J. Phys. Chem. 1995,99, 13796-13799
13796
Photoisomerization Dynamics and Spectroscopy of the Polymethine Dye DTCI Roberto E. Di Paolo? Lucia B. Scaardi,' Ricardo Duchowicz,*$*and Gabriel M. Bilmesl Centro de Investigaciones Opticas (Clop), CIC-CONICET, Casilla de Correo 124, 1900 La Plata, Argentina Received: November 2, 1994; In Final Form: May 11, 1995@
Laser absorption spectroscopy, laser-induced fluorescence, and pulsed-laser optoacoustic spectroscopy were used to characterize photophysical and spectroscopic parameters of the photoisomer of the cyanine dye 3,3'diethylthiacarbocyanine iodide (DTCI). Values for the photoisomerization and back-photoisomerization quantum yields (&p = 0.20 f 0.03 and C$PN = 0.16 z t 0.03,respectively), as well as values for absorption cross sections and the energy content of the ground state of the photoisomer, were obtained. The quantum efficiency of fluorescence of this species was found negligible. Similar to the findings for DODCI and Merocyanine 540, these results suggest that intemal conversion without passing through an intermediate twisted state is the main deactivation step for the excited photoisomer.
Introduction The photophysics of cyanine dyes has attracted great interest because of its importance in scientific, technological, and industrial areas. Since the initial discovery of this type of compound in the early 1950s,their properties were investigated by many authors. Photoisomerization, an important mechanism characteristic of a large number of molecules of this family of compounds is of considerable interest from both experimental and theoretical points of The cyanine dye 3,3'-diethylthiacarbocyanine iodide (DTCI) (Figure 1) is an example of the type of molecule for which the N P photoreaction O C C U ~ S . N ~ is the thermally stable form of the molecule, and P is a short-lived photoisomer. The isomerization is produced by rotation of the molecule around a double bond of the polymethinic chain after irradiation with wavelength resonant with the first or higher N excited states. DTCI was previously studied under different conditions using several technique^.^-^ Ponterini and Momicchioli4 found that trans-cis photoisomerization of this molecule occurs in both highly polar and hardly polar solvents, showing that isomerization kinetics is only slightly affected by solvent polarity. On this basis, the authors discussed different theoretical models for the photoisomerization processes. They found that the model using explicit CS-INDO formulated by Momicchioli et calculations agreed with the experimental data. Figure 2 represents the potential energy curve model proposed to explain the photophysics of this molecule. 6 is the rotation coordinate responsible for the isomerization. :S and Sy are the thermodynamically stable ground state and the first excited singlet state of N. S,' and SB are the ground state and the first excited singlet state of P, and t is a nonspectroscopic twisted intermediate. After light absorption, photoisomerization starts from the Sy state potential energy curve minimum. Its deactivation is mainly dominated by an activated processes at room temperature
Et
Et
Figure 1. 3,3'-Diethylthiacarbocyanine iodide (DTCI).
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~~~
* To whom correspondence
should be addressed. ' Fellow of Comision de Investigaciones Cientificas de la Provincia de Buenos Aires (CICPBA), Argentina. Researcher of Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), Argentina. Researcher of CONICET and Physics Department of Universidad Nacional de La Plata, Argentina. lResearcher of CICPBA and Universidad Nacional de La Plata, Argentina. Abstract published in Advance ACS Abstracts, August 1, 1995.
*
N 0"
t
P
90"
180"
m
8 (TWISTING COORDINATE)
Figure 2. Potential energy surface diagram for DTCI photoisomerization: N, normal form; P, photoisomer; t, twisted state.
EN^ = 24 kJ/mol in EtOH7). The decay to the ground state surface occurs mainly from the 90" minimum (ht= 0.930.954,7) and by fluorescence emission (#== 0.077). intersystem crossing and SY-S: intemal conversion are negligible. P returns to N via S:-S: or via ST-S: (backisomerization process). The first one is also a thermally activated step (EPN= 63 kJ/mol in EtOH'). Absorption and fluorescence cross sections of normal and photoisomeric species are strongly o ~ e r l a p p e d . ~This . ~ is one of the reasons why the characterization of the kinetics and spectroscopic parameters of the photoisomer P is difficult. Up to now, little information is available about these species. Following the investigation line described in previous studies performed with DODCI?*9*'0 DTDCI," and Merocyanine 540,12 in this work we use laser absorption spectroscopy and laserinduced fluorescence under steady state conditions and pulsed laser optoacoustic spectroscopy to analyze the photoinduced cis-trans isomerization of DTCI. Absorption and fluorescence measurements were performed at variable excitation fluences. At low fluences, the population of P is negligible, while at high
0022-365419512099-13796$09.00/0 0 1995 American Chemical Society
Sy-c
J. Phys. Chem., Vol. 99, No. 38, 1995 13797
Spectroscopy of the Polymethine Dye DTCI values it reaches a maximum (“saturation ~ondition”’~). Results obtained under the latter conditions allowed us to determine the absorption cross section of the photoisomer at various wavelengths as well as the quantum yield of back-isomerization from excited P to the N form and to obtain limiting values for the quantum efficiency of fluorescence of P. By means of optoacoustic measurements, we could determine the value of the energy content of P.
