J. Phys. Chem. 1994,98, 3165-3173
3165
Temperature Dependence of Fluorescence and Photoisomerization in Symmetric Carbocyanines. Influence of Medium Viscosity and Molecular Structure+ Pedro F. Ammendfa,' R. Martin Negri, and Enrique San RomPn INQUIMAE, Departamento de Quimica Inorghzica, Anafitica y Quimica Fiiica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellbn 2, Ciudad Universitaria, 1428 Buenos Aires, Argentina Received: June 10, 1993; In Final Form: December 20, 1993'
The temperature dependence of fluorescence emission and photoisomerization of five symmetric carbocyanines was studied in a group of primary n-alcohols from 0 to 70 OC. The trans-cis isomerization on the excited state potential surface was studied by steady-state fluorescence emission and flash photolysis. The back isomerization rate constant on the ground-state potential surface from the photoisomer to the normal form was also studied by flash photolysis. In all cases fluorescence quantum yields were found to diminish with temperature and to increase with viscosity (q), while isomerization quantum yields showed the opposite behavior. All deactivation rate constants of the excited singlet state were calculated, and activation energies for the processes of excited and ground states were obtained. The solvent does not affect radiative and internal conversion rate constants of the excited singlet state. The only effect of the solvent on the isomerization rates (&A)of both processes is through q . Results indicate that the dependence of ki, on q is ki, =f(q) exp(-Eo/RT), where Eo was found to be solvent independent. Fits off(q) by the Kramers equation (Physica 1940,7,284) and by an activationvolume-based model were performed. Kramers's equation provides a good fit of the data for the carbocyanines with the smallest EO,but for other cases it shows systematic deviations already reported for other carbocyanines, which are discussed on the basis of the hypothesis involved in the model. The activation-volume-based model fits very well the data in all cases. A comparison is made between excited- and ground-state isomerizations. The influence of structural properties of the dyes on the photophysical parameters is also discussed.
Introduction Carbocyanines are cationic dyes that have a wide variety of applications including photography,' optical probes in membranes or model membrane systems,Z active or mode-locking substances in dye lasers,' and initiators in photo polymerization^.^ They show a very strong II lI* absorption which can be tuned through the visible and NIR region by varying the length of the polymethinic chain.' This transition has a radiative lifetime of a few nanosecond^,^^ so processes effectively competing with fluorescence emission should be very fast. The short singlet lifetime and the low yield of intersystem crossingl.5 assure the required photostability for membrane monitoring and laser applications. In solution, carbocyanine dyes, with no bulky substituent in the polymethine chain, adopt an all-trans configuration.8 The photophysics of these dyes has been extensively studied,>lg and it is well established that following light absorption the dyes isomerize from the first excited singlet state, IN, to a groundstate photoisomer, P, which has been proposed to have a mono cis conformation.8 The kinetic scheme of Figure 1 is normally adopted to account for the photophysical behavior of carbocyanine~~l-148 is the rotation coordinate responsible for the isomerization, N is the thermodynamically stable groundstate, IN is its first excited singlet state, P is the ground-state photoisomer, and t is a nonspectroscopic partially twisted intermediate excited state, which deactivates very fast nonradiatively to the ground-statehypersurface. Deactivation processes of IN take place in the pico- to nanosecond time ~ c a l e ,depending ~.~ on the compound and on the medium, whereas the back reaction of P to N occurs in the micro- to millisecond.5J1J2J4 The two isomerization processes, IN t P and P N, involve the same coordinate. Both are activated processes, and
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--
* To whom correspondence should be addressed.
