The Journal of
Physical Chemistry
0 Copyright, 1984,by the American Chemical Society
VOLUME 88, NUMBER 23
NOVEMBER 8,1984
LETTERS Lifetime Shortening of the Photoisomer of a Cyanine Dye by Inclusion in a Cyclodextrin Cavity As Revealed by Transient Absorption Spectroscopy Kazuo Kasatani, Masahiro Kawasaki, and Hiroyasu Sato* Chemistry Department of Resources, Faculty of Engineering, Mi.?University, Tsu, 51 4 Japan (Received: May 22, 1984; In Final Form: September 14, 1984)
The rate of back-reaction of the photoisomer to the original conformer of a cyanine dye, 3,3'-diethyloxadicarbocyanine iodide, is enhanced by inclusion of the dye in the cyclodextrin cavity. The transient absorbance of the photoisomer followed a singleexponential decay, showing the fast establishment of an equilibrium between the photoisomer included in the cyclodextrin cavity and that in solution. From the equilibrium constant and its temperature dependence, thermodynamic quantities related to the equilibrium are obtained.
Introduction The effect of inclusion of organic molecules in the cavity of cyclodextrin (CD) on their photophysical and photochemical properties has been studied. Among those studies are emission intensity enhancement,'S2 intramolecular excimer or exciplex f ~ r m a t i o n ,excimer ~.~ fluorescence due to dimers included in the cavity,5 fluorescence quenching due to the inclusion of the fluorophore and quencher in the same cavity,6s7 emissive triplet stabilization,* a change in the rate of proton di~sociation,~ etc. (1)Ueno, A.; Takahashi, K.; Osa, T. J . Chem. SOC.,Chem. Commun. 1980,921.Ueno, A.; Takahashi, K.; Hino, Y.; Osa, T. J . Chem. Soc., Chem. Commun. 1981. 194. (2) Hoshino,~M.; Imamura, M.; Ikehara, K.; Hama, Y. J. Phys. Chem. 1981,85, 1820. (3)Turro, N.J.; Okubo, T.; Weed, G. C. Photochem. Phobiol. 1982,35, 325. ._.
(4)Emert, J.; Kodali, D.; Catena, R. J. Chem. SOC.,Chem. Commun. 1981,758. Cox, G.S.;Turro, N. J. J . Am. Chem. SOC.1984,106,422. (5)Yorozu, T.;Hoshino, M.; Imamura, M. J. Phys. Chem. 1982,86,4426. (6)Kano, K.; Takenoshita, I.; Ogawa, T. Chem. Lett. 1980,1035. Kano, K.; Takenoshita, I.; Ogawa, T. J . Phys. Chem. 1982,86, 1833. (7) Kobashi, K.; Takahasi, M.; Muramatsu, Y.; Morita, T. Bull. Chem. SOC.Jpn. 1981,54, 2815.
The emission spectra of 2,2-bis(a-naphthylmethyl)- 1,3-dithiane revealed the effect of CD on the conformational equilibria.1° The influence of C D on photoisomerization is another interesting subject. The photoisomerization reaction of cyanine dyes such as 3,3'-diethyloxadicarbocyanine iodide (DODC) has been studied in fluid solvents in relation to the important role of the photoisomer in the mode-locking action of these dyes in picosecond lasers."J2 The photoisomer is unstable and returns to the original conformer by a first-order process in homogeneous solutions. The rate of this back-reaction is dependent on the temperature and on the solvent vis~osity.'~In this paper, the effect of a-,p-, and y(8) Turro, N. J.; Bolt, J. D.; Kuroda, Y.; Tabushi, I. Photochem. Photobiol. 1982,35, 69. Turro, N.J.; Okubo, T.; Chung, C-J. J . Am. Chem. SOC. 1982,104,1789. Turro, N.J.; Cox, G. S.; Li, X . Photochem. Photobiol. 1983, 37, 149. (9)Yorozu, T.;Hoshino, M.; Imamura, M.; Shizuka, H. J. Phys. Chem. 1982,86,4422. (10)Arad-Yellin, R.;Eaton, D. F. J . Phys. Chem. 1983,87, 5051. (11)Schmidt, W.;Schafer, F. P. Phys. Lett. 1968,26A,558. (12)Arthurs, E. G.;Bradley, D. J.; Roddie, A. G. Appl. Phys. Lett. 1972, 20, 125. (13)Jaraudias, J. J . Photochem. 1980,13, 35.
0022-3654/84/2088-545 1$01.50/0 0 1984 American Chemical Society
Letters
5452 The Journal of Physical Chemistry, Vol. 88, No. 23, I984
O R I G l NAL
4.0
I
Oh\
o
‘
CONFORMER
T W I S T I N G COORDINATE
)cllc
Figure 2. A schematic model for DODC photophysics (solid lines: DODC in the aqueous phase (D or P in the text); broken lines: DODC in the inclusion complex with 8-CD (D-CD or P-CD in the text)). The relative positions of solid and broken lines which are determined in this Letter are only those for the ground state of the photoisomer and for the barrier top of the twisted ground state.
