavy atom in enhancing the phosphorescence of a pyridinic nitrogen compound at 77K. BOUTILIER et al. [9] found silver introduces strong phos-

phorescence in purine and pyrimidine nucleosides at 77 K.

In order to preclude possible protonation effects and to examine the possibility of complex formation, a silver phenazine precipitate and pure, crystalline phenazine were examined using front surface lumines- cence. This matrix eliminates the possibility of triplet state prototropic equilibria that phenazine is reported to exhibit [lo].

These pyridinic nitrogen heterocyclics, particularly phenazine, strongly phosphoresce at 77 K [lCrl5]. It was considered highly probable that phenazine would phosphoresce in the absence of external heavy atoms due to the increased spin-orbit coupling [lo] and vibronic coupling [ 1 l] induced by the lone pair elec- trons on the two aza nitrogens. This was confirmed using sodium dodecyl sulfate micellar medium with no external heavy atoms.

Singlet state studies of the probe molecules were conducted to determine acid/base effects of the micel- lar microenvironment on the dissolved solute. Protonation equilibria for acridine producing species of different singlet state energies have been obser- ved [ 16, 171. The dynamic and complex environment provided by the micelle can produce large variances in the local pH experienced by a species depending on its solubilization site of the probe. In order to ascertain which excited state species (free base, first or second protonated species) exist in the micelle, the singlet state wavelength maxima and quantum yields were determined.

EXPERIMENTAL

Sample preparation

Sodium dodecyl sulfate (NaDS), either electrophoresis grade (Bio-Rad Laboratories, Richmond, CA) or specially purified for biochemical work (BDH Biochemicals, Poole, England), was dissolved in distilled deionized water or in 50: 50 water/methanol (Fisher spectroanalysed) using sonification (Branson Cleaning Equip., Shelton, CO, Model B-22-4

643

644 RODNEY WOODS and L. J. CLINE LOVE

ultrasonic cleaner, 125 watt). The heavy counterion surfactant was prepared by metathesis of either silver nitrate (J & S Scientific Co., Crystal Lake, CO, A.C.S. crystal) or thallous nitrate (Fisher Scientific Co., Fair Lawn, NJ, purified) and sodium dodecyl sulfate in water. The vacuum filtered (Reeve Angel filter paper, Clifton, NJ, grade 934AH glass fiber filter paper) crystals were recrystallized twice in 507” aqueous methanol and dried overnight in a vacuum oven at 373 K. The heavy atom micellar solutions were 0.08 M total dodecyl sulfate (50: 50Na/AgDS) or 0.15 M total dodecyl sulfate 70:30 NapIDS). All solutes (Aldrich Chemical Co,) were recrystallized where necessary from ethanol. High concen- tration stock solutions of solute (0.01 M) were prepared in either methanol or methyl ethyl ketone (J. T. Baker, spec- troanalysed). Working concentrations were prepared by spik- ing the micellar solutions with microliter aliquots of the concentrated stock solutions followed by sonilication to obtain typical concentrations of 5 x 10-s M. Nitrogen (AGL Welding Sup. Co., Clifton, NJ, high purity) passed through an oxygen trap (Alltech Associates Inc.) was bubbled through the sample in the cuvet for 15 min to deaerate the solution.

The silver : phenazine complex was prepared by metathesis of equimolar amounts of silver nitrate and recrystallized phenazine in SO:,, aqueous methanol. The precipitate was collected through vacuum filtration and vacuum oven dried overnight at room temperature.

All organic solvents used for the quantum yield determi- nations were supplied by Fisher and were spectrograde. Hydrochloric and sulfuric acids were Fisher A.C.S. reagent grade. Quantum yields were determined using the compara- tive methods [ 181 using quinine sulfate in O.lN sulfuric acid (r#~ (fl) = 0.55)as the standard. Quantum yield precision varied from k 0.01 to f 0.004 going from 0.48 to 0.016 observed quantum yields.

A combination electrode (Fisher Combination Electrode, No. 13-639-97, AgCl reference) and a Leeds and Northup 7410 pH meter were used to measure pH. All pH measure- ments were observed at immersion because of deleterious effects of the surfactant on the electrode response and therefore are approximate. Immediate pH readings were necessary because of the electrode KCI insolubility in NaDS micellar solution which resulted in a slow drift of less than 1 pH unit per min of immersion.

