J. Phys. Chem. 1995, 99,
Influence of “Inert” Counterions
on
6525-6529
6525
the Photochemistry of Some Cationic Polymethine Dyes
A. S. Tatikolov,* Kh. S. Dzhulibekov, L. A. Shvedova, and V. A. Kuzmin Institute of Chemical Physics, Russian Academy of Sciences, Kosygin
St. 4,
117977 Moscow V-334, Russia
A. A. Ishchenko Institute of Organic Chemistry, Ukrainian Academy of Sciences, Murmanskaya Received: April 11, 1994; In Final Form: December 30, 1994®
St. 5,
252660 Kiev 94, Ukraine
The effects of “inert” counterions on some photochemical processes involving cationic polymethine dyes have been studied in ion pairs formed in low-polar media. In particular, the counterion effects on the back isomerization kinetics of photoisomers of benzimidacyanine and indotricarbocyanine dyes and on the triplet state decay of the benzimidacyanine dyes have been investigated by flash photolysis. The increase in the rate constant and decrease in the activation energy of back isomerization of benzimidacyanines are observed when dye—counterion ion pairs are formed in low-polar media, whereas the increase in the activation energy is observed for indocyanines under the same conditions. For benzimidacarbocyanine with the I- counterion, ion pair formation gives rise to an increase in the decay rate constant of its triplet state due to interaction of the dye cation with the 1“ heavy atom; meanwhile, there is no appreciable rise in Si T intersystem w*— The enhancement of the internal in conversion ion has been revealed for all Si So crossing. pairs with the I" counterion. benzimidacyanines 1-
Introduction
In contrast with the cases of the indocyanine dyes, an increase in the rate constant of back isomerization with ion pair formation is observed for cationic benzimidacyanines. Our present work
The influence of ion pair formation on the photochemistry of ionic polymethine dyes has nowadays become a matter of comprehensive investigations. A number of works of Schuster and coauthors, which appeared in recent years, are devoted to the photochemistry of cationic cyanines in ion pairs with “active” borate anions.1-5 These anions have low oxidation potentials and may participate in fast electron-transfer processes involving dye cations. The geometric structure of the bulky borate counterions was shown to be one of the most important factors controlling the rate of the electron transfer between the borate counterions and the dye cations.5 On the other hand, it is known that even “inert” counterions, which cannot be involved in photochemical reactions with cyanines, such as CIO4-, BF4-, Cl-, etc., are capable of affecting the spectroscopic and fluorescent properties of cationic cyanines in ion pairs by means of their electrostatic influence on the electronic structure of the dyes.6 Hence, we may expect to reveal the effects of “inert” anions on cyanine photochemistry as well, but such studies are very scarce. For instance, the influence of the Ianion on the decay and quantum yield of the cyanine triplet states in ion pairs has been studied.7·8 Recently we have discovered the influence of “inert” counterions on some photochemical processes in indocyanine dyes, in particular on their isomerization around one of the polymethine bonds.8 We have shown that the rate constant of back isomerization of indotricarbocyanine dyes (back isomerization of the photoisomer formed under light excitation) decreases significantly with changing the Solvent from a polar to a nonpolar one at the expense of ion pair formation. This effect was supposed to be the result of the interaction of the dye cation with the counterion in ion pairs formed in nonpolar solvents. On the basis of quantum chemical calculations, we concluded that the interaction of the counterion took place with the terminal carbon atom of the indotricarbocyanine polymethine chain carrying a maximum positive charge. We supposed that it led to a distortion of the symmetry of the dye cation and to an increase in the bond order around which the isomerization
deals with the investigation of several photophysical and photochemical processes involving these dyes and their dependence on the ion pair formation in the solvents of different polarity by flash photolysis techniques. In detail, we have measured the rate constants (k¡) and Arrhenius activation parameters of back isomerization, as well as the rate constants of the triplet state decay (£ ). Also we have measured the activation parameters of back isomerization of indotricarbocyanines. The data for benzimidacyanines are compared with those for indocyanines.
