J. phys. Chem. 1982, 86, 1788-1794
1788
TABLE I: Calculated Parametersa
35 30 25 20 15 10 a
99 100 100 100 100 100
9 330 9 590 9 900 10 400 10 600 10 900
17 680 17 890 18 240 18 960 19 040 19 330
d s E / d t = 0 ; d6 /dt - 12.10 - 0.02t; d6c/dt = 14.46- 0.01t. - 6 c&d)2j '"/;4 (24 data points).
7.5 7.5 7.5 7.5 7.5 7.6
11.39 11.52 11.61 11.68 11.80 11-90
Units: (mole fraction)-'.
14.11 14.16 14.21 14.26 14.31 14.36
1.0 1.3 1.8 2.5 3.1 4.0
Standard Deviations:
{8(6o b d
and to obtain the unique set of ICs they introduced the third assumption. The assumption that the chemical shift of a proton involved in hydrogen bonding is independent of temperature is quite difficult to accept, and it contradicts the theoretical work of Muller and Reiter.39 By inspection of the results of their calculations, it seems that they ignored the boundary condition, Kdrm< 1, in the procedure of curve fitting. Without this condition I also found that many sets of parameters resulted in the best-fit curve. The calculated values of chemical shifts of cyclic dimer and monomer are lower than those previously reported. This may suggest that the values of earlier works are shifted to high field by the influence of moisture. The ratio of the squared fraction of E proton to the fraction of hydrogen-bonded proton, [ 2 X , + Cn=2m(n (39)Muller, N.;Reiter, R. C. J. Chem. Phys. 1965,42,3265.
l)X,,]/(X,,, + Cn=2mXn)2, may be the quantity equivalent to the equilibrium constant of earlier works in which only monomer-dier equilibriumhas been assumed. This ratio agrem with the results of infrared experiments, at 25 OC.18 The values of AH, derived from the plot of In K vs. 1/RT, are -32.3 kJ/mol for AHl and -33.6 kJ/mol for AHc The values of AHcand AHl are in the range of the variety of literature values. Kpdecreased with increasing temperature, but its magnitude was negligibly small. Thii behavior of Kp may be a perplexity which is implied in the second assumption. The second assumption is generally used to analyze the data of acoustic absorption or dielectric relaxation studies on polymerization of alcohols and results in similar problems. The model used in this investigation has some limitations; however, it is a quite useful model for deduction of important information from experimental data obtained from these complicated systems.
Viscosity-Dependent Isomerization Yields of Some Cyanine Dyes. A Picosecond Laser Spectroscopy Study Vllly Sundstrh and Tomas Glllbro' Divlslon of physical Chmlstry, Unlverstty of UmeB. SO01 87 UmeB, Sweden (Received: Aprll24, 1981; In Final Form: November 9, 198 1)
Short-lived photoisomers with red-shifted absorption spectra are formed in 1,lr-diethyl-4,4'-cyanine, pinacyanol, and DQOCI after picosecond laser pulse excitation. The yield of photoisomer is shown to be strongly viscosity dependent. This is interpreted as the existence of two different excited-state deactivation channels having differing viscosity dependences. No significant activation energy for the relaxation process is found.