Front surface luminescence studies were run on an alum- inum block with the surface at 45 degrees from excitation and emission. The illuminated surface of the block was covered with grease (Dow Corning High Vacuum Grease) and the mortar ground solid sample was applied to this greased surface. The same procedure was followed for both the Ag: phenazine complex and phenazine. A blank of the high vacuum grease yielded no appreciable luminescence in the observed spectral region.

Luminescence measurements

Corrected spectra measurements employed a Spex Industries (Metuchen, NJ) Fluorolog 2 + 2 spectrofluorom- eter equipped with a Hamamatsu R928P photomultiplier tube operated at -900V. Emission correction factors ob- tained from the manufacturer were stored in a Spex Datamate computer. Digital integration of the corrected photon count signal was used to calculate the number of photons emitted for quantum yield determinations. Excitation corrected spectra were measured using a ratio of the sample photo- multiplier tube signal divided by a reference Rhodamine B quantum counter photomultiplier tube signal, which cor- rected for source power, flicker, and excitation monochro- mator throughput variations with wavelength. The corrected spectra were stored in the Datamate or on disk and then output to a digital printer plotter (Houston Inst. Co., Hi- Plot). All reported phosphorescence spectra are uncorrected due to rapid PMT response changes in the 70&9OOnm spectral region. To get corrected phosphorescence spectra the 125 stored correction factors in the Datamate would have to be modified giving poor correction by interpolation over a spectral region of 300-900 nm. A 450 watt continuous xenon source was used for all singlet state and all total luminescence studies. Precision of maxima measurements using the 450 watt source was * 2 nm.

Acridine phosphorescence appears as a shoulder on the residual fluorescence tail and is not well resolved under the conditions used for the data shown in Fig. 1. The fluorescence can be compensated for by a simple background subtraction procedure [19]. Fluorescence subtraction entails measure- ment of the spectrum of the sample in the presence and absence of oxygen, a triplet state quencher, followed by

5 16E04 I ’

I \a

OCOEOO 60000 750 00

Wavelength (nm)

Fig. 1. Emission spectra of acridine: (a) deaerated, phosphorescence and residual fluorescence; (b) aerated, residual fluorescence; (c)background subtracted phosphorescence spectra, i.e. subtraction of(b) from (a); 3.3 x lo- 5 M in 0.08 M Na/AgDS, Ex. 359 nm, slits, 14.4 nm Ex., 7.2 nm Em.; photon counting 1 s average per

0.5 nm increment, 1 scan; Spex Fluorolog 2 + 2, 450 watt continuous xenon source, uncorrected emission spectra.

Phosphorescence of anthracene, acridine and phenazine 645

subtraction of the aerated spectra from the deaerated spectra. This difference spectrum is due to phosphorescence, although some fluorescence may be present (under subtraction) if the fluorophore is oxygen sensitive. Over subtraction of the phosphorescence spectra results from the removal of the long lived triplet state pathway which allows for greater cycling through the absorption and fluorescence emission pathways and results in slightly higher fluorescence (0.5-l %) in the presence of oxygen. The uncorrected phosphorescence wave- length maximum for acridine does red shift several nm in the absence of the residual fluorescence spectral interference. Acridine luminescence spectra using this technique are de- picted in Fig. 1.

The phenazine phosphorescence signal decreased after several minutes of scan time with a concomitant production ofa fluorescent decomposition product. Temporal resolution, as per Fig. 3, was added to minimize decomposition and discriminate against prompt fluorescence [20]. Temporal resolution was provided by a 150 watt pulsed xenon source (Spex Industries, Digital Phosphorimeter Model 1934). The pulse source was operated at 20 gashes per s and the photon containing signal was sampled for 3 ms after the 30 PCS delay per lamp llash. Temporal resolution reduces photodecompo- sition due to low duty cycle (20 Hz, 3 ps width at half height) light pulses resulting in less than 1 ms irradiation time per s of scan time. Longer integration times (averaging times per scan increment in nm) and multiple scans were frequently em- ployed with the pulse source to enhance the signal-to-noise ratio. A stable phenazine phosphorescence signal was ob- tained using this regime, but the sensitivity and resolution were reduced due to the lower light intensity and wider slit employed. The lower signal-to-noise ratio of the pulse source made the wavelength maxima readings less precise when compared to the continuous source. The phosphorescence wavelength maxima obtained using this technique were in agreement within experimental error with those obtained by the fluorescence background subtraction method.