Experimental Section The experiments were carried out for benzimidacyanine (1— 3) and indotricarbocyanine (4 and 5) dyes having structural
formulas
Ph
_
Ph
n=1 (1)2(2)3(3) X- CIO4-, ci-1=
occurs. ®
in toluene, acetonitrile, chloroform, and acetonitrile—toluene and
Abstract published in Advance ACS Abstracts, March 15, 1995.
0022-3654/95/2099-6525$09.00/0
©
1995 American Chemical Society
Tatikolov et al.
6526 J. Phys. Chem., Vol. 99, No. 17, 1995
Difference absorption spectra of the photoisomers of dyes and 2 (4 and 5) with the CIO4" counterion in acetonitrile (1 and 4), chloroform (2), and toluene (3 and 5).
Figure 1
1.
(1-3)
chloroform—hexane mixtures of various composition. Toluene was dried by distillation over metal sodium; acetonitrile was purified by distillation over KMnOa, NaaCCH and P2O5; and chloroform was purified from traces of acid by passing it through a column of alumina. The difference absorption spectra of dye photoisomers and the rate constants of back isomerization (k¡) were measured in air-saturated solutions using conventional flash photolysis (flash energy 50 J, pulse duration 7 x 10"6 s). The photoexcitation of dye solutions was performed at the longwavelength absorption band, the dye concentration being about (1—5) x 10"5 M. The activation parameters of back isomerization were obtained at temperatures from —10 to +60 °C. Inasmuch as the triplet quantum yield of the dyes studied is low, the triplet states were generated by the energy transfer from anthracene, the dye concentration being sufficiently high (2 x 10"5 to 1 x 10"4 M) for effective quenching of the hydrocarbon triplet state. Low rate constants of triplet decays (kj < 2 x 104 s"1) were determined by conventional flash photolysis, while 1t>2x 104 s-1 values were determined by laser photolysis using a PRA-LN-1000 nitrogen laser as an excitation source = 337 nm, W = 0.8 mJ, r = 1 ns, the laser beam being 1 (Aexc mm in diameter). The solutions were deoxygenated in a vacuum unit or by bubbling argon through the solutions. The dissociation constants (K 80%, the alterations of Ea, log(Ao), and log k, become more sharp (in some cases the alterations change their sign when = 80%, see Figure 3) under the influence of passing through m ion pairing. Herewith, the enhancement of ion pair formation with increasing m has opposite effects on Ea (and on k¡ at room temperature) for benzimidacyanines and indocyanines: it decreases Ea for the former dyes and increases Ea for the latter ones. For all dyes except 2 the most steep alterations of Ea are observed at m > 95%, when more tight ion pairs are formed with a strong effect of counterion on the dye cation. 3,
60
40
20
0
m
(%
80
100
toluene)
Figure 3. Dependences of Ea (a) and log(A0) (b) on the composition of the acetonitrile toluene solvent mixture for dyes 1 (1), 2 (2), 3 (3), and 5 (4) with the CIO4- counterion. -
when m > 80%, a steep increase in k¡ is observed due to formation of dye ion pairs with counterions. For indotricarbocyanines the opposite ion pair effect was revealed, a steep decrease in k¡ in toluene—2-propanol mixtures with m > 50%.8 Unlike the case of indocyanines, there is no distinct dependence of k¡ for benzimidacyanines on the nature of the counterion. A monotonous alteration of k¡ for benzimidacyanines is observed with variation of the solvent mixture composition, as was revealed for indotricarbocyanines in toluene—2-propanol mixtures.8 Herewith, the decay kinetics of photoisomers follows first-order law and contains only one exponent. Apparently these experimental data indicate the existence of a complex equilibrium between the completely dissociated ions, solventseparate ion pairs of variable composition, and the tight (contact) ion pairs:
dye+ + A” *= [dye+·
·
*S·
·
· _] *= [dye+* ·8· ·
~] ^
...