I
I
lo-* CP-CDl/
M
Figure 1. [@-CD]dependence of the lifetimes of the photoisomer. [DODC] = 1.0 X 10” M: (0) 10.2 O C ; ( 0 )17.9 OC; (A) 22.0 “C; (A) 32.3 OC. The solid lines are simulated curves.
This is apparently due to the increase of DODC included in the p-CD cavity (diameter 0.70 nm14). Such a change in decay rate was not observed for a-CD (cavity diameter, 0.45 nmI4). The change was smaller for y C D (cavity diameter, 0.85 nmI4). The observed lifetimes (7obd’s) are plotted against [p-CD] in Figure 1.
cyclodextrin on the rate of this reaction for DODC was studied with transient absorption spectroscopy.
Experimental Section
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The exciting light was obtained by a flash-lamp-pumped dye laser (- 1 %I, 1 mJ/pulse, 0.8 ps, 6 Hz). The dye used in the dye laser was Rhodamine-6G (Kanto, laser grade). The monitoring light a t 632.8 nm from a He-Ne laser (Spectra-Physics 155, 0.95 mW) passed through the sample cell (Pyrex, 10 mm X 10 mm) coaxially with the exciting light. The attenuation of the monitoring light was measured as a function of time after the exciting light pulse with a pin photodiode (HTV S-1188). The signal averaged by a transient memory averager (Kawasaki Electronica, M-50E/TMC-400) was analyzed by a microcomputer. The temperature of the sample solution was controlled by setting the cell in a jacket through which water was circulated. DODC (Dojin, laser grade) and a-, p-, and y C D (Nakarai) were used as received. Water was distilled twice. The concentration M in all experiments unless stated of the dye was 1.0 X otherwise. Results Absorption Spectra. The absorption maximum of DODC in water is located at 575 nm. On addition of p-CD, this peak shifted to 577 nm, and another absorption band appeared at 531 nm. The 531-nm band must be due to dimers, since its intensity increased with the concentration of dye at the expense of the intensity of the 577-nm band. Dimerization of the dye is enhanced by inclusion in the p-CD cavity. At this total dye concentration dimers are present exclusively in the p-CD cavity. Relative Fluorescence Quantum Yield in the Absence and Presence of p-CD. The relative fluorescence quantum yield on the excitation of the monomer absorption band (570 nm) was monitored for [DODC] = 5.0 X 10” M. It was found to be essentially constant for [p-CD] = 0 and 5.0 X 10-4-5.0 X M. Transient Absorption Spectra. Those spectra were measured by a flash-photolysis apparatus, using a 10-cm cell with a dye concentration 5 X lo-’ M. The peak of the transient absorbance of the photoisomer of DODC in the aqueous solution appeared near 615 nm, Le., on the longer-wavelength side of the absorption band of the original conformer at 575 nm. In the presence of /3-CD (5 X M) the peak of the transient absorption shifted to 617 nm. Decay of the Transient Absorption. The observed decay of the transient absorbance of the DODC-p-CD system was found to be single exponential for the temperature and [p-CD] range studied ([p-CD] I20 mM, 10-30 “C). DODC molecules included in the /3-CD cavity and those in the aqueous phase are in equilibrium in this system. The decay rate increased with [p-CD].
PHOTOISOMER
Discussion Model. The observed single-exponential decay behavior can be accounted for by the following model:I5 P t CD
i
kl = p-CD k9
kw
D t CD
1
‘CD
D-CD
D, P, D-CD, and P-CD in this scheme are the original conformer and photoisomer in the aqueous phase and those in the CD cavity, respectively. The species P and P-CD appear as a result of the photoisomerization in the excited state deac(excitation tivation into the ground state of the photoisomer) process of DODC as shown in Figure 2, drawn after RuliBre.16 The excitation was made by a 0.8-ps pulse. The photoisomerization processes which follow the excitation occur on a time scale of picoseconds. k’s in the scheme are relevant rate constants. k l and kl are those for the association and dissociation of the photoisomer and /3-CD. kw and kcD are those for the return of the photoisomer to the original conformer in the ground state. The rate equations for P and P-CD according to the model are d[P]/dt = k-l[P-CD] - (kw kl[CD])[P] (1) d[P-CD]/dt = kl[P][CD] - (kcD k-,)[P-CD] (2) The Decay of the Transient Absorbance due to P and P-CD. Although P and P-CD have overlapping transient absorption at the monitoring wavelength, the observed decay was single exponential. To account for this decay, we have to assume that the equilibrium between P C D and P-CD is always maintained. In other words, kl and are much larger than the other rate constants. On the basis of this assumption we have (3) [PI or [P-CD] a eXP(-t/70bsd) 1/Tobsd = akCD + (1 - a)kW (4) a = K[CD]/(l + K[CD]) (5) K = kl/k-l = [P-CD]/([P][CD]J (6)
-
-
+
+
+
~~
~
(14) Bender, M. L.; Komiyama, M. “Cyclcdextrin Chemistry”; SpringerVerlag: New York, 1977. (15) This model does not take into account dimer formation in the ground state. This is allowable since the observed decay rate of the photoisomer was found to be invariant with change of DODC concentration and change of excitation wavelength from the monomer band to the dimer band. (16) Rulikre, C. Chem. Phys. Lett. 1976, 43, 303.