Limit of detection studies employed a Varian SF 330 spectrofluorometer which is source corrected. The Varian instrument source was a 150 watt continuous xenon source and the photomultiplier tube was a Hamamatsu R928. The input from this instrument was run to a strip chart recorder (Fisher Recordall Model 5000).

Absorption measurements

Absorption spectra emp1oyed.a Beckman (Irvine CA) Acta III or DB-GT spectrophotometer. Accuracy of the wave- length readings observed on the Acta III are k 1 nm.

RESULTS AND DISCUSSION

Form of emitting species

The fluorescence study was done to determine the form of emitting singlet state species in the fluid micellar solution. Figure 2 shows the fluorescence spectra of anthracene, acridine and phenazine in NaDS micellar solution. The fluorescence maxima and quan- tum yields are reported in Table 1.

Phenazine fluorescence wavelength maximum and quantum yield are similar to values reported for the free base in ethanol. Phenazine excited state pK, values are 6 _+ 0.5 (singlet [ lo]) and 4 k 0.3 (triplet [lo]). The observation of the free base maximum and the lack of observation of prototropic equilibria suggests the local micellar pH is > 6 in NaDS micellar solution. Anthracene wavelength maxima and quantum yields are similar to those reported in ethanol.

The fluorescence studies indicate that prototropic equilibria are occurring for acridine in NaDS which has a bulk solution pH of 8. In this micellar solution, the fluorescence wavelength maximum at 488 nm is the same as that observed for the protonated species in 0.1 N HCl, and the quantum yield is increased over the quantum yield observed in the aprotic media. However, in Na/AgDS, which has a bulk solution pH of 5, the singlet state wavelength maximum is at 446 nm which is closer to the free base singlet state wavelength maximum at approximately 430 nm. The pK and pK* (singlet) for the acridinium cation are

b

Wavelength (run)

Fig. 2. Corrected fluorescence spectra of (a) anthracene (1.9 x 1O-6 M, 359 nm Ex., display x 1); (b) acridine (1.9 x 10m6 M, 359nm Ex., display x 3) (c) phenazine (2.0 x 10e6 M, 359 nm Ex., display x 200) in 0.08 M NaDS micellar solution. Slits, 5.4 nm Ex., 0.72 nm Em.; photon counting, 1 s average per 0.5 nm increment, 1

scan; Spex Fluorolog 2 + 2, 450 watt continuous xenon source with background subtraction.

646 RODNEY WOODS and L. J. CLINE LOVE

Table 1. Fluorescence wavelength maxima and quantum yields*

Wavelength maxima?

Solute Medium (nm) Quantum yields*

Phenazine NaDS 485 O.ooO6 Ethanols 476 0.00086

12 N H,SO, 526 II Anthracene NaDS 407 0.37

Ethanol? 406 0.27 + 0.05 Acridine Methanol 428 0.016 & 0.002

Acetone 431 0.001 Butanol 425 0.007 Cyclohexane 425 < 0.0002 Water 458 0.3 0.1 N HCl 488 0.48 + 0.02 NaDS 488 0.1

Na/AgDS 446 II

*All solutes 5 x 10m6 M or less, Spex Fluorolog 2 + 2 instrument; 450 watt continuous xenon source, corrected emission spectra, slits 5.4 nm Ext., 0.72 nm Em.; photon counting 1 s average per 0.5 nm increment.

t +2nm for all maxima except for phenazine in ethanol and 12 NH,SO, f 4 nm.

$See Ref. 1181, comparative method using quinine sulfate (0, = 0.55) in O.lN HZS04.

§See Ref. [lo].

IiNot measured. ISee Ref. [20].

reported at 5.45 and 10.65 f 0.05, respectively [ 151. The bulk solution pH values suggest that electronically excited acridine in both micellar media should exist as the protonated species and that in Na/AgDS proto- nation should be more complete than in NaDS. The expected increase of protonation in Na/AgDS with decreasing pH was not observed; clearly, some other form of interaction is occurring for acridine in Na/AgDS micellar solution. In Na/AgDS, neither the free base or protonated form clearly exist, and the emitting species is a different micelle/heavy atom perturbed entity.

