7996
J . Phys. Chem. 1992,96, 7996-8001
(25) Littman, M.; Metcalf, H. Appl. Opt. 1978, 17, 2224. (26) Krogh-Jmpersen, K.; Westbrook, J. D.; Potenza, J. A,; Schugar, H. J. J. Am. Chem. Soc. 1987,109,7025. Westbrook, J. D.; Krogh-Jespersen, K.ESPPAC, an electronic structure program for the calculation of excited stare properties; Rutgers University: New Brunswick, NJ, 1989. (27) (a) Ridley, J.; Zerner, M. C. Theor. Chim. Acra 1973.32, 1 1 1. (b) Edwards, J. D.;Zerner. M. C. Theor. Chim. Acra 1987, 72, 347. (28) Linderberg, J.; Ohm, Y. Propagators in Quantum Chemistry; London: Academic Press, New York, 1973. (29) Krogb-Jespcrscn, K.; Ratner, M. A. Theor. Chim. Acta 1978,47,283. (30) MMX(89) may be. obtained from Serena Software, c/o K. Gilbert, P.O.Box 3076, Blwmington, IN 47402. (31) The twisted nature of a 1,6,7,12-tetrachlorinatedperylene is demonstrated by the X-ray structure of the NJV-di-n-butyl derivative: Iden, R.; Seybold, G.; Stange, A.; Eilingsfeld, H. Forschungsberichr T 84-164; Bundesministerium fuer Forschung und Technologic, 1984; see also ref 23. (32) Cosmo, R.; Hambley, T. W.; Sternhall, S.Tetrahedron Leu. 1987, 28, 6329. (33) It is not possible to obtain direct information about torsional energies in SIby molecular mechanics modeling in MMX, but presumably the torsional
potentials for the pendant groups are very similar in So and Si. (34) The fluoreacein dianion (a"= 91%)and the decarboxylatedhomolq 9-phenylfluorananion (0" = 20%) stand in a similar relationshipwith regard to internal conversion promoted by the released torsion of a conjugated aromatic group: Lindquist, L.; Lundeen, G. W. J . Chem. Phys. 1966,44,1711. (35) IUPAC Solubility Dara Series: Battino, R., Ed.;Pergamon Press: New York, 1981; Vol. VII, 312. (36) Hadley, G. S.;Keller, R. A, J . Phys. Chem. 1969, 73, 4351. (37) Engel, P. S.; Monroe.,B. M. Complications in Photosensitized Reactions. In Advances in Photochemistry; Pitts, J. N., Hammond, G., Noyes, W. A., Eds.; 1971; Vol. VIII, p 245. (38) Lobmannsroben, H. G.; Langhals, H. Appl. Phys. 1989, B-48,449. (39) Ito, M.; Ebata, T.; Mikami, N. Annu. Rev. Phys. Chem. 1988, 39, 123. (40) Pavlopoulos, T. G.; Hammond, P. R. J . Am. Chem. Soc. 1974, 96, 6568. (41) Bird, G. R. Unconventional Approaches to Efficient Loser Dyes; Paper N-6at Laser 90, San Diego, CA, 1990. (42) Colour Index, 3rd ed.; Society of Dyers and Colourists: Bradford, Yorkshire, England, 1971; Vol. IV, Chemical Constitutions, see p 4590.
Effect of Sdvent on Nonradiatlve Processes in Xanthene Dyes: Pyronln B In Alcohols and Alcohol-Water Mlxtures Yavuz Onganer and Edward L. Quitens* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409 (Received: August 13, 1991; In Final Form: May 18, 1992)
The fluorescence lifetime rf and quantum yield & of pyronin B in water, n-alcohols, and alcohol-water mixtures were measured as a function of temperature. The nonradiative rate constant k, was calculated from rf and &. At 25 O C , k, was q u a l to (4.1 f 0.2) X lo8 s-l in water and ranged from (1.3 f 0.3) X lo8 to (3.9 f 0.4) X lo8 s-l in alcohols and from (3.6 f 0.7) x lo8 to (6.0 f 0.5) X lo8 s-I in mixtures. The natural logarithm of k, increased linearly with the solvent polarity parameter ET(30) but with a steeper slope for alcohols than for mixtures. The nonradiative activation energy E,, which was obtained from Arrhenius plots of k,, was q u a l to 4.0 f 0.8 kcal mol-' in water, 5.1 f 0.3 kcal mol-' in alcohol-water mixtures, and 6.0 f 0.3 kcal mol-I in alcohols. The dependence of the nonradiative rate on solvent polarity can be explained by a model involving a planar-to-pyramidal change at the xantheneamine bond. On the basis of this model, the dependence on solvent polarity is due to specific solute-solvent interactions at the amino group.