Experimental Section DTCI (Exciton) was used in solutions of analytical grade ethanol at room temperature. Low-concentration samples (< M) were employed to avoid dimerization and inner filter effects. Absorbances were measured with a Beckman spectrophotometer DU-65. The experimental setup used for laser absorption, laserinduced optoacoustic measurements, and laser-induced fluorescence was previously d e s ~ r i b e d . ~ ,For ~ * ’laser ~ absorption and laser-induced fluorescence measurements, DTCI solutions flowed through standard fluorescence cuvettes. CW laser excitation was carried out with an Ar ion laser at 501.7 and 514 nm and an argon-pumped Rhodamine 6G dye laser at 562, 557, and 550 nm. The excitation source for the optoacoustic measurements was a flash lamp-pumped dye laser (Chromatix CMX-4, 1 ps pulse duration and 1 mJ maximum energy). For this experiment Rhodamine 640 was used, with the laser tuned at 577 nm. The heat integrating time of the whole system was 1 ps, and iodine in ethanol was used as calorimetric r e f e r e n ~ e . ’ ~ . ’ ~
20
3 b v a‘
0
-20
40
OEtO
2E-3
I
I
4E-3
6E-3
I /a
Figure 3. Absorption measurements as a function of the absorbed photon flux a for three wavelengths. Solid lines represent fitting by using eq 1.
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61
I
Results Absorption Measurements. By using laser absorption spectroscopy, we analyzed the dependence of the photoequilibrium between both isomers as a function of the kinetic parameters of the system and the excitation fluence. Basically, we studied changes in the absorbance of the sample as a function of the absorbed photon flux a [photons s-’1 of a chopped laser beam. A low-power beam of the same laser was used as the probe beam. The change in the absorption coefficient of the sample when the excitation beam is “on” or “off” can be written as2
where B = up/o~(ON and u p are the absorption cross section of the N and P species, respectively) is wavelength-dependent. p = C#JPN/&P is the ratio between isomerization and backisomerization quantum yields, k is the rate constant of the thermal P N isomerization, and w is the rate of sample renewal within the irradiated volume of the solution by circulation. By plotting o$(a - a,) as a function of l/u at various wavelengths, p , &A), and &p can be univocally determined from the ordinate and the slope of the straight line fitted to the experimental points. Figure 3 shows experimental values and the corresponding adjusted curves for solutions of DTCI-EtOH ranging between 2 x and 10 x M. Best fits were obtained for p = 0.78 f 0.08, &p = 0.20 f 0.03, B(514 nm) = 1.16, B(557 nm) = 0.56, and B(562 nm) = 0.50. B(550 nm) = 1 was determined observing the wavelength for which the absorption difference was zero. Around this point, changes in the sign of the signal can be clearly observed. With the values for p and &p. ~ P = N 0.16 & 0.03 was evaluated.
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450
600
550
600
650
700
WAVELENGTH lnml
Figure 4. (a) Absorption cross section of DTCI normal form. (b) Absorption cross section of DTCI photoisomer (points were determinated in this work, solid line was calculated by using previous flash photolysis results4).(c) Corrected fluorescence emission of DTCI normal form.
With the values of B for different wavelengths, absorption cross section of the photoisomer can be determined as u p = BUNwith UN values taken from ref 16. Calculated up values (circles) are shown in Figure 4. To compare our results with those obtained by using flash photolysis measurements, we recalculated the spectral distribution of u p with data from ref 4. For this purpose, the differential absorbance AA is written as
AA(2.303/1) = (ON - 0p)PO = ON( 1 - B)PO where 1 is the path length in the sample, POis the photoisomer population, independent of the probe wavelength, and AA are the values taken from ref 4. Then, for two different probe wavelengths (AI and il 2), we obtain
Using eq 2 and a B(AI ) value obtained with the absorption technique, we calculated a whole curve for up. The results are shown in Figure 4 (curve b, dashed line). The good agreement
Di Paolo et al.