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5453
Letters
5 t
I
I
I
-I
E 2\
:' --
.-.
t; 0.5 LL
3 0.2 0.1
'
I
Figure 3. Arrhenius plot of
I
I
3.3
3.5
3.4 k K/T T~
(0)and
T~~
(0).
5.2
I
\
s
5.0
4.8
(7)
we obtain from Figure 4 AH = 12 f 2 kJ mol-' and A S = 84 f 7 J mol-' K-I. Effect of the CD Cavity on the Photoisomerization of the Dye in the Ground and Excited States. A remarkable fluorescence enhancement has been reportedZ for aqueous solutions of some molecules, e.g., benzene derivatives on the addition of CI). This is due to the fluorescence enhancement of these molecules by inclusion in the C D cavity and has been used to determine the equilibrium constants of these molecules and CD. In the system presently studied, however, little change in the fluorescence quantum yield ($F) of the monomer dye was observed on the addition of p-CD. Two explanations are possible for this finding, and we cannot decide which is the case at present. The first is as follows: $F varies little upon inclusion of the dye in the C D cavity. It is known that the nonradiative decay of the first excited singlet (S,) state of DODC in solution at room temperature is essentially dominated by the photoisomerization process,I8 because the quantum yield of the intersystem crossing to the triplet state is very low (hC 5 X in ethanol)lg and because the direct SI So internal conversion is not important compared to photoisomerization at room temperature and above.*O So, the slight variation of $F implies little effect of CD on the photoisomerization in the SI state. C D accelerates the back-reaction in the ground state, however. The alternative explanation is as follows. Monomers of the original conformer of the dye are almost exclusively present in the aqueous phase. In other words, the equilibrium D-CD @ D CD is shifted to the right to a very substantial degree. Then, the molecules giving fluorescence are almost exclusively those in the aqueous phase. The (excitation photoisomerization in the deactivation into the ground state of the phoexcited state toisomer) process occurs only in the aqueous phase. (Recall that this process occurs within the picosecond time scale.) The resulting photoisomers are distributed among the p-CD cavity and the aqueous phase, as evidenced by the dependence of the photoisomer lifetime on the CD concentration.
-
n 7
y
AG=AH-TAS=-RTlnK
I
3.3
I
3.4 k K/T
I
3.5
-
Figure 4. Temperature dependence of K.
We simulated the T,,bsd - [p-CD] plots (Figure 1 ) with kcD and K as adjustable parameters." We obtained, for example, 7CD = kcD-l = 0.29 ms and K = 164 M-' a t 22.0 'C. The lifetime is much shortened by inclusion in the p-CD cavity, in comparison with the value in the aqueous solution ( T ~ )1.18 , ms at 22.0 'C. Activation Energies and Thermodynamic Quantities. From the Arrhenius plots (Figure 3), the activation energies were deD-CD process termined to be 45 f 2 kJ mol-' for the P-CD and 54 f 2 kJ mol-' for the P D process. The value of equilibrium constant K increases with temperature. Using the relation
-
+
-
(17) The lifetime of the photoisomer increases with the solvent viscosity (ref 13). A small elongation of the lifetime was observed for a-CD, which does not take the dye into the cavity (e.g., 2.04 ms for [a-CD] = 10 mM at 16.7 "C) compared to that in water (1.96 ms at 16.7 "C). This is due to a small increase in the solvent viscosity by the addition of (u-CD. So that this small viscosity effect would be taken into account, the T~ = kw-' values used in the simulation are those measured in the presence of a-CD with the temperature and concentration the same as b-CD used in the experiment.
-
Acknowledgment. The present authors are grateful to Professor Shunji Kat0 and Dr. Akio Yoshimura of Osaka University for measurement of the transient absorption and to Messrs. H. Takasaki and A. Takayasu for their assistance in the experiment. Registry No. a-CD, 10016-20-3; 8-CD, 7585-39-9; -/-CD, 1746586-0; DODC, 14806-50-9. (18) Correctly, this process is the crossing over of molecules to the twisted intermediate state over a potential barrier, as shown in Figure 2. A fraction of molecules in the intermediate state gives the photoisomer and the rest the original conformer in the ground state. This process is called photoisomerization for simplicity. (19) Dempster, D. N.; Morrow, T.; Rankin, R.; Thompson, G. F. J . Chem. SOC.,Faraday Trans. 2 1972, 68, 1479. (20) Velsko, S. P.; Fleming, G. R. Chem. Phys. 1982, 65, 59.