Introduction Although the photophysics of rhodamine B and related xanthene dyes in solution has been well studied, the mechanism for &-So internal conversion, the main nonradiative process in these dyes,'V2 is still contr~versial.~ Internal conversion in these dyes is associated with the rigidity of the xantheneamine bond.'.2 This rigidity controls the temperature dependence of the dye fluorescence. For example, the fluorescence quantum yield is unity and independent of temperature when the diethylamino groups are rigidly fixed to the xanthene ring by methylene bridges, as in rhodamine 101, but decreases with temperature when the groups are not constrained, as in rhodamine B.e7 Several models have been proposed to explain the effect of solvent and molecular structure on internal conversion in xanthene dyes: the intramolecular rotation the twisted intramolecular charge-transfer (TICT) model,8-10 and the umbrellalike-motion (ULM) In the intramolecular rotation model, internal conversion is mainly governed by solvent viscosity, whereas in the TICT solvent, internal conversion is determined mainly by solvent polarity. In the ULM model, internal conversion is determined by specific solutesolvent interactions. In this paper, we describe a study of the nonradiative rate of pyrOnia B (PyB) in water, n-alcohols,and alcoholwater mixtures. PyB differs from rhodamine B in that the substituent group at the 9-carbon atom on the xanthene ring is an H atom instead of a carboxyphenyl (PhCOOR) group (Figure l).I**9l7 The goal of this study, in part, was to see which of these models beat explains internal conversion in xanthene dyes. The solvents were chosen 0022-3654/92/2096-7996$03.00/0
to compare with the previous work on the acid form of rhodamine B (FtBH+).18J9The fluorescence lifetime rfand the quantum yield t$f of PyB in the alcohols and alcohol-water mixtures were measured as a function of temperature. The nonradiative rate constant k,, for PyB was calculated from rf and t$fi Activation energies E, were obtained from Arrhenius plots of A two-state kinetic mechanism, which was previously developed in our laboratory to analyze the photophysics of RBH+ in a l c o h o l ~ , l ~ ~ ' ~ provides a mathematical framework to obtain quantitative relationships between k,,, E,, and ET(30). The molecular rationale for these relationships is provided by the ULM model, which attributes the solvent polarity dependence to specific soluttsolvent interactions at the diethylamino groups on the xanthene ring.
Experimental Details w o n i n B (Fluka, Standard grade) showed a single spot on a TLC plate and was used without further purification. The solvents (except water) were distilled and dried over molecular sieves prior to use. Distilled water that was passed through deionizing filters was used. The solvent viscosities were obtained from the literature or measured with a viscometer. Alcohol-water mixtures were prepared so that their viscosities were all equal to 1.4 cP. Values of the polarity parameter ET(30) for pure solvents were obtained from the literature.*O Values of E ~ ( 3 0 )for the alcoholwater mixtures were calculated as described previ~usly.'~ PyB was stored in the dark as a concentrated stock solution ( 2 5 mM) in methanol. Samples were prepared by evaporating 10-20 pL of the stock solution and then redissolving with 5 mL of solvent. The final 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 7997
Nonradiative Processes in Xanthene Dyes
TABLE I: Compahoi~of Solveat Roptrtiea rad Pbotopbysid Parameters of Pyropin B at 25 'C CP
Ed30): kcalmol-l
hb/
solvent
nm
nm
methanol ethanol 1-propanol 1-butanol 1-pentanol 1-hexanol 91.5% water-8.5% ethanol' 87% water-13% methanol' 32% water-68% methanol' 6% water-94% ethanol" water
0.55 1.14 1.95 2.73 3.75 5.10 1.40 1.40 1.40 1.40 0.89
55.13 52.00 50.35 49.53 49.12 48.60 63.10" 62.86d 58.67d 53.12d 63.20
552 553 554 554 554 554 554 554 554 553 553
574 577 577 579 580 579 579 579 579 579 576
q?
kr,'
&m: Tf,
1.52 f 0.07 1.77 0.08 1.97 k 0.07 2.12 f 0.07 2.24 f 0.08 2.39 f 0.01 1.34 f 0.02 1.37 f 0.03 1.54 f 0.03 1.66 f 0.01 1.58 0.02
knr?