13798 J. Phys. Chem., Vol. 99, No. 38, 1995
it
40 -I
0.2
1L-LI
I
’
2
‘ i I
I 3
0.0
IODINE
0.1
//
0.3
0.2
0.4
0.6
1-10-*
Log (4
Figure 6. Energy normalized optoacoustic signal HE vs fraction of
Figure 5. Dependence of the ratio between the fluorescence intensity and the absorbed photon flux (rfla) as a function of a, for three different
absorbed energy for sample (DTCI) and reference (iodine), both in ethanol. A,,, = 577 nm.
excitation wavelengths, as indicated. can be seen between this calculated curve and the points obtained in this work (circles). Fluorescence Measurements. Figure 4 (curve c) shows a typical corrected fluorescence spectrum of DTCI. This emission curve did not change either with the intensity or with the wavelength of the excitation. The intensity ranged between 0.25 and 25 W/cm*. Figure 5 shows the dependence of the ratio between the fluorescence maximum intensity and the absorbed photon flux (a) as a function of a for three different excitation wavelengths. Assuming that the emission spectra of both isomeric species are almost fully overlapped, the fluorescence intensity at any wavelength is a combination of the emission of both species, then2 Zda = K( 1
+ Gu/u,)(1 -I- a/u,)-’
(3)
where K is a constant independent of a, G is defined as G = (p BI(p lIB), f = r$I&, and a, is the saturation absorbed photon flux defined as the absorbed flux necessary to obtain the half value of the maximum photoisomer population.2 Fitting the experimental results shown in Figure 5 , we obtained the G values for three different wavelengths. Using the p and B values derived from the absorption measurements allowed us to determine f I 0.05. Since 4: = 0.07,7 the photoisomeric fluorescence quantum yield is 4; 5 0.004. Photoacoustic Energy Content Determination of DTCI Photoisomer. Pulsed laser photoacoustics was used to determine AE, the molar energy content difference between the ground states of N and P (Figure 2). As it can be seen from Figure 4, the ratio between the cross section of both isomers shows a maximum for excitation wavelength used in this experiment (577 nm). In this region, the absorption by P can be neglected. In this case, eq 4, derived from simple energy balance considerations is used:I7
+
+
E, = 4
k + aEa+ 4”pAE
(4)
Ea is the absorbed molar energy. The first term is the energy dissipated as fluorescence (Efis the molar energy of the fluorescent state); a is the fraction of energy deposited in the medium as prompt heat, within the heat integration time of the experiment (ca. 1 p s ) . The third term on the right side of eq 4 is the energy stored by the photoisomer, which in this case has
a lifetime of about 7 ms,7918Le., much longer than the heat integration time of the experiment. The total stored energy, &pAE, is calculated measuring a and knowing the light emission properties of the sample. Provided rp”p is known, AE is derived. As is well-known, the energy normalized photoacoustic signal is given by2.93’7
where H is the amplitude of the first acoustic signal, E is the excitation energy, K is a constant containing the thennoelastic parameters of the solution and instrumental factors, and A is the absorbance of the sample (S). By using a calorimetric reference (R),I7 a is determined from measurements of HIE, performed under the same experimental conditions for sample and reference: (H/E),/(HIE), = a
if aR= 1
In our case a was determined from the slopes of linear plots of H vs E for sample and reference at different absorbances, and iodine was used as calorimetric reference (Figure 6). From the slopes of the two lines, a value of a = 0.88 f 0.02 was determined. From eq 4 and taking into account that 4: = 0.07 f0.007, & = 580 nm, and &p = 0.20 f 0.03 (vide supra), a value AE = 0.5 f 0.4 eV results for the molar energy content difference between N and P ground states. This value is higher than the previously reported one in ref 19 evaluated with the same experimental data. The reason for this difference lies in the fact that in ref 19, calculations were performed using I$”P = 0.82 as reported by Chibisov,8 which is a value much higher than the one determined by absorption technique in this work (&p = 0.20 f 0.03, vide supra). Unfortunately, due to the value of the parameters involved in the determination, the precision of the AE value is strongly limited. Discussion The value determined for the formation quantum yield of P, &p = 0.20 f 0.03, confirms previous laser flash photolysis measurements in ethanol-methanol solutions4 (&p = 0.25 f 0.04) and is much lower than that previously reported by Chibisov (0.82).8 The absorption cross sections for the DTCI