*=?
[dye+A~]
where dye+ is the dye cation, A" is the counterion, and S is the solvent. This equilibrium is shifted to the direction of either the free ions or the tight ion pairs with the alteration of the solvent mixture composition. Consequently, the observed roughly exponential decay kinetics of the photoisomers is really the mean value, averaged over the exponents corresponding to
different species involved in the equilibrium. The changes of both the activation energy Ea and preexponential factor Aq of back isomerization may be responsible for observed alterations of k,. For elucidating the alterations of these parameters and their influence on k¡, the dependences of Ea and log(Ao) on the composition of the toluene—acetonitrile mixture were studied both for benzimidacyanines and for indotricarbocyanines (Figure 3; for dye 4, which is not included in Figure
The cause of the influence of counterion on the Ea of the benzimidacyanine and indocyanine back isomerization is apparently the same as was proposed earlier for the thermal cis— trans and trans—cis isomerization of simple cyanines;15·16 it is the electrostatic influence of the anion on the potential-energy surface of the isomerization path. This influence may be accomplished in two ways. The first one is the electrostatic influence of the counterion on the isomerization barrier in the twisted (roughly perpendicular) state of the cyanine cation. In this model, localization of the counterion near the polyenic fragment of the cation, which bears high positive charge in the perpendicular state, leads to stabilization of this state and lowering of the isomerization barrier, whereas its localization near the low-charged polymethinic fragment may lead to an increase of the isomerization barrier16 (such a charge distribution in the twisted perpendicular state was shown by quantumchemical calculations15·17). If we suppose that the counterion moves freely by electrostatic forces along the polymethine chain during isomerization, then when the cation approaches the twisted state, the counterion should move from the low-charged polymethinic fragment to the strongly charged polyenic one, which should always cause a lowering of the isomerization barrier. Hence, this model can easily explain only a decrease (but not an increase) of Ea induced by the interaction with a counterion.
The second possible way the counterion may affect Ea is by change in bond orders within the polymethine chain induced by the interaction of the cyanine cation with the counterion. It may cause the bond alternation of the dye polymethine chain and either increase or decrease the activation barrier of the isomerization (see below). a
4. Calculations of Charge Distribution within the Dye Cations. Table 1 contains the results of the quantum-chemical CNDO/2 calculations of the charge distribution within benzimidacyanine cations. It is evident that the maximum positive charge is located on the terminal carbon atoms of the polymethine chain (C-2), which are common with the heteroresidues of the dye molecule, as it takes place for the indocyanine dyes.8 Hence, the counterion should be located close to one of these atoms. The interaction of the dye cation with the counterion may cause the asymmetrical distribution of the charge in the dye cation and may lead to an alternation of the bonds within the polymethine chain of the symmetrical dye, forming ion pairs. Such coordination of counterion close to the positively charged heterocycle of the dye cation was also shown by NMR with the use of lanthanide chelates as shift reagents.18·19 The bond alternation in the dye polymethine chain caused by its interaction with the counterion located near its end was experimentally
Tatikolov et al.