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This publication is part of the Ph.D. Thesis of R. M. Negri, Universidad de Buenos Aires, 1991, and is dedicated to Prof. Alejandro J. A d a on the occasion of his 65th birthday. .Abstract published in Aduclnce ACS Abstracts. February 1, 1994.
the rate constants that describe these processes, kN, and kpN, have a dependence on temperature and medium viscosity which has already been reported for other compounds.11~'2~'5-18~20 The equationproposed by Kramersm to account for the kinetics of barrier crossing in a viscous medium is normally used to quantitativelydescribe the temperature and viscosity dependence of isomerization rate constants ( k h ) . In this model, the height of the energy barrier for isomerization,EO,is an intrinsicmolecular property and is independent of solvent. Under the assumptions that EO>> RT, that the motion takes place in only one coordinate (rotation 8 in Figure I), and that the medium-friction effect is a Markovian process, the model renders for k h (which can be either kNt or kpN)
ki, = wO(4?ro'~,)-'[(1
+ ( ~ w ' T , ) ~ ) ' / ~11- exp(-Eo/RT)
(1)
where wo is the frequency related to the curvature of the energy surface at the minimum and o' is the correspondingvalue for the maximum. 7,. = p / f is the relaxation time for the velocity along the rotation coordinate 8,p is the mass factor associatedwith the rotating group, and the friction coefficient 6 is the proportionality constant between the friction force and the velocity. To apply Kramers' equation, it is necessary to postulate a dependence of f on q, the solvent viscosity coefficient. The hydrodynamicmodel very often applied assumes a direct proportionality of f on q. Under these conditions, eq 2 results: ki,(tl,T) = A ( v / N [ ( + ~ ( B / v ) ~ ) '-' ~11 exp(-Eo/RT)
(2)
with A = o0/2uand B / q = ~ w ' T , . Equation 2 fails sometimes to account for the quantitative descriptionof kh.11J7J8J1.*2Alternative descriptionsof k h have been proposed which render a good quantitative approach2s26 but involve more than two parameters which should be fit to the experimental rate constants. On the other hand, eq 3 is in many cases a good description of k h but provides little microscopic insight into the molecular process: 11.17.1 8.22.26
0022-3654/94/2098-3 165$04.50/0 0 1994 American Chemical Society
3166
Aramendla et al.
The Journal of Physical Chemistry, Vol. 98, No. 12, 1994
K
\
R
Carbocyanine n
N
t
O0
90'
P > 1 soo
0 Figure 1. Potential energy surface diagram for cyanine photoisomerization: 8,rotation coordinate;N, ground state of the normal form; IN, first excited singlet state of the normal form; t, twisted state; P, ground state of the photoisomer.
In eq 3, D and a are adjustable parameters, with 0 Ia I1. Equation 3 has a theoretical background on diffusion theories involving activation v o l ~ m e s . z ~ ~ ~ ~ ~ ~ * Accurate description of kN, and kpN have been achieved for carbocyanines, but the measurements are restricted to few compounds, namely, 3,3'-diethyloxadicarbocyanine iodide (DODCI)" and 3,3'-diethyloxacarbyanine iodide (DOCI).18 In this work we undertake a determination of the viscosity and temperature effect on k~~ and kpN for a series of symmetric carbocyanines differing in the polymethine chain length and/or the nature of the substituent in the position 1 of the indoline moiety. We obtain for all of them the parameters of the energy surface and estimate all the rate constants in Figure 1,correlating them to the various structures. We integrate our results with literature values for DODCI, which matches the characteristics of the other compounds. The substances studied, together with their structure, are shown in Figure 2.