MS-RTP studies

As can be observed from Tables 2 and 3, MS-RTP spectra are generally redshifted compared to 77 K data in ethanol, as is seen in this data, and solutes in Na/AgDS micellar solution are further redshifted. As an example, there is a slight redshift of 2 nm for arithracene in Na/TlDS; however, in Na/AgDS there is a further shift of 26 nm. To further investigate this

phenomena, phenazine phosphorescence was measured in NaDS, Na/AgDS and Na/TlDS micellar solutions. Phenazine was chosen because it is the only compound with observable phosphorescence in all three media. Temporally resolved phosphorescence spectra for phenazine in NaDS and Na/AgDS are shown in Fig. 3. Phosphorescence wavelength maxima and relative intensities of all three micellar solutions are given in Table 2. The phosphorescence wavelength maximum for phenazine in NaDS is at 666 nm which is slightly redshifted over the maximum at 77 K in

Table 2. Phenazine phosphorescence wavelength maxima and relative intensities with various solution media, chemical

state and temperature

Medium

Wavelength* maxima

(nm)

Relative intensity

NaDS Na/AgDS Na,TlDS

77 K Ethanol? Ag: phenazine FSLS

Phenazine FSL$

666 0.37 712 1.0

666,712 0.06

652 II 712 II

§ II

FSL is front surface luminescence. *All phosphorescence maxima are uncorrected for PMT

response and are k 2 nm. tSee Ref. [lo]. *Front surface luminescence of the crystalline solid. §No phosphorescence signal observed.

IiNot measured.

ethanol due to temperature and solvent effects, as has been observed for carbocyclics and pyrollic nitrogen probes 115-71. In Na/AgDS a further redshift of 46 nm is observed for this solute. In Na/TlDS two low intensity phosphorescence maxima are observed which correspond to those observed in NaDS and AgDS. Clearly two forms of emitting species are observed. For phenazine in micellar solution several possible modes of phenazine interaction are metal or metal-micelle perturbed, or undergoing prototropic equilibria.

Phosphorescence of anthracine, acridine and phenazine 647

Table 3. Phosphorescence wavelength maxima, limits of detection and percentage fluorescence quenched of acridine, anthracene, phenazine

Solute Medium

Wavelength* maxima

(nm) LODt % FluorescenceS

x lo-’ M quenched

Acridine NaDS 0 Na/AgDS 653 8.3 74 Na/TIDS 77 K EtOHll

§ 625

Anthracene NaDS § Na/AgDS 708 90 Na/TIDS 682 1.2 82 77 K EtOHq 680

Phenazine NaDS 666 Na/AgDS 712 1.1 40

*Phosphorescence wavelength maxima background subtracted, uncorrected. See Figs 2, 5 for conditions.

t LOD = Limit of detection where signal-to-noise ratio = 3 determined on Varian SF300 instrument with 150 watt xenon source at maximum emission intensity wavelength.

$See text for definitions. No observable phosphorescence.

/See Ref. [ 131. ISee Ref. [23].

OOOEOO 60000 75000 90

Wavelength (nml

Fig. 3. MS-RTP of phenazine in (a) 0.15 M NaDS; (b) 0.08 M Na/AgDS (3 x lo-’ M, 359 nm Ex.; slits, 14.4 nm Ex. and Em.; photon counting, 5 s average per 1 nm Increment, 2 scans; Spex Fluorolog 2 + 2, 150 watt pulsed xenon source, 20 Hz, delay 30 ps, sampling time 3 ms, uncorrected emission spectra with

temporal resolution.

Front surface phosphorescence studies

Front surface phosphorescence of Ag : phenazine precipitate and crystalline phenazine were conducted to preclude possible prototropic and other solution equilibria effects. Figure 4 depicts front surface phos- phorescence of Ag : phenazine precipitate and phena- zine in Na/AgDS micellar solution. The phosphores- cence intensity of the Ag : phenazine precipitate which appears as a shoulder on the residual fluorescence signal is low which could be due to many possible triplet state deactivating mechanisms observed in solid

state luminescence. No phosphorescence is observed for pure, crystalline phenazine using this technique (spectrum not shown) which probably is a result of the difference in lifetimes. A heavy atom interaction can decrease the lifetime by a factor of 1000 making deactivation mechanisms less competitive with a much shorter-lived triplet state radiative pathway, and in- creased intersystem crossing. Phenazine solid would be expected to have a much longer radiative lifetime than the silver precipitate and could be more easily de- activated. That the phosphorescence maximum of the Ag : phenazine precipitate observed using this tech-

