J. Phys. Chem. 1994, 98, 1776-1782
1776
ARTICLES Combination of Laser-Induced Optoacoustic Spectroscopy (LIOAS) and Semiempirical Calculations for the Determination of Molecular Volume Changes: The Photoisomerization of Carbocyanines Maria S. Churio,? Klaus P,Angermud,$ and Silvia E. Braslavsky'J Max-Planck-Institut fir Strahlenchemie, Postfach 10 13 65, 0-45413 Miilheim an der Ruhr, Germany, and Max-Planck-Institut fir Kohlenforschung, 0-45466 Miilheim an der Ruhr, Germany Received: August 10, 1993'
The volume change for the E-Z photoisomerization reaction of the 3,3'-diethyloxadicarbocyanine iodide (DODCI) at 562 nm, and the 3,3'-diethyloxacarbocyanine iodide (DOCI) at 460 nm were determined in ethanol-water mixtures. The methods used were the solvent- and temperature-dependent laser-induced optoacousticspectroscopy (LIOAS) with nanosecond excitation and ca. 1-ps heat-integration time. Contractions of 52 and 29 mL/mol, respectively, were measured for the DODCI photoisomerization. No photoisomerization volume change was observed for DOCI. The parameters obtained from calculations of the known initial and possible final conformations led to the conclusion that the volume change observed is mainly due to the rearrangement of solvent molecules around the DODCI isomers with different dipole moments.
Introduction
SCHEME 1
Photothermal techniques, in particular laser-induced optoacoustic spectroscopy (LIOAS), have been used for the determination of molecular movements concomitant with photoinduced reactions.'-5 The proper selection of the heat integration time permits to assign the movement to a particular molecular step.5 The time can be selected from several picoseconds6 to micro- or milliseconds,14depending on the particular technique used.5With LIOAS, time windows from several nanoseconds to microseconds can be studied. Techniques like the study of pressure effects on the kinetics of photochemical reactions and on quantum yields, on the other hand, afford information on volume changes, the assignment of which to individual molecular events is d i f f i ~ u l t . ~In- ~addition, the relatively high pressures needed to produce measurable effects make the application of this technique difficult for the study of fast photophysical and photochemical Callis et al.1 drew attention to the fact that there are two possible contributions to the volume change in a photoinduced reaction in solution: (a) the difference in volume between products and reactants, and (b) the expansion or contractionof the medium upon the release of heat. Only the second part depends on the thermoelastic parameters of the solution (cp,the heat capacity at constant pressure; /3 = (aV/aT)p(l/V), the volume expansion coefficient; and p , the solution density).1.5 Optoacoustic measurements as a function of these parameters can separate both terms, thus allowing the calculationof molecular volume changes. This method has been already applied to various photoinduced reactions.2-496J0-12 In order to test the potential and limitations of this type of measurement,we have studied by LIOAS the photoisomerization of two symmetric carbocyanines in solution: 3,3'-diethyloxadicarbocyanineiodide (DODCI) and 3,3'-diethyloxacarbocyanine iodide (DOCI) (see Scheme 1). To whom correspondence should be addressed.
t Max-Planck-Institutfur Strahlenchemie. t
Max-Planck-Institutfur Kohlenforschung. Abstract published in Advance ACS Absrracts, February 1, 1994.
Et
Et
i
(33' dirthybxa&mbccyanine bdide)
I
\
Et
Et -
I
Doc1 (3,3' .diethyioxacarkyanine iodide)
Cyanine dyes have been extensively studied in relation to their practical application as laser materials and saturablemode-locking absorbers,13-15as well as spectral sensitizers in photography.1618 Moreover, their structural similarity with many biological pigments makes them useful as models of more complex photobiological Upon irradiation, cyanines undergo a reversible E-2 isomerization in their polymethinic chain.22 For some unsubstituted carbocyanines,the isomerization has been shown to occur via the excited singlet state. Fluorescence competes with isomerization, whereas the quantum yield of intersystem crossing is extremely 10w.23~24
The photoisomerization of DODCI and DOCI is a thoroughly investigated p r o c e s ~ . ~ In ~ 3 spite ~ of this, the experimental
0022-3654/94/2098-1776$04.50/00 1994 American Chemical Society
Determination of Molecular Volume Changes
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1777
0.1
Figure 1. Term scheme for the excitation and deactivation of a cyanine normal species N producing a photoisomer P. The excited state lN* passes through the 9O0-twistedintennediateformZtbeforeeitherreturning to N or isomerizingto P. Solid arrows indicate radiative absorption and emission processes. Broken arrows show nonradiativetransitions. A E N ~ is the energy differencebetween the photoisomer ground state, P and the ground state of the normal form N.