6528 J. Phys. Chem., Vol. 99, No. 17, 1995
TABLE 1: CNDO/2 Calculations of Charge Distribution Atoms of Benzimidacyanines 4
N-l C-2 N-3 C-3a C-4 C-5 C-6 C-7 C-7a C-8 C-9 C-10
C-ll
3
1
2
-0.155
-0.157
0.321
0.317
0.159 0.314
-0.158
-0.160
-0.161
0.088
0.089
0.089
-0.015
-0.016
-0.017
0.014 0.015
0.012 0.013
0.012 0.012
-0.015
-0.016
-0.017
3
0.091
0.090
0.090
-0.186
-0.182
-0.181
0.172
on
0.153
0.148
-0.125
-0.118 0.134
revealed in the crystal phase.20-22 It was also concluded on the basis of spectral and fluorescent properties of the dyes in low-polar solutions.6 The same conclusions concerning the position of the counterion and the polymethine bond alternation induced by its influence can be drawn from the results of MNDO calculations made by Krossner and Dietz for simple cyanines.16
An enhancement of the ion pair formation (with a decrease in the polarity of the medium) may result in an increase in the alternation of the bond orders: bond order C-2—C-8 decreases, C-8—C-9 increases, etc. During the isomerization the rotation around the bond with higher order is hindered and that around the bond with lower order is facilitated. It influences the activation energy of isomerization: the activation energy should increase if isomerization occurs around the bond with higher order and should decrease if isomerization takes place around that with lower order. Really, our experiments with indotricarbocyanine dyes 4 and 5 reveal rising £a in toluene compared to that in acetonitrile (see Figure 3) and, as a result, a significant lowering of k, at room temperature with changing the solvent from acetonitrile to toluene, as was first found earlier.8 It may show that for these dyes the isomerization occurs around the bond whose order increases at ion pair formation (a decrease of £a owing to the interaction of the counterion with the twisted state is apparently less significant). Unlike indocyanines, in case of benzimidacyanines the experiment reveals lowering £a at ion pair formation. It may indicate the decrease in the order of the bond around which the isomerization occurs in ion pairs. So we may conclude that in benzimida- and indocyanines photoisomerization occurs around different polymethine bonds. Alternatively, photoisomerization in benzimidacyanines may occur around the same bond as in indocyanines, but for benzimidacyanines the decrease of £a due to the influence of the counterion on the twisted state may dominate over the effect of a bond order increase. In contrast with indocyanines, for benzimidacyanines the values of k¡ are practically independent of the nature (nucleophilic activity) of the counterion (see Figure 2). It may be due to higher positive charge on C-2 atoms of benzimidacyanines (0.314—0.321) compared to indotricarbocyanines (0.236s). Consequently, the high energy of the counterion interaction with the C-2 atom in the benzimidacyanine cation may smooth away the differences in the nucleophilicity of counterions. As can be seen from Figure 3, the effect of counterion on £a for carbocyanine 1 is less pronounced and is observed in more
m
(%
toluene)
Figure 4. Dependences of log k¡ on the composition of the hexanechloroform solvent mixture for dyes 1 (1—3), 2 (4), and 3 (5) with Cl~ (1), I- (2), and CIO»- (3-5) counterions.
tight ion pairs than for dicarbo- and tricarbocyanines 2 and 3. It is probably connected with a lesser length of the polymethine jr-electron system of carbocyanine and its lower polarization under the influence of a counterion in comparison with dicarboand tricarbocyanines. Along with £a, ion pair formation results in the alteration of the preexponential factor of back isomerization (Ao) atm > 80%. An initial rise of Ao with formation of ion pairs (growing m from 80% to 95-100%) is observed for all dyes studied (Figure 3). It seems unlikely that a drastic rise of Ao observed for dyes 1, 3, and 5 could be caused by the alteration of torsional frequencies (the moments of inertia) of the dye molecule.23 A more probable explanation is connected with the changes in entropy of the isomerization twisted transition state as a consequence of ion pairing. This state has been shown to possess higher charge localization than the initial state;17 hence, it should be strongly solvated by acetonitrile molecules in polar media. Its change is partially compensated by the oppositely charged counterion in the initially formed loose ion pairs, which leads to a desolvation of the transition state and a raising of its entropy. The decrease in Ao for dyes 1 and 3 at m > 95% may be the result of ion pair reorganization, changing the ion pair structure from a loose to a more tight one with a fixed counterion in nonpolar media, leading to a decrease in entropy of the transition state. In contrast with the toluene-acetonitrile mixtures, in the hexane—chloroform ones a monotonous increase in k; is observed with growing amounts of hexane in the mixtures due to the shift of the dissociation equilibrium to the direction of ion pair formation (Figure 4). Here we do not reveal any initial decrease in k, as we observed in the toluene—acetonitrile mixtures. It is likely explained by a sufficiently low dielectric constant of chloroform (4.02), which corresponds to the right (rising) branch of the plot for the toluene—acetonitrile mixtures
(at
m >
80%).