Introduction The possible existence of geometrical isomers in cyanine dyes has long been a lively subject of discussion.'+ The first experimental observation of this possibility was the chromatographic separation on a column of the dye 3,3'diethylthiatricarbocyaninechloride into three components The spectra of these three considered as stere~isomers,.~ components became, however, identical when placed individually in separate solutions at room temperature. This was attributed to the rapid equilibration of isomer com~
(1)G. Scheibe, Angew. Chem., 52,631 (1939). (2)L. G.5. Brooker, F. L. White, R. H. Sprague, 5. G. Dent, Jr., and G.Van Zandt, Chem. Rev., 41,325 (1947). (3)L. G. S. Brooker, F. L. White, D.W. Heseltine, G.H. Keyes, S.G. Dent, Jr., and E. Van Lare, J. Photogr. Sci., 2, 173 (1953). (4) H. Kuhn, Helu. Chim. Acta, 34, 1308 (1961). (5)L. Zechmeister and J. H. Pinckard, Experientia, 9, 16 (1953). 0022-3654/82/2086-1788$01.25/0
position at this temperature. Later, several chain-substituted dyw were shown to exist as an equilibrium mixture of different stereoisomers in s o l ~ t i o n . ~ ~ ~ Photochemical production of isomers, stabilized at low temperature, followed by thermal reconversion, was also Flash photolysis experiments have also indicated the formation of unstable isomers.'JoJ1 The use of cyanine dyes as saturable absorbers in mode-locked picosecond lasers has triggered many picosecond spec(6)W.West, s.Pearce, and F. Grum, J . Phys. Chem., 71,1316(1967). (7)J. T.Knudtson and E. M. Eyring, J . Phys. Chem., 78,2355(1974). (8) F. Baumgiirtner, I. Gijnter, and G. Scheibe, 2.Elektrochem., 60, 570 (1956). (9)G. Scheibe, J. Heiss, and K. Feldman, Ber. Bunsenges. Phys. Chem., 70,62 (1965). (10)Von F. Dbr, J. Katachy, and J. Kauser, Ber. Bunsenges. Phys. Chem., 69,11 (1965). (11)P. J. McCartin, J. Chem. Phys., 42,2980 (1965).
0 1982 American Chemical Society
The JOWM~of phvslcel Chemlsity, Vol. 86, No. 10, 1982 1789
Isomerlzatlon Ylelds of Some Cyanlne Dyes
C
0
30
90
60
120
05
US
D
0
30
60
90
120
US
0
M
60
90
120
VS
Flguro 1. Recorded GSR klnetlcs at low vlscostty and room temperature: (A, 6 ) plnacyanol in methanol; (C, D) 1,1'-4,4'-cyanine In ethanol.
troscopy studies of cyanine dyes (see, for example, ref 12-14). In this paper we wish to discuss the viscosity and temperature dependence of formation of photoisomers in the dyes, l,l'-diethyl-4,4'-cyanine iodide, pinacyanol, and DQOCI after picosecond pulse excitation.
Experimental Section The kinetics of ground-state recovery (GSR) of absorption after picosecond pulse excitation is measured as a function of solvent viscosity and temperature. The picosecond pulse source is a synchronously mode-locked cavity dumped dye laser, previously described in detail elsewhere.1s*20In the pump and probe techniquels used in this work, the same wavelength is employed for the excitation and analyzing light. Thus,the pulse train emitted from the cavity dumped dye laser is split into two parts. Approximately 95% is chopped at a speed of lo00 Hz and used as excitation light. The modulation of the analyzing light imposed by the chopped excitation light is detected with a photomultiplier and a phase-sensitive amplifier. Time resolution is achieved by a motor-driven delay in the excitation beam. The signal from the phase-sensitive amplifier is directly recorded on a chart (12) M. A. Duguay and J. W. Haneen, Opt. Commun., 1, 254 (1969). (13) J. R. Taylor, M. C. Adams, and W. Sibbett, Appl. Phys., 21,13 (1980). (14) C. J. Tredwell and C. M. Keary, Chem. Phye., 43, 307 (1979). (15) V. SundstrBm and T. Gdbro, Appl. Phys., 24,233 (1981). (16) E. P. Ippen and C. V. Shank, Top. Appl. Phys., 18, 83 (1977). (17) R C. Weast, Ed., 'Handbook of Chemistry and Physics", 56th ed., CRC Press, F k a Raton, n, 1975. (18) T. Tao, Biopolymers, 8,609 (1969). (19) H. E. Lmdag and A. von Jena, Chem. Phye. Lett., 42,213 (1976). (20) V. SundsWm and T. Gillbro, Chem. Phys., 61,257 (1981).