648

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RODNEY WOODS and L. J. CLINE LOVE

Wavelength (nm)

Fig. 4. Phenazine phosphorescence: (- ) solid Ag : phenazine precipitate employing front surface technique, display x 14; (----) phenazine 5 x lo5 M in 0.08 M Na/AgDS micellar solution, display x 1, 368 nm Ex.; slits, 14.4 nm Em., 7.2 nm. Ex.; photon counting, 1 s average per 0.5 nm increment; Spex

Fluorolog 2 + 2, 450 watt continuous xenon source, uncorrected emission spectra.

nique is identical to the phosphorescence maximum of phenazine in Na/AgDS micellar solution is strong evidence for the formation of a complex between silver

and phenazine.

Absorption studies

Absorption studies were run in 0.08 M NaDS and Na/AgDS micellar solutions for each of the three solutes in order to examine the possibility of ground state complex formation. An absorption spectrum shifted approximately 10 nm to the red was observed for the nitrogen heterocycles in Na/AgDS micellar solution indicating possible ground state complex formation. The 10 nm stabilization of phenazine and acridine in Na/AgDS micellar solution is consistent for reported ground state complexes of quinolines and silver in excess silver [22] and silver toluene complexes [23].

Met&solute complex formation

The micelle acts to organize reactants, in this case silver and the pyridinic nitrogen or carbocyclic system. Silver interacts strongly with amines[22] and BOUTILIER and WINEFORDNER had observed a “charge transfer complex” between silver and several probe molecules studied at 77 K. The complex remains solubihzed in Na/AgDS as is evidenced by the observa- tion of complex stabilized MS-RTP for the heterocyc- les. The stabilized phosphorescence wavelength maxi- mum in the presence of silver was also observed using nucleosides at 77 K [9]. Additional evidence of com- plexation and solubilization between silver and pyri- dinic nitrogens is the observation of a singlet state charge transfer complex between silver and S-hydroxy- quinoline, as has been observed in our laboratory in micellar solution.

BOUTILIER and WINEFORDNER [8] discuss the nature of the bonding in silver complexes with olefins, aromatics and nitrogen compounds when silver(I) is used as the heavy atom. For an aromatic molecule, the bonding with silver can be considered as an overlap of the n electron density of the aromatic molecule with a 0 acceptor orbital on the silver ion, and backbond from filled d(xz) or dn-pz hybrid orbitals into the 7c* orbitals of the carbons or ring system. The donation of K electron density to the metal G orbital is the stronger effect so the interaction is an aromatic donor and metal ion acceptor complex. For pyridinic nitrogen, where a higher amount of the x electron density is localized and with a lone pair of electrons, one would expect a preferential nitrogen-silver interaction in these “donor-acceptor” complexes. BOUTILIER and WINEFORDNER [8] note that ring binding occurs at a lower concentration of silver ion than the complex with the aromatic portion of the molecule. This implies stronger binding at the pyridinic nitrogen than at any

of the carbons in the heterocycle, or at any of the carbons in the carbocychc analogue, anthracene. A stabilizing interaction is occurring between silver and the pyridinic nitrogen which is greater than that normally observed between a carbocyclic and thallium(I). The phosphorescence spectra of an- thracene is redshifted in Na/AgDS as compared to the Na,TlDS micellar solution, and is considerably lower in intensity. The absorption data are equivocal for this solute as to whether a ground state complex is formed. The redshifting of the phosphorescence spectrum is frequently observed for the linear ring carbocyclics along with a loss of residual vibronic structure when silver is used as the heavy atom vs the use of thallium in heavy atom micellar solution. A complex is most likely formed between the aromatic carbocyclic and silver,

Phosphorescence of anthracene, acridine and phenazine 649

but silver is not the heavy atom of choice for promot- ing intense phosphorescence from the carbocyclics

observed to date.

Heatiy atomjluorescence quenching

The percentage of fluorescence quenched upon addition of the heavy atom to a NaDS micellar solution is a measure of the effectiveness of the external heavy atom in quenching singlet state emission. The values given in Table 3 for phenazine and to a lesser extent acridine are lower than values usually reported for carbocyclics. This is due to the increased spin orbit coupling provided by the nitrogen lone pair and also to the higher degree of vibronic coupling resulting from the mixing of singlet and triplet states (n-n* and n-x* type) which result in faster intersystem crossing rates over their carbocyclic analogue anthracene [10-l l] and less fluorescence to be quenched by addition of the heavy atom.

