Vol. 16 No. 1 CHINESE JOURNAL OF CHEMISTRY 1998

Intermolecular interaction between squarylium cyanine and porphyrin

Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, China

Abstract In this paper, the interaction between squarylium cyanine and porphyrin in chlw roform is investigated by absorption and fluorescence spectroscopy. Emphasis has been put on the mechanism of intermolecular energy transfer. The overlap integral J between the absorption spectrum of squarylium cyanine and the fluorescence spectrum of porphyrin was calculated, which reveals that the singlet-singlet energy transfer may occur &om porphyrin to squarylium cyanine in solution. In comparison of the observed rate constant [k~1=6.1~10'3 (rnol/L)-'.s-l] for fluores- cence quenching of porphyrin by squarylium cyanine with the diffusion rate constant in chloroform [k~ i~= l . l~ lO '~ (mol/L)-'-s-'] and the rate of energy transfer [k.t16.7xlO4 (rnol/L)-'.s-' in the experimentally dilute solutions] estimated from Forster formula, the possibility of energy transfer by electron exchange or/and coulombic mechanism could be excluded. So it has been definitely convinced that the intermolecular energy transfer between them is performed through long-distance radiative mechanism which is in good agreement with the results in experimentally dilute concen- tration mol/L). Meanwhile, the fluorescence of squarylium cyanine can also be quenched by porphyrin, which suggests that photoinduced charge transfer with other process may occur. In the contrast, the results can be partly contributed to understand the reason why the energy transfer efficiencies of porphyrin 4:; are always lower than its fluorescence quenching efficiencies 4:.

Keywords transfer

Squarylium cyanine, porphyrin, interaction, energy transfer, photoinduced charge

Introduction

As an important class of functional materials, squarylium cyanines possess so many advantages as stable structure, outstanding photoconductivity and small ther- mal conductivity that they have extensive application perspects in the fields of organic solar cells, laser dyes, optical recording media, xerographic photoreceptors, nonlinear optical and imaging materials e t ~ . ' - ~ As porphyrins present excellent phG toelectric ch~acter i s t ics ,~-~ recently increasing interests have been confined to the

Received February 18, 1997; Revised November 28, 1997. Project supported by the National Natural Science Foundation of China.

8 Chinese Journal of Chemistry Vol. 16 No. 11998

aspect where porphyrins are used as a model system to mimic the primary process in natural photosynthesis. Both squarylium cyanines and porphyrins have strong abil- ity to harvest light energy, and the combination of their absorption spectra covers up the whole visible regions, which suggests high academic quality in solar energy storage and utilization. However, there is few information on the interaction be- tween squarylium cyanine and porphyrin up to now. In this paper their interaction mechanism in organic solvent is discussed by means of spectroscopic technology, and attempts to provide scientific basis and theoretical guidance for manufacture of novel photoelectric functional materials have been made.

Experiment a1

Materials

The structures of squarylium cyanine (I) and porphyrin (11) are shown in Fig. 1. The syntheses of I and I1 refer to the literature^',^ respectively. Chloroform (Beijing Chemical Factory) of analytical grade was used.

Fig. 1 Structures of the compounds: squarylium cyaine I, pbrphyrin 11.

Instruments and methods

The absorption spectra were recorded on a Hitachi-557 spectrometer. The cor- rected fluorescence spectra were recorded using a Hitachi-850 spectrofluorometer with an excitation wavelength of 410 nm for porphyrin and 600 nm for squarylium cyanine, excitation slit 5 nm and emission slit 5 nm. The fluorescence quantum yields of the dyes in chloroform were determined by comparison to aqbase form

ZHAO et d. Squarylium cyanine 9

of TTP in DMF (&=0.128, where TTP is tetratolylporphyrin)8 and squaraine in isopropanol (c#Jf=O.lO, where Sq. is 5,5’-dichlor~l,l’-dirnethyl-3,3,3’,3’-tetramethyl squarylium ind~cyanine.)~ respectively. Fluorescence lifetimes were measured using a Horiba NAESl 100 timecorrelated singlephoton counting instrument.

Results and discussion

Ground state absorption spectra

The electronic absorption spectra of squarylium cyanine I, porphyrin I1 and the equimolar mixture solution of I, I1 are exhibited in Fig. 2. It is indicated that the absorption spectrum of the 1-11 mixture is simple fold interrelated to that of I or 11, showing no obvious absorption spectral changes. So there is no indication of ground-state interaction between I and I1 in chloroform.

Fig. 2 Absorption spectra of squarylium cyanine I (- - - -), porphyrin I1 (-.-.-) and 1-11 mixture (1:l) (-) in chloroform.