recorder. An average laser power of 0.2-0.5 mW, at a pulse repetition rate of 82 kHz, focused to a spot of ca. 200-pm diameter in a 0.2- or 0.5" sample cell, is used. This results in an excitation degree of ca. 1%. At this low degree of excitation, the recorded signal is directly proportional to the absorbance changes (AA) in the sample caused by the excitation pulses. The recorded signals shown in this paper (Figures 1 and 2) have an arbitrary AA scale because no methods of normalization have been employed. In experiments where the sample is photochemically bleached, a flow cell system based on a 0.2-mm cell and a peristaltic pump is used. The flow rate commonly employed results in an exchange of the sample contained in the excitation volume once every 30 pulses. In experiments performed below room temperature an Oxford instrumenta DN704 cryostat maintained constant temperature to within f 1 OC. All dyes (Koch-Light Ltd) were used without further purification, and the solvents were of analytical or spectroscopic grade. Solvent viscosities below room temperature were obtained from the literature" and viscosities of the glycerol/methanol mixtures at room temperature (296 K) were measured by us. Results All three investigated dyes, l,l'-diethyl-4,4'-cyanine iodide, pinacyanol, and DQOCI, behave kinetically quite similarly. Two recovery rates with widely separated lifetimes are detected in low-viscosity solvents such as methanol and ethanol at room temperature. Examples of the recorded kinetics are shown in Figure 1. The initial fast recovery, with an inverse rate constant of the order of 10 ps, is followed by a much slower recovery on the
I
,
I
1
!
I
0
30
90
60
120
1 150
0
30
60
90
120
PI
PI
Figuro 2. Recorded QSR kinetics at high Viscosity and low temperature: (A) plnacyanol In methanol; (B) l,lf-4,4'-cyanine in ethanol; (C) 1, lr-4,4'cyanine in 25% (v/v) glyceroi/methanoi; (D) 1,1'-4,4'-cyanine in 60.2% (v/v) glycerol/methanol.
nanosecond time scale. The initial faster (a100 ps) decay of this long component, seen in Figure 1, is due to rotational relaxation. This is checked by repeating the experiment with the polarization of the analyzing light rotated to the "magic angle", 54.7O, relative to the polarization of the excitation light.16Jg The complete recovery of the transient bleaching before the arrival of the next excitation pulse (no constant background is seen in the recorded traces) indicates a lifetime on the microsecond to nanosecond time scale for this slower component. When a much longer optical delay is used, capable of covering up to ca. 3-115 delay times with maintained perfect beam overlap in the sample, the recovery time of the slow component is measured to be 4 ns in the case of 1,l'-4,4'cyanine in methanol at 21 O C . The sharp spike in all dynamic spectra at t = 0 is due to a coherent coupling between the pumping and probing beams16 that is always present when deriving both pumping and probing light from the same pulse. From Figure 1 the value of the ratio between the recorded intensities of slow and fast components, respecis seen to be largest tively, extrapolated to zero time (12/11) for 1,1'-4,4'-cyanine, while pinacyanol and DQOCI both have considerably lower values. When I z / I l is measured at the blue part of the absorption spectrum of the respective dyes, as shown in Figure 1, A and C, for pinacyano1 and l,l'-4,4'-cyanine, the ratio 12/11is found to be independent of wavelength (results not shown). This indicates that there is no photoisomer absorption at these wavelengths. Consequently the measured values of 12/11 can directly be used as a relative measure of the concentration of photoisomer produced by the excitation pulse.
TABLE I: Amax of Investigated Dyes and Isosbestic Wavelengths of the Corresponding Photoisomers
dye
solvent
nm
isosbestic wavelength, nm
l,l'-4,4'-cyanine pinacyanol
EtOH MeOH MeOH
592 605 593
605 616 607
Amax,
DQOCI
The magnitude and the sign of the slowly decaying component change with wavelength. All three dyes behave similarly in that the long-lived component is seen as a bleaching at short wavelengths and as an increased absorption at long wavelengths. The approximate isosbestic points are given in Table I. Examples of the recorded kinetics showing the slow component in absorption are also given in Figure 1. The experiments presented in this paper are all conducted by using the pump and probe technique.'* Since both pumping and probing wavelengths are the same the measured wavelength dependence of the relative magnitude of the slow kinetic component can be due to the change in either analyzing or excitation wavelength. In the Discussion we come to the conclusion that pinacyanol exist in only one ground-state species (see below). Thus, it seems plausible that the I z / I l wavelength dependence is due to the changing wavelength of the analyzing light. The intensity of the slow recovery decreases rapidly with decreasing temperature and increasing viscosity. This is illustrated for pinacyanol and cyanine in Figure 2. The spectra of Figure 2 should be compared with the corre-
The Journal of Physical Chemlstty, Vol. 86, No. 10, 1982 1791
Isomerization Yields of Some Cyanine Dyes
5.0
6
c
2o
B
10
6
I
:*-Et’
e z”
;: L 9
05
L
\
30
LO
35
DQOCI Et/
~
8’
a l l - trans
5’
4
-
11’- Diethyl L,L’- cyanine
I
50
L5
tooo/r
Pinacyanol a l l - trans Et
O2
.