Spectroscopy of the Polymethine Dye DTCI photoisomer determined here are also consistent with those derived from the differential spectrum obtained from flash phot~lysis.~ The lower limit obtained for the fluorescence quantum yield of the photoisomer of DTCI (4; 5 0.004) confirms the general notion that the cis isomers of carbocyanines have negligible fluorescence at room temperature. The results can be discussed in terms of the kinetic scheme in Figure 2. The branching ratio to P from the twisted state should be the same whether the molecules come from the cis or from the trans geometry. The actual value of this branching ratio is r = &p/c#J"t = 0.21. Under the assumption that photoisomerization occurs from both the trans and cis sides through a common perpendicular @ u p ) t intermediate and that the trans-to-perp process is activated, the cis-to-perp process seems to be a nonactivated, intrinsically barrierless process. This result is supported by the fact that the values for &N = 0.16at room temperature determined by us and $JPN = 0.15 at 190 K, calculated by Ponterini et show a negligible dependence of this parameter with temperature. Due to the low value of c$, deactivation of excited P is mainly a radiationless process. The low value of &N suggests that, in contrast to the deactivation of excited N, in which most of the nonfluorescent molecules go to SO through the twisted state, nonradiative transitions of excited P follow a different pathway. Internal conversion appears to be the most important deactivation step. This conclusion is supported by the value of the quantum efficiency of deactivation of excited P to the twisted state, & = 0.20, calculated assuming the same branching ratio for the trans-cis and the cis-trans photoisomerization processes, Le., 1 - r = &N/@R, taking into account that the quantum yield of fluorescence is negligible for this species. In this way only one of every four molecules from excited P decays to the twisted intermediate. The behavior of the photoisomer of DTCI resembles that of the photoisomer of DODCI (3,3'-diethyloxadicarbocyanine iodide), a cyanine with one additional double bond in the polymethinic chain and 0 instead of S as heteroatom. For this molecule, intemal conversion without passing through the
J. Phys. Chem., Vol. 99, No. 38, 1995 13799 twisted state seems to be the main deactivation channel for excited P, as it was proposed by Bilmes et and later c o n f i i e d by other studies.20,21Similar results were also found in the case of the anionic polymethine dye Merocyanine 540.12 ~
1
.
~
3
'
~
Acknowledgment. We thank J. 0. Tocho and P. F. Aramendfa for fruitful discussions and comments during the preparation of the manuscript. This work was supported by PID 3973/92of CONICET (Argentina). References and Notes (1) Korobov, V. E.; Chibisov, A. K. Russ. Chem. Rev. 1983, 52, 27. (2) Tocho, J. 0.;Duchowicz, R.; Scaffardi, L.; Bilmes, G. M.; Di Paolo, R. E.; Murphy, M. Trends Phys. Chem. 1992, 3, 31. (3) Momicchioli, F.; Baraldi, I.; Berthier, G . Chem. Phys. 1988, 123, 103. (4) Ponterini, G.; Momicchioli, F. Chem. Phys. 1991, 151, 111. (5) Rentsch, S.K.; Fassler, D.; Hampe, P.; Danielius, R.; Gadonas, R. Chem. Phys. Lett. 1982, 89, 249. (6) Gadonas, R.; Danelyus, R.; Piskarskas, A,; Rentsch, S. Sou. J. Quantum Electron. 1983, 13, 186. (7) Aramendia, P. F.; Negri, R. M.; San RomBn, E. J. Phys. Chem. 1994, 98, 3165. (8) Chibisov, A. K. J. Photochem. 1!?76/77, 6, 199. (9) Bilmes, G. M.; Tocho, J. 0.; Braslavsky, S. E. Chem. Phys. Lett. 1987, 134, 335. (10) Bilmes, G. M.; Tocho, J. 0.;Braslavsky, S. E. J. Phys. Chem. 1988, 92, 5958. (11) Bilmes, G. M.; Tocho, J. 0.;Braslavsky, S. E. J. Phys. Chem. 1989, 93, 6696. (12) Aramendia, P. F.; Duchowicz, R.; Scaffardi, L.; Tocho, J. 0. J. Phys. Chem. 1990, 94, 1389. (13) Duchowicz, R.; Scaffardi, L.; Di Paolo, R. E.; Tocho, J. 0.J. Phys. Chem. 1992, 96, 2501. (14) Tam, A. C.; Patel, C. K. N.; Kerl, R. J. Opt. Lett. 1979, 4, 81. (15) Nonell, S.; Aramendia, P. F.; Heihoff, K.; Negri, R. M.; Braslavsky, S. E. J. Phys. Chem. 1990, 94, 5879. (16) Brackmann, U. Data Sheets of Laser-grade Dyes, 1st ed.; Lambda Physik GmbH: 1986; 111 -129. (17) Braslavsky, S. E.; Heibel, G. E. Chem. Rev. 1992, 92, 1381. (18) Negri, R. M. Private communication. (19) Bilmes, G. M.; Murgida, D. H.; Erra-Balsells, R. E. J. Phys. 1V 1994, C7, 4, 417. (20) Rentsch, S.;Dopel, E.; Petrov, V. Appl. Phys. B 1988, 46, 357. (21) Zhu, X.R.; Harris, J. M. Chem. Phys. 1988, 124, 321. JP942979E