Fluorescence quenching of porphyrh I1 by squaryhm cyanine I

A series of 11-1 mixture in chloroform were made up for fluorescence quenching of porphyrin I1 by squarylium cyanine I. The concentration of I1 fixed at 2 . 0 ~ 1 0 - ~ mol/L, with that of I in the range examined (0 to 2 . 7 ~ 1 0 - ~ mol/L) (Fig. 3). Exci- tation wavelength of porphyrin I1 in the mixture solutions was set at 410 nm, where squarylium cyanine I has no absorption.

A gradual decrease in the emission at 610 nm for porphyrin I1 was observed

10 Chinese Journal of Chemistry Vol. 16 No. 11998

while 660 nm fluorescence peak of I1 vanishes more slowly with the increasing con- centration of I (Fig. 3). The reason lies in the evidence that the emission of I is contained mainly within the 660 nm fluorescence region (Fig. 6). The trend above is most favourable for interpreting energy transfer from excited porphyrin I1 to ground-state squarylium cyanine I.

1 n

A/nm

Fig. 3 Fluorescence spectra of porphyrin I1 in 1-11 mixture chloroform solutions (pp.11 concen.=2.0x lo-' mol/L) (&,=410 nm) sq.1 concen. (mol/L): 1) 0; 2) 2.1X10-6; 3) 6.3X10-6; 4) l . l ~ l O - ~ ; 5) 1 . 5 ~ 1 0 - ~ ; 6) 1.9~10-'; 7) 2.3~10-'; 8) 2 . 7 ~ 1 0 - ~ .

Since there is considerable overlap between the emission spectrum (560-770 nm region) of I1 and the ground-state absorption spectrum (520-700 nm area) of I (Fig. 4, normalization), singlet-singlet energy transfer should be effective in terms of energy transfer theory. I1 is the energy donor while I as energy acceptor when they coexist in chloroform.

ZHAO et al. Squarylium cyanine 11

Fig. 4 Absorption spectrum of mquarylium cyanine I (-) and fluorcacence spectrum of porphyrin I1 (- - - -) in chloroform (X~,=410 nm, intensity normalized).

The mechanism of energy transfer will be discussed as follows. The Stem-Volmer from the fluorescence quenching of I1 by I (the rel-

ative emission intensities of I1 in the absence or presence of I cp/IPp versus [sq,I] is good linear (Fig. 5 ) , the slope is kixsfi=8.6x1O4 (mol/L)-l, so the observed rate constant was calculated to be kiz=6.1 x 1013 (mol/L)-'.s-' by the mean fluorescence time of I1 sP=1.40 ILS.

Fig. 5 Plot of the Stern-Volmer curve for porphyrin I1 fluorescence quenching by q.1 the relative fouorcacence intensities of pp.11 in the absence or presence of sq.I(I&/Ipp) venrun [sq.I].

Obviously, kil is much larger than the diffusion-controlled limit [kdif=l . l x 10"

12 Chinese Journal of Chemistry Vol. 16No. 11998

(mol/L)-l.s-’ in chloroform]. Then it is impossible to perform singlet-singlet en- ergy transfer via a simple overlap electron exchange mechanism in the dilute working concentration ( mol/L) giving large intermolecular separation between I1 and I. Furthermore, the fluorescence lifetime of I1 is unvarying with changing concentra- tion of I, which indicates that the fluorescence quenching of I1 by I is not a dynamic process in favor of the above result of k:I>kdil.

Moreover, on the basis of Forster theory, the rate of energy transfer ket by the dipole-dipole resonance mechanism can be estimated by Eq. ( l).12-15

ket = (1/.)(c/co)2 (1)

where ~=1.40 11s is the mean fluorescence lifetime of I1 in chloroform, the total solution concentration C=CII+CI, Co is ‘‘critical concentration” by the expression

Eq. (2).

Ci = (8 x 1O3.rr3n4/NK2(b~hl0) J-’ (2)

where 4~=0.044 is the fluorescence quantum yield of I1 in chloroform in the absence of acceptor I, n is the refractive index of the solvent (n=1.4459 for chloroform), N is Avogadro’s constant and K is an orientation factor equal to 2/3. The overlap integral J is given as follows.

J = Lrn FD(X)EA(X()X~~X (3)

where FD(X) is the energy donor I1 fluorescence distribution (in quanta normalized to unity on a wavelength scale), E A ( X ) is the molar extinction coefficient of the accep tor I, and X is the wavelength. Thus we obtain the numerical value of J = 4 . 3 2 ~ 1 0 - ~ (mol /~) - l xm3.