Et
Pinacyanol
2,g-mono-
CIS
Et 3
“+e
Pinacyanol
9,lO - mc;no - L I S ~
05
10
20
50
100
200
VISCOSITV/cP
Flgure 3. (A) Relative concentratlon of longlhred component as a functkn of temperature. Upper curve is 1,lf-4,4‘-cyanlneIn ethanol: (0)586 (0) 588 nm. Lower curve is pinacyanol in methanol: (A)595 and (0)619 nm. The plnacyanol curve has been scaled by multlpllc a m wlth a factor 10. The unlts of concentration are arbltrary unlts. (B) Relathre concentrationof long-ihred component as function of vi& cosity, no matter the method of viscosity variation. Upper curve is 588 nm, EKm; ( 0 )597 nm, l,lf-4,4‘cyanine: (0)586 nm, EtoH; (0) glycerol/MeOH; and (B) 585 nm, glycerol MeOH. Lower curve Is plnacyanoi: (A)595 nm, MeOH; and (0)619 nm, MeOH. The pinacyanol w e has been scaled by multipllcatlon with a factor 10. The unlts of concentration are arbitrary unlts.
sponding spectra of Figure 1 at room temperature. The relative concentration of long-lived component of pinacyanol and l,l’-4,4’-cyanine extrapolated to time zero is plotted as a function of temperature in Figure 3A. The decrease in the relative amount of slow component with decreasing temperature is seen to be faster for pinacyanol than for cyanine. The decrease in temperature also results in an increased viscosity. To separate temperature and viscosity effects, the viscosity is varied by varying the composition of a glycerol/methanol mixture at constant temperature. Similar results are obtained with both methods (see Figure 2, C and D). In Figure 3B the relative concentration of long-lived component is plotted vs. viscosity, irrespective of method for viscosity variation. All data points are seen to obey the same viscosity dependence within experimental error. This suggests that there is no significant activation energy barrier to be surmounted along the relaxation pathway. The recovery dynamics of the initial fast component and factors influencing it has not been treated in this paper but is the subject of further investigations.20
Discussion Cyanine dyes may exist in a variety of stereochemical forms that can be reached by rotation about bonds in the
Flgura 4. Molecular structure of the investigated dyes. Possible isomeric forms of a carbocyanlne dye are illustrated In the case of plnacyanol.
polymethine chain. As an example, let us consider a carbocyanine dye such as pinacyanol. The molecule may exist in all-trans, 2,9-monocis, and 9,lO-monocisforms (see Figure 4). Several X-ray crystal structure determinations of cyanine dyes have been reported in the literature. The dyes with the shortest polymethine chain, 1,l’-diethyl-2,2’cyanine and l,l’-diethyL4,4’-cyanine,for example, are shown to deviate from planarity having the quinoline rings twisted 4 4 O and 40°, respectively, relative to each othera21P The N atoms are cis relative to the chain. The twisted conformation is a result of the steric hindrance between the H(3) and the H(3’) in l,l’-4,4’-cyanineand in addition between H(9) and the ethyl H atoms in l,l’-2,2’-~yanine,~~~~~ For the atom numbering, see Figure 4. The cis form is unlikely to exist for these two dyes because of the severe overcrowding present in this conformation.2l When space filling molecular models are used, it is possible to get a relatively good estimate of these steric interactions. Chain-unsubstituted dyes with longer chains, such as carbocyanines, are shown to adopt an all-trans configuration with the N atoms cis relative to the chain. The quinoline ring planes are found to be roughly paralle1.2*26 (21) H. Yoehioka and K. Nakatsu, Chem. Phys. Lett., 11,255 (1971). (22) K. Nakatau, H. Yoshioka, and H. Marishita, Acta Crystallogr., Sect. B, 33,2181 (1977). (23) T. Kaneda, S. Yoan, and J. Tanaka, Acta Crystallogr., Sect. B , 33, 2065 (1977). (24) J . A. Potenza, L. Zyantz, and W. Borowski, Acta Crystallogr., Sect. B, 34, 193 (1978). (25) K. Nakao, K. Yakeno, H. Yoehioka, and K. Nakatau, Acta Crystallogr., Sect. B, 35, 415 (1979). (26) P . J . Wheatley, J. Chem. Soc. 3245, 4096 (1959).