9(
108 s-1
0.41 0.49 0.63 0.54 0.57 0.71 0.20 0.28 0.39 0.40 0.35
2.7 f 0.3 2.8 0.5 3.2 f 0.4 2.5 k 0.3 2.5 f 0.3 2.9 f 0.3 1.5 f 0.2 2.0 f 0.2 2.5 f 0.3 2.4 f 0.2 2.2 0.2
ns
*
kim'
108 s-1 3.9 f 0.4 2.8 f 0.5 1.9 k 0.4 2.0 f 0.5 2.0 f 0.4 1.3 f 0.3 6.0 f 0.5 5.3 f 0.7 4.0 f 0.7 3.6 f 0.7 4.1 0.2
10'2
s-1
9.8 7.0 4.8 5.0 5.0 3.3 3.3 2.9 2.2 2.0 0.3
'Volume percentages. bObtained from CRC Handbook or measured with a viscometer. cValues for pure solvents obtained from ref 20. dValues calculated as described in ref 18. 'Uncertainty = f 1 nm. 'Uncertainty = &lo%. #Calculated from eq 2a. *Calculated from eq 2b. 'Calculated from eq 8 with E, = 6.0 kcal mol-l for alcohols, E, = 5.1 kcal mol-' for mixtures, and E. = 4.0 kcal mol-' for water. El
kl
Q El'
El
El
"El
(b)
(.)
El
El
A?
El0'
21
i
.
0.
'+ -+.
( C )
Figure 1. Resonance structures of pyronin B.
PyB concentration was = l e 2 0 pM. Emission spectra were recorded on an SLM Aminco 4800C fluorometer. The samples were contained in a 1-cm cuvette that was maintained to f l "C with a temperature-controlled water circulator (Neslab RTE-4). Fluorescence quantum yields from corrected fluorescence spectra were calculated by using the equation2'
A A(Ds/Dr)(%2/q2)[(1 - lo-ODf)/(l - lo-OD~)I (1) where D is the integrated area under the corrected fluorescence spectrum, n is the refractive index of the solution, and OD is the optical density at the excitation wavelength (X, = 520 nm). The subscripts s and r refer to the unknown and reference solutions. The values of the OD were 0.104.15. The values of D were obtained by numerically integrating the areas under the corrected fluorescence spectra. Rhodamine 101 (RhlOl) was used as the reference. The fluoresctnccquantum yield of RhlOl (4, 1.00) is relatively independent of temperature in alcoholic solvents."*22 The fluorescence spectra of PyB and RhlOl were obtained in the same solvent and at the same temperature. Under thest conditions, the refractive index correction, %2/q2,was equal to unity. Fluorescence lifetimes were measured by phase fluorometry with the SLM Aminco 4800C. The excitation and emission monochromators were set, respectively, at 520 and 575 nm. A polarizer set to 54.7" ("magic angle") with respect to the polarization of the excitation light was placed between the emission monochromator and the sample to remove molecular reorientation effects. Rhodamine 101 in ethanol was used as the reference for the fluorescence lifetime measurements. Our measured value of rf for RhlOl in ethanol at 25 "C compared well with the literature value of 5.3 ns5 when we used other dyes with known fluorescence lifetimes as references. The values of Tf for RhlOl obtained by using rhodamine 6G in ethanol (rf = 3.85 ns),I5 RhB in acidic ethanol (Tf = 2.3 ns),1618923 and RhB in basic ethanol (Tf = 2.7 ns)16923 as references were 5.30, 5.26, and 5.31 ns, respectively. Rdt8 The photophysical parameters Tf and t$f and the absorption and emission maxima and & at 25 OC are listed in Table I. The radiative and nonradiative rate constants k, and k, listed in Table I were obtained from and #f by using the equations
ab
kr
&/Tf
(24
knr = (l/Td - kr (2b) Arrhenius plots of k, (Figure 2) were linear (correlation 1 0.98) over the temperature ranges of the measurements (15-60 OC). Limar of In kvs l/Twas used to extract the activation energy E, and preexponential factor A,, (Table 11). The cal-
t .hexanal
- - water
. 87%wateril3%melhanol 3.0
3.2
3.4
3.6
IOOO/T(K-~)
Figure 2. Representative Arrhenius plots of the nonradiative rate constant k, for pyronin B. Arrhenius parameters are listed in Table 11.