Triplet sfafe inductive e$ects

Intercalation of pyridinic nitrogens into the ben- zenoid ring results in inductive effects which can be observed using phosphorescence wavelength maxima. Figure 5 depicts phosphorescence spectra of acridine and phenazine in Na/AgDS and anthracene in Na,TLDS. Anthracene in Na/AgDS is not shown due to low intensity and lack of spectral featuring. The phosphorescence spectrum for acridine is blue shifted relative to that of anthracene due to the insertion of an electron withdrawing substituent in the ring [14]. Electron density maps for pyridinic nitrogens [lo] in a benzenoid ring system always show a greater electron density on the nitrogen atom than on any of the carbons. For phenazine the electron withdrawing capability of a single ring nitrogen is reduced by

I 29E 05

0 OOE OC

inserting a competitive nitrogen in the para position. GOODMAN and HARRELL [ 151 published ground state maps for pyridine and pyrazine, the single ring ana- logues of acridine and phenazine, respectively. Their calculations, considering the K clouds as one unit, show the localization of l/3 of the electron density at the nitrogen for pyridine. On the insertion of the second nitrogen in the ring (pyrazine) the density drops to no more than l/6 at any one of the nitrogens. The para

substitution reduces the induced dipole effect of the monosubstituted pyridinic nitrogen and the para di- nitrogen compounds will be redshifted compared to the monosubstituted compound.

Using similar reasoning the phosphorescence wave- length maximum for phenazine should lie between that of acridine and anthracene. In Na/AgDS the phos- phorescence maxima for acridine, phenazine and an- thracene are at 653 nm, 712 nm and 708 nm. For these molecules in Na/AgDS micellar medium, a simple inductive effect is inadequate to explain the data. A second type of interaction involves complexation effects between the solute and silver in this matrix. That the phenazine phosphorescence wavelength maximum is redshifted relative to anthracene can be due to a greater degree of stabilization due to this proposed complexation.

CONCLUSIONS

Thallium as a heavy atom does not promote intense phosphorescence from any of the pyridinic nitrogen heterocycles. The low intensity phosphorescence spectra observed for phenazine in Na/T’lDS micellar solution indicate two phosphorescence maxima whose wavelengths correspond to the free base and the complexed species. This indicates that the

t I: b 1 I

’ I I

1 : 1 I

! I

Wavelength (nm)

Fig 5. MS-RTP of (a) acridine Na/AgDS, 5 x 10m5 M, 359 nm Ex.; (b) anthracene Na/TIDS, 5 x 10m5 M, 345 nm Ex.; (c) phenazine Na/AgDS, 5 x 10m5, 368 nm. Ex.; slits, 14.4 nm Ex., 7.2 nm Em.; photon counting, 1 s average per 0.5nm increment, 1 scan; Spex Fluorolog 2+2, 450 watt continuous xenon source,

uncorrected emission spectra.

650 RODNEY WOODS and L. J. &NE LOVE

Tl : phenazine complexation is less favored, producing

appreciable concentrations of both the complex and free phenazine. In addition, it appears that the strength of the heavy atom-pyridinic heterocycle complex is an indicator of the intensity of the induced room tempera- ture phosphorescence. Silver appears to serve two roles in the promotion of pyridinic nitrogen phosphores- cence. Silver acts as an external heavy atom in the production of strong phosphorescence and forms a complex, most likely through the nitrogen moiety, to keep the molecule in the vicinity of the heavy atom counterion micelle.

Aclinowledgements-This work was supported in part by National Institute of Health Grant No. GM-27350, National Science Foundation Grant Nos. PRM-8111335 and CHE- 8216878 and the Environmental Protection Agency. Although the research described in this article has been funded, in part, by the United States Environmental Protection Agency under assistance agreement number R 809474 to L. J. CLINE LOVE, it has not been subjected to the Agency’s required peer and administrative review and, therefore, does not necessarily reflect the view of the Agency and no official endorsement can be inferred. This work was presented, in part, at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, 12 March 1982, Atlantic City, NJ, U.S.A., Abstract No. 711.

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