1702
The Journal of phvslcel Chemistry, Vol. 86, No. 10, 1982
The equilibrium between the different possible isomers in solution has been discussed by several author^.^^^" Analogies to spectral properties of polyenes and carotenes have been used to assign observed spectral bands to alltrans and cis isomers. Thus,the all-trans isomer is believed to correspond to the visible absorption band of longest wavelength having the greatest transition moment, while the cis isomers are weaker bands blue-shifted relative to the all-trans band. The differences in A, of the absorption bands attributed to the different isomers, as well as the differences in extinction coefficients, are however small enough that differences in planarity between the isomers and bending of the polymethine chain might obscure the spectral differences. Solvent interaction in solution might cause such distortions and cause an isomer to absorb in an unexpected wavelength region. The half-width of the main absorption band at low temperatures is a useful indication of the structural homogeneity of cyanine dyes.40 The half-width of this band in pinacyano1 in alcoholic solution suggests the presence of a single isomer.40 These considerations lead us to the conclusion that pinacyanol exists in only one ground-state configuration prior to excitation. Consequently only one ground-state species is excited and the observed kinetics is due to this species alone. Several transient absorption'J0J1*2g3B% and time-resolved f l u ~ r e s c e n c e ~ measurements ~ ~ ~ ~ ~ " - ~ ~have been performed that clearly demonstrate the formation of unstable photoisomers in cyanines which reconvert to the normal isomer in the dark. Photoisomers with transient spectra bathoor hypsochromically shiftchromically ed''~'~ relative to the normal form have been shown to exist. The rate of formation of the photoisomer is in most cases faster than the time resolution of the technique employed. For DODCI a formation rate constant k, = 6.45 X 10'' s-l was calculated from measured fluorescence yields.% The decay of the photoisomer ground state back to the normal isomer, on the other hand, varies from milliseconddoJ1W to fractions of a microsecond.% In the assignment of observed spectral features to certain isomers, the considerations briefly outlined above have been used. The shape of potential energy surfaces as a function of twist angle about a C-C double bond has received attention in explaining the photoisomerization behavior of substituted ethylenes such as On the basis (27)D. F. O'Brien, T. M. Kelly, and L. F. Costa,Photogr. Sci. Eng., 18, 76 (1974). (28)D.N.Dempster, T. Morrow, R. Rankin, and G. F. Thompson, J . Chem. SOC.,Faraday Trans. 2,68,1479(1972). (29)E. G. Arthurs, D.J. Bradley, and A. R. Roddie, Chem. Phys. Lett., 22,230 (1973). (30)E. G. Arthurs, D. J. Bradley, and A. R. Roddie, Opt. Commun., 8,118 (1973). (31)D.M a d e and M. W. Windsor, Chem. Phys. Lett., 27,31 (1974). (32)C.V. Shank and E. P. Ippen, Appl. Phys. Lett., 26, 62 (1975). (33) J. Jaraudias, J. Photochem., 13, 35 (1980). (34)J.-P. Fouassier, D.-I. Lougnot, and 3.Faure, Chem. Phys. Lett., 35, 189 (1975). (35)J. P. Fouassier, D.-I. Lougnot, and J. Faure, Opt. Commun., 23, 393 (1977). (36)V. A. Kuzmin and A. P. Darmanyan, Chem. Phys. Lett., 54,159 (1978). (37)C Rullisre, Chem. Phys. Lett., 43, 303 (1976). (38)G. R.Fleming, A. E. W. Knight, J. M. Morris, R. J. Robbins, and G. W. Robinson, Chem. Phys. Lett., 49,l (1977). (39)J. C. Mialocq, A. W. Boyd, J. Jaraudias, and J. Sutton, Chem. Phys. Lett., 37,236 (1976). (40)W.West, P. Lovell, and and W. Copper, Photogr. Sci. Eng., 14, 52 (1970). (41)G. Orlandi and W. Siebrand, Chem. Phys. Lett., 30,352(1975). (42)J. Saltiel, J. DAgoatino, E. D. Megarity, L. Metta, K. R. Neuberger, M. Wrighton, and 0. C. Wiriou, Org. Photochem., 3,1 (1973). (43)P.Tavan and K. Schulten, Chem. Phys. Lett., 56, 200 (1978).