Experimental Section A. Chemicals. The carbocyanine~-3,3'-diethyloxacarbocyanine iodide (DOCI), 3,3'-diethylthiacarbocyaocyanine iodide (DTCI), 3,3/-diethyloxadicarbocyanineiodide (DODCI), 3,3'-diethylthiadicarbocyanine iodide (DTDCI), 1,3,3,1',3',3/-hexamethylindodicarbocyanine iodide (HIDCI), and 3,3'-diethyloxatricarbocyanine iodide (DOTC1)-were obtained from Lambda Physik or Exciton (Laser Grade), and their purity was checked by HPLC (Shimadzu system 6, RP18,S Mm, 25 X 0.46 cm, MeOH, 4 mL/ min). Rhodamine 101and cresyl violet perchlorate wereobtained from Lambda Physik. All the alcohols employed-ethanol 95% (EtOH 95%, Merck), methanol (MeOH, Merck), pentanol (PeOH, BDH or Merck), octanol (OcOH, BDH or Merck), and decanol (DecOH, Fluka or Merck)-were freshly distilled. Solutions were not degassed, and in all cases the concentration was less than les M in order to avoid dimerization. No photochemical or thermal decompositionof the dyes was observed in the range 0-70 OC, except for DOTCI. Solutions of DOTCI were thermally degraded above 35 "C, but the rate of decomposition did not significantly affect the results (i.e., upon making a temperature cycle, the same Ifcould be measured), except in DecOH where it was faster. Data on refractive index, thermal
DTCI DOCI DTDCI DODCI HIDCI DOTCI
1 1
2 2 2 3
X
R
s 0 s 0 C(CH,), 0
Ethyl Ethyl Ethyl Ethyl Methyl Ethyl
Figure 2. Molecular structure of the symmetric carbocyanines studied in this work.
expansion coefficients, and viscosity of solvents as a function of temperature were obtained from l i t e r a t ~ r e . ~ ~ B. Absorption and Fluorescence Emission. Absorption spectra were recorded on a Cary 2300 (Varian) spectrophotometer. The absorbances in the fluorescence quantum yield determinations were measured at room temperature, and no significantvariations between 20 and 27 OC were observed. Absorbances were measured with an error of 0.001 unit. Todetermine the absorption coefficients in the different alcohols, 100 gL of an ethanolic solution of known concentration was diluted to 5.00mL with the other alcohol, and the absorbance of the resultant solution was measured. Steady-state corrected fluorescencespectra were measured on a Perkin-Elmer LS5 spectrofluorometer (2.5-nm bandwith). The absorbance of sample and reference was always less than 0.05 (1-cm path length) in the whole wavelength range in order to avoid reabsorption effects.30 The solutions were contained in a quartz cuvette: 1-cm path length for absorption and 0.4 cm for emission and were thermostatized by water flow from a Lauda RC-6 cryostat-thermostat. The temperature was measured with a PtlOO thermometer inside the cuvette. Spectra were recorded between 0 and 70 OC except in methanol and decanol, where boiling and freezing point of the solventrestricted the temperature range. Fluorescencequantum yields (&) were measured relative to rhodamine 101 in ethanol (& = 1.003l) and cresyl violet in methanol (& = 0.5432). The uncertainty in the determinations is within 5-10%. Fluorescence quantum yields at 25 O C were determined by integrating the whole emission. As the emission spectral distribution of the substances did not change with temperature, the fluorescence intepsity at a particular wavelength range was proportional to the whole emission. This relativevalue was considered to compute & at all other temperatures using eq 4:
where If is the fluorescence intensity measured under identical conditions at T and at 25 OC. Some of our determinations of & are compared with literature values in Table 1. C. Flash Photolysis. Conventional photolysis and laser flash photolysis were used in order to obtain photoisomer lifetimes and relative values of photoisomerization quantum yield (+p). Both setups with absorption detection have been previously described.33.34 Measurements were performed between 0 and 65 "C (except for DOCI and DTCI, for which the range was 18-60 "C) with the same restrictions as for fluorescencemeasurements.
The Journal of Physical Chemistry, Vol. 98, No. 12, I994 3167
Temperature Dependence of Fluorescence
TABLE 1: Comparison of Fluorescence Quantum Yield
0.6
Determinations with Literature Data' "pd
cresyl violet
DODCI DODCI DODCI DODCI DTCI DTDCI DTDCI a
solvent MeOH MeOH EtOH PeOH DecOH MeOH MeOH EtOH
temp ("C) 25 20 25 20
20 25 20 25
4Jf this work
lit.