evidence on the conformationalnature of the isomers is scarce.33-35 It has been proposed that cyanineswith unsubstituted polymethine chains in solution are in a fully extended conformation with the nitrogen atoms in 3yn.28.3638 The infrared and spontaneous Raman spectra for DODCI and analogues suggest the possibility of a considerable difference of the structures in the crystalline form and that in solution, as well as a weakened bond alternation in the central conjugated chain that is more important for DODCI than for DOCI molecules.39 Upon excitation of the normal form N, the photoisomer P is formed, in which one of the polymethinic bonds has changed its conformation. A kinetic model put forward by RulliCre for the photoisomerization is the most appropriate in order to explain the photophysical and photochemical data reported to date.m According to this model, the twist angle around an ethylenic bond is the reaction coordinate. From a twisted intermediate, It, the molecules relax to the ground state of either N or P (Figure 1). The deactivation of IN* takes only a few nan0seconds,2~J~Jl whereas the photoisomer P is a relatively long-lived species (lifetimesintherangeofmillisecondsaJ7J9).Thus, P isconsidered the final state in our measurements since the heat integration time in the LIOAS experiments5 is shorter than the lifetime of P in all cases studied. The absorption characteristics of P and N are such to permit preferential excitation of N. Excitation of both isomers within one pulse frequently occurs in many systems, in particular with biological photoreceptors, making difficult the evaluation of the individual kinetic parameters.m.41 Semiempirical calculations were achieved in order to interpret the volume changes measured. Possible structural changes involved in the isomerizations were calculated and correlated to the experimental results allowing as well the identification of P's conformation. Materials and Methods Samples. DODCI (Radiant Dyes Chemie) and DOCI (Lambda Physik) were used as received, in mixtures of distilled ethanol (96%) with deionized and Millipore-fiiltered water. Solutions were all freshly prepared. Cyanine concentration kept under 3 X 1od M minimized possible dimerization effects. Absorbances (10.3) were matched within f0.005 units to those of the CoC12 (Merck) solutionsused as a calorimetric referen~e.~ Bromocresol green (Sigma) and Evans blue (Aldrich) were used as provided. The samples were thermostatted to f0.1 OC. A PTIOO thermoelement placed directly into the sample registered the temperatures. Absorption Spectra. Absorption spectra were recorded on a Perkin-Elmer Model 356 spectrophotometer. Fluorescence. Corrected emission spectra were obtained with a Spex Fluorolog spectrofluorometer (5 nm bandwidth). Quantum yields were determined using rhodamine 101 in ethanol (@f = 1.042) and cresyl violet in methanol (@f * 0.5443) as references for DOCI and DODCI, respectively. Reference and sample had matched absorbances within 0.002 absorbance units, with a value
L
-0.1 1
_/'
I
410
450
430
470
490
510
530
550
I
Wwclcngoh [m]
-
Figure 2. Ground state N (- -) absorption spectrum of DOCI in ethanol ( 10-6 M),and (-) smoothed transient differenceabsorption spectrum of N and P, 5 ps after excitation of the solution with a 15-ns and 460-nm N
pulse. Thebleachingobserved in the differencespectrumclearlyrescmblej the absorption spectrum of the ground-state N mainly in the blue flank. The absorption of P becomes important from 480 nm to the red. around 0.05 at the excitation wavelength (slit = 1.2 nm). Fluorescence emission spectra were integrated within the 735575-nm range for DODCI and within 714472 nm for DOCI. The correction for the difference in the refractive index of the solvent was included in all cases.44 Optical Transient Detection. ( a ) With Optical Multichannel Analyzer (OMA).The transient absorption spectrum of DOCI was measured with an optical multichannel analyzer (1460-OMA I11 system, PARC). The detector gate of 100 ns was opened 5 ps after the 15-nsand 308-nm laser pulse produced by a FL2000 Lambda Physik-EMG 101 MSC excimer laser (XeCl), pumping a coumarin 47 dye laser (A = 460 nm). The signal was an