Sundstrom and Gillbro
B
W W
n MHEDRPL ANGLE
Figure 5. Potentlei surface diagram used to discuss the photolsomerlzation of cyanlne dyes.
of these results, Rulli5refl has proposed a model to explain the photochemical behavior of DODCI. Similar schemes have been used for other polymethine dye^.^"*^^ In the models of ref 41-43,the potential energy surfaces assume the shape of a double-minimum potential. For a theoretical description of the rate constant for passage over the barrier in such a double-minimum potential, a FokkerPlank equation and approximations thereof have recently been used.u4 To explain the photochemical behavior of the dyes studied in this paper, it is possible to use a similar set of potential surfaces (see Figure 5). The recorded kinetics summarized in Figures 1 and 2 can be rationalized on the basis of the scheme in Figure 5. The fast component of the decay is the sum of the deactivation rates of Sl(kOM= kiC + k,,) assuming k,' and k,," >> k,, or alternatively the absorption from the twisted ground state could be very similar to the normal groundstate absorption, where hi, and k, represent the rate coefficients for direct internal conversion back to So and conformational change to a twisted, excited state, respectively. Of the molecules reaching the twisted excited state t-S,, a certain ratio will complete the conformation change and end up in the photoisomer ground state, p-S,. The rest will go back to the normal-form ground state, n-S* The ratio between the number of molecules that isomerize and return to n-So from t-Szis given by k
A quantitative description of spherical diffusion and capillary shielding effects in reverse pulse voltammetry is presented. Experimental results obtained at a stationary spherical mercury electrode for both solution-soluble and mercury-soluble products correlate well with theoretical predictions. A novel method is presented for the rapid and accurate evaluation of diffusion coefficients of mercury-solubleproducts and of unstable solution-soluble products.
Reverse pulse (RP) voltammetry is a modification of normal pulse (NP) voltammetry in which the initial potential is chosen at a point where the electrochemical reaction of interest takes place, and the products of the reaction are characterized by the Faradaic response to potential pulses in the reverse direction.l2 This technique, originally termed scan reversal pulse polarography by Oldham and Parry,' has features in common with the Kalousek commutator method3 and with cyclic voltammetry: in that they all employ in situ generation of in+ Anderson
Physics Laboratories, Urbana, IL 61801. 0022-3654/82/2086-1794$01.25/0
termediates and products of electrode reactions. RP voltammetry is superior to cyclic voltammetry, however, in its range and control of timing parameters and reaction conditions, and in its discrimination against charging current. RP voltammetry has been applied to the unambiguous characterizationof electrochemical reversibility,'2s5 ~~
(1) K. B. Oldham and E. P. Parry,Anal. Chem., 42, 229 (1970). (2)Janet Osteryoung and E. Kirowa-Eisner, Anal. Chem., 52, 62 (1980). (3) M. Kalousek, Collect. Czech. Chem. Commun., 13, 105 (1948). (4)R.S. Nicholson and I. Shain, Anal. Chem., 36, 706 (1964).
0 1982 American Chemical Society