ref
0.52 0.39 0.42 0.54 0.73
0.54 0.36 0.42 0.58 0.68
0.06
0.05
0.32 0.35
0.36 0.35
32 16 42 16 16 43 19,43 5
0.4 c
8
0.2
Thevalutsinthisworkweremeasurcdwithrhodamine 101 inethanol
as a standard.27
In both cases temperature was controlled and monitored inside the cuvette with a PtlOO thermometer. The conventionalequipment has two coaxial xenon lamps with emission between 250 and 600 nm (maximum at 450 nm).33 The cuvette (12-cm path length) was surrounded with colored plastic filters to diminish the excitation pulse energy (En). The photoisomer lifetimes and relative values of 4pof DOC1 and DTCI were measured with this equipment. The excitation pulses were attenuated to thevalues where a linear relationship between the transient absorbance difference immediately after the light pulse and En was obtained. En was varied within this range by regulating the flash lamps discharge voltage. The square of this latter value was taken as a measure of the pulse energy. For the other carbocyanines a 15-11s Nd:Yag laser was used, operating in its second harmonic (532 nm) and pumping either a DCM or a pyridine 1 dye laser.34 The sample was contained in a 1 cm X 1 cm quartz fluorescence curvette. The energy was varied with neutral density filters and was measured with a pyroelectric energy meter.
ResultS A. Absorption and EmissionSpectra. In all cases, the spectral distribution for absorption and emission was independent of temperature. The absorption coefficient at the wavelength of the maximum did not change with the solvent. In spite of the fact that the solutions employed to determine the absorption coefficient contained 2% v/v of EtOH, the absorption spectrum in these solutionswas always identical to that in the pure alcohol. Thus, we conclude that differential solvation plays a negligible role. A red shift of absorption and fluorescencespectra of ca. 10 nm was observed on going from MeOH to DecOH. In every case, the emission spectral distribution kept a good mirror-image relation to the absorption spectrum,independentlyof theexcitation wavelength within the principal visible absorption band. In all cases, a reversiblesystematicdecreasepfabsorption with temperature was observed, which amounted sometimesup to 10% between 10 and 60 OC. Up to 5% absorbance decrease is accounted for by solvent thermal expansion,the other contribution arising probably from the presence of variable amounts of a configurational isomer together with the predominant all-trans form.8.35 The small change in absorbance registered in the temperature range of work precluded any quantitative study of this possible thermal equilibrium. Nevertheless, the actual absorbance at each temperature was used to compute either & or &, Thus, eq 4 was corrected by the factor A(25OC)/A(T). For the q5p correction, see below. Absorptionand emission spectra wereused tocalculate radiative rate constants (kf in Figure 1) according to the procedure proposed by Strickler and Berg.36 This procedure is expected to render good results when applied to this kind of compounds. Comparison of theoretical and experimental results for DODCI and merocyanine 5405.37 supports this assumption. The little shift in absorption and fluorescencewavelength mentioned above
I
0
I
20
I
1
40
1
60 Temperature (''2)
1
80
Figure 3. Temperature dependence of the fluorescence quantum yield of HIDCI in the different alcohols: (0)MeOH, (v)EtOH, ( 0 )PeOH, (A) OcOH, and (0)DecOH.
did not reflect an appreciable variation of kf in the different solvents. B. Excited-StateIsomerizationRate Constants. F l u o " ~ QuantumYieldsand RelativePhotoisomerhationQu~ahun Yields. In all cases the fluorescencequantum yield was found to increase on lowering the temperature in a given solvent and with solvent viscosity at constant temperature. This behavior is shown in Figure 3 for HIDCI as an example case. (We choose HIDCI as the typical compound for which results are shown. Similar behavior is observed in other cases, although with quantitative differences.) c#q values under different conditions for all compounds are summarized in Table 1. According to Figure 1, & is given by
6f = kf/(kf +
+ kNt)
(5)
If we reasonably assume that neither kf nor kiCis temperature dependent, then at low temperatures and high viscosity kNt 0.10. The relative value of 4p was found to display the opposite behavior to &, namely to increase with temperature in a given solvent (except for DTCI where the slight changes observed could hardly be distinguished from experimental error) and to decrease with viscosity, as is expected according to the kinetic scheme proposed. For 4p it is possible to write eq 1 1 (following the terminology of Figure 1 )
c5 I
0 4
\