average of 20 pulses (Figure 2). (b) With Photomultiplier. The laser flash photolysis system has been previously d e ~ c r i b e d . ~ 5The . ~ 154s and 460-nm laser pulse, described above, with 4-mm beam diameter was directed into a 1-cm path length quartz fluorescence cuvette. The analyzing beam was generated by a 150-W Xe lamp and passed through the cuvette at right angle with the laser beam. The signal was fed into a transient recorder (Biomation 4500, Gould) andsuccessiveshotswereaveragedbya VAX-LSI11/73 computer system. The product of photoisomer quantum yield times its absorption coefficient pep) for DOCI in ethanol (&a = 510 nm) was obtained by the comparative method.47 The triplet production of mercurochrome in aqueous solution (@T = 0.23; A e r . 5 ~= 6.57 X lo3M-l ~ m - l ) and ~ * eosin in ethanol (@T = 0.43; A E T , J=~1.02 ~ X lo4 M-l ~ m - l )used ~ ~ ,as references, were observed at 590 and 580 nm, respectively. Purified mercurochrome and eosine were kindly provied by Professor K. Gollnick (Munich). Both reference solutions, with absorbances matched to within f0.005 with the DOCI solution of 0.20 at 460 nm, were degassed by three freezepump-thaw cycles. The measurements were performed at room temperature. ep, the molar absorption coefficient for the photoisomer, was determined from the difference absorption spectra (Figure 2) by using the ground-state depletion method.50 LIOAS System. The LIOAS setup has been described p r e v i o ~ s l y . ~The 5 ~ 562-nm excitation pulse was provided by an 8-11s Nd:YAG/dye (rhodamine 6G) laser system described elsewherem with 5-Hz repetition rate. For A, = 460 nm in the DOCI measurements, the 15-ns excimer laser described above was used at a 3.3-Hz repetition rate. The energy fluence of the laser pulses was varied by a variable neutral density filter, and the energy values were determined with a pyroelectric energy meter (Laser Precision Corp. RJ7620 and RJP-735), the output of which was registered by the computer system. The laser beam diameter was set with a pinhole of 1 mm diameter, so that the effective acoustic transit time (7: = d/ua;d = diameter of beam;
1778 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
-" I
/
P
Churio et al. Thus,
I
kPV,2
H = -(AVth VO
0.2
0.1
0
E, WI Figure 3. Signal amplitude H, as a function of the absorbed energy E, for (full symbols) DODCI and for (empty symbols) CoCl2, in (squares) water, (circles) 30% ethanol, and (triangles) 100%ethanol. T = 25 OC, Lc= 562 nm, A562 = 0.14. The slope of each plot is the corresponding energy normalized signal, Hn.Inset: Optoacousticsignal of DODCI in = 562 nm, A562 = 0.14. ethanol at 25 OC,bxc
u, = sound velocity)$ ranged between 680 and 990 ns (cases of
neat water and neat ethanol, respectively). The sound wave was detected with a 4-mm PZT ceramic transducer (Vernitron) attached to the wall of the 1-cm path length quartz cuvette.$' The signals were amplified 10 times (Comlinear E103). The average of 100-200 signals was stored in a transient recorder (Tektronix 7912) and transfered to a VAX station 3100 coupled to a VAX mainframe. LIOAS Signal Analysis. The signals were analyzed as previously described.52 The difference between first maximum and minimum was taken as the signal amplitude, H (Figure 3, inset). As already mentioned, the LIOAS signal after the absorption of the laser pulse is due to two p r o c e s s e ~ . ~ .One ~ - ~ is . ~derived from the thermally induced volume change in the solution, AVh, and is described in terms of the thermoelastic parameters of the sample (/3/cpp), as in eq 1,where a is the fraction of absorbed energy,
E,, released as heat within the effective acoustic transit time 7:. A second contribution to the volume change may arise from photoinduced molecular changes. Such changes, described as AV,, include any conformational variation due to photochemical processes and/or rearrangement of the solvent, and are expressed in terms of the molar reaction volume change AVR (eq 2),where
@R is the reaction quantum yield and n is the number of absorbed einsteins. In order to correlate the volume change with the induced pressure, one resorts to the compressibilityK = -(dV/dP)T(l/ V). Thus, since AV