It is necessary to point out that in neat toluene the photoisomer quantum yields fall down to zero for benzimidacyanines with the I- anion. Simultaneously the sharp decrease in the quantum yield of fluorescence ( 0 is observed for these dyes in toluene as compared to acetonitrile and chloroform (Figure 5; in = chloroform 0.13 and 0.28 for dyes 1 and 2, respectively). Moreover, the triplet quantum yields of these dyes remain very low. We did not obtain such effects for dyes with other counterions. (The moderate increase in < of dye 1 in tolueneacetonitrile mixtures with m = 50—90% and in chloform in comparison with that in neat acetonitrile is likely due to the more rigid structure of the dye cation in ion pairs.) Thus, in
Photochemistry of Some Cationic Polymethine Dyes
J. Phys. Chem., Vol. 99, No. 17, 1995
6529
analogous effect for indocarbo- and indotricarbocyanine dyes with the I- anion.
Conclusions
It can be concluded that ion pair formation even with “inert” counterions can have a dramatic effect on the photochemistry of cationic cyanines. These counterions can disturb the electronic structure of the dye cation by electrostatic interaction with one of the terminal polymethine atoms and thereby change the potential barrier for its thermal isomerization. In particular, they can either increase or decrease the activation energy of back isomerization of cyanine dyes. Consequently, ion pair formation has a pronounced effect on the rate constant and activation energy of back isomerization of benzimidacyanines on the Figure 5. Dependences of the fluorescence quantum yields composition of the acetonitrile-toluene solvent mixture for dyes 1 (1) and 2 (2) with the I- counterion.
and indotricarbocyanines: it decreases £a for benzimidacyanines and increases £a for indotricarbocyanines. For benzimidacarbocyanine with the I- counterion, as well as for indocarbo- and indotricarbocyanines, the enhancement of intersystem crossing Ti w.—- So (I- heavy atom effect) is observed in ion pairs. It has been shown that for all benzimidacyanines with the I~ counterion the interaction of dye cations with the I- anion in ion pairs leads to the enhancement of the internal conversion Si
So·
Acknowledgment. We are gratefully indebted to the Russian Fund for Fundamental Research (93-03-4217) for financial support.
References and Notes
m
(%
toluene)
of log ky on the composition of the acetoniFigure trile-toluene solvent mixture for dye 1 with C104~ (1) and I™ (2) 6. Dependences
counterions.
toluene the tight ion pair formation with the I- counterion affects the excited singlet states of benzimidacyanines, increasing the internal conversion Si vw— So but having no influence on the T. It can probably be attributed intersystem crossing Si to the formation of charge-transfer complexes between the dye cation and the I- anion in the ion pair, which is not revealed in the absorption spectra in the visible region. 5. Triplet State Decays of Benzimidacyanines. We have also investigated the triplet states of benzimidacyanine dyes, i.e. the T—T absorption spectra and the rate constants of the triplet state decay ky. The triplet absorption spectra of these dyes are observed in the wide range from the long-wavelength edge of the ground-state absorption spectra up to =900 nm. The values of ky for these dyes except 1 with the I" anion are found to be independent of either the counterion or the solvent and are revealed within 103 to 104 s-1 (for dye 3 ky = (1—2) x 104 s-1)· There is a dependence of ky on the dye concentration at low ky due to the self-quenching of the triplet by the dye. for dye 1 with the I- anion increases However, the value of with amount of toluene (m) in the acetonisteeply increasing trile-toluene mixture at m > 70% (Figure 6; when ky > 104 s“\ the self-quenching is negligible; at m = 90% ky = 2 x 105 s-1). The most likely origin of this phenomenon is the interaction of the heavy atom (I- anion) with the polymethine chain of the dye in ion pairs, leading to the increase in the rate of the triplet state decay. In our work8 we described the
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