Substituting these values into Eq. (2) results‘in C0=3.0x mol/L. Replacing the maximum total concentration CmaX=C~ax+C~aX=2.9x mol/L, according to Eq. (l), kZaX=6.7x1O4 (mol/L)-l.s-l. Since k2”.<<ki1=6.1 x1013 (mol/L)-l.s-’, the coulombic interaction can be eliminated. So it is clear that the intermolecular singlet-singlet energy transfer between I and I1 occurs via radiative mechanism in the dilute experimental concentration range

The energy transfer efficiencies of I1 4:; by the Eq. (4) and its fluorescence quenching efficiencies 4;’ according to Eq. (5) are summarized in Table 1.

mol/L).

ZHAO et al. Squarylium cyanine 13

where SA is the fluorescence area of excited I as energy acceptor, which can be obtained from the rest area with deduction of the free I1 fluorescence of the same concentration in the total normalized fluorescence area in the mixture solution; SD is the fluorescence area of I1 in the absence of I; 4~ is the mean fluorescence quantum yield of I, 4~=0.077.

where $JDA and t $ ~ are the corresponding fluorescence quantum yields of I1 in the presence or absence of I.

Table 1 Comparison of 4;' and q5ft us. Isq.11 in dilute chloroform

0

2.1x10-6

1.1~10-5

6 . 3 ~

1 . 5 ~ 1 0 - ~

1 . 9 ~

2 . 3 ~

2 . 7 ~ 1 0 - ~

0

9.2

20.7

29.0

37.0

43.1

48.5

51.4

0

3.8

7.9

10.8

13.2

14.5

15.5

16.3

The results show that 4;' and 4:: present increasing tendency with growing concentration of I. Nevertheless, the phenomena that 4:: are always lower than 4;' demonstrates the I1 fluorescence quenching by I not only belongs to intermolecular 'energy transfer but also ends in other processes.

Fluorescence quenching of squarylium cyanine I by porphyrin I1

A set of 1-11 mixture in chloroform was prepared. The concentration of I re- mained constant at 2.1 x mol/L in connection with the concentration of I1 given at 0, 2 . 0 ~ 1 0 - ~ , 4 . 0 ~ 1 0 - ~ , 6 . 0 ~ 1 0 - ~ , l . o ~ l O - ~ , 1 . 4 ~ 1 0 - ~ , 2 . 2 ~ l O - ~ and 3 . 0 ~ 1 0 - ~ mol/L respectively. The fluorescence spectra of the mixture solutions are shown in Fig. 6 on excitation of 600 nm in case the I1 emission is weak enough to be negligible.

14 Chinese Journal of Chemistry Vol. 16 No. 11998

I I ! The I fluorescence is quenched

by degrees with the increasing con- centration of 11, indicating perfect I

2 : 5 6

4 linear regularity where Stern-Volmer 7 curve refers to Fig. 7. As a re-

* a sult, the slope of the straight line gives ki ~;=4.1x10~ (mol/L)-l, so k;=l.l x 1014 (mol/L)-'.s-l upon in- troducing the mean fluorescence life- time of I ~;=0.365 ns (experimental value).

The I fluorescence is quenched by I1 probably via photoinduced charge transfer and other related processes. Further study is going on. ki (the a p parent rate constant of the processes) is comparable with kkl (the apparent

the contrast the experiments may sup port the above evidence that 4:: are

I I

A/nm

Fig. 6 Fluorescence spectra of squqlium CP nine 1-11 mixture chloroform solutions

rate constant of energy transfer), in

([sq.I]=2.1~10-~ mol/L, X~,=600 nm) porphyrin I1 concen. (mol/L): 1) 0; 2) 2.0~10-~; 3) 4.0~10-~; 4) 6 . 0 ~ 1 0 - ~ ; 5) ~ . O X ~ O - ~ ; 6) 1.4~10-~; 7) 2.2~10-'; 8 )

- always lower than 4:.

Fig. 7 Plot of the Stern-Volmer curve for squarylium cyanine I fluorescence quenching by pp.11 the relative fouorescence intensities of sq.1 in the absence or presence of pp.II(I:q/Isq) versus (pp.II].

ZHAO et 41. Squarylium cyanine 15

Summary

The interaction between squarylium cyanine I and porphyrin I1 in chloroform is investigated. It is confirmed that they can perform singlet-singlet energy transfer via radiative mechanism where I1 is energy donor while I as acceptor. The reason why the porphyrin energy transfer efficiencies 4:: are always lower than its fluores- cence quenching efficiencies 4: may be verified in the experiments of the squarylium cyanine I fluorescence quenching by porphyrin 11.

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