TABLE [I: Summary of N m d i a t i v e ArrMw Panmeters for pymaia BeC
solvent methanol ethanol 1-propanol 1-butanol 1-pentanol 1-hexanol 91.5% water-8.5% ethanol 87% water-13% methanol 32% water-68% methanol 6% water-94% ethanol water
A,, 10'2 s-I 4.57 f 0.4 7.16 f 0.3 5.31 f 0.2 5.86 f 0.2 2.64 0.4 5.31 f 0.3 2.51 f 0.2 2.38 A 0.2 9.67 f 0.5 2.16 f 0.4 0.30 f 0.1
*
E,,
kcal mol-l 5.6 f 0.8 6.0 f 1.0 6.1 f 1.0 6.1 f 1.1 5.7 f 1.1 6.3 f 1.9 5.0 f 0.7 4.8 0.8 5.4 f 1.2 5.2 f 1.4 4.0 0.8
*
'Correlation of fits 1 0.98. *The temperature range over which the Arrhenius parameters were determined was 15-60 O C . CCalculationof errors is described in the Appendix.
culation of the errors in the values of Ea is described in the Appendix.
Discussion Viscosity Effects. In the intramolecular rotation model,'*2 rotation about the xanthene-amine bond causes internal conversion. The rate of this rotation, and hence the rate of internal conversion, decreases as the solvent shear viscosity 7 and the size of the amino groups are i n c r e a ~ e d . ~The ~ . ~decrease ~ in k,, for PyB in alcohols can be correlated to an increase in 7 (Table I). The values of k, can be empirically fitted by C/vUwith C = 2.95 X 108 PI, a = 0.434, and a correlation coefficient of 0.90. This value of a lies within the range of values of 0.23-1.00 found previously for other molecules that undergo photoinduced conformational changes and show dynamic solvent effects.% Recent theoretical and experimental studies of processes involving photoinduced torsional motion about chemical bonds, such the p h e toisomerization of stilbene,27 have shown that the fractional viscosity dependence is a general signature that the solvent shear viscosity must be replaced by the frequency-dependent friction.28 The variation of k,, in the isoviswity mixtures with solvent
Onganer and Quitevis
7998 The Journal of Physical Chemistry, Vol. 96,No. 20, 1992
t - +I
1
18.51
50 52 54 ET(30) kcallmol
48
F '
'
'
54
56
56
60
62
50
55 60 E ~ ( 3 0 )kcallmol
65
Figure 4. Plot of the activation energy E, VI ET(30) for pyronin B in alcohols (box), alcohol-water mixtures (O), and water (u). Slope = 0.10 & 0.05, intercept = 1 1 i 3 kcal mol-', correlation = 0.87.
" ' " ' , ' 1
2 0 . 2 1 (b)
52
t
56
64
ET(30) kcallmol
Figure 3. Plot of In k,, vs E ~ ( 3 0 for ) pyronin B in (a) n-alcohols [ ( l ) CH@H, CZHsOH, (3) C,H@H, (4) C J W H , (5) CJHIIOH,(6) C6Hl3OH]and (b) alcohol-water mixtures ((1) 91.5% H20-8.5% C2HSOH, (2) 87% H20-13% CH3OH, (3) 32% H Z W 8 % CHJOH,(4) 6% &0-94% C2HSOH. For alcohols, intercept = 11.6 i 0.3, slope = 0.14 f 0.02 mol kcal-I, correlation = 0.94. For mixtures, intercept = 17.1 f 0.4, slope = 0.05 f 0.03 mol kcal-I, correlation = 0.94.
composition (Table I) further confirms that the solvent shear viscosity has very little bearing on the dynamics of internal conversion in PyB. This behavior is consistent with what has been previously observed for most rhodamine dyes.3,'2-16,'8~'9~23 For example, k,for rhodamine B has the same value in ethylene glycol as in ethanol, even though ethylene glycol has a much higher viscosity than ethanol.I2 Furthermore, T f and & are greater for dyes with monoethylamino group than for dyes with diethylamino
group^.^*'^ Sdvent Polarity Effects. The isoviscosity data (Table I) strongly suggest that the effect of solvent on internal conversion in PyB depends more on solvent polarity than on solvent shear viscosity. Indeed, plots of In kn, vs ET(30) for alcohols (Figure 3a) and for the alcoholwater mixtures (Figure 3b) are linear. However, the slopes of these plots are very different: the slope is 0.14 f 0.02 mol kcal-' in alcohols and 0.05 f 0.03 mol kcal-I in alcohol-water mixtures. The nonradiative rate constant in water seems low in comparison to the alcohol-water mixtures. Extrapolation of the linear fit of In k,, vs ET(30) for the mixtures to water yields k,, = 5.5 X IO8 s-' in water, instead of the measured value of (4.1 f 0.2) x 108 s-1. When plotted together, the activation energies (Figure 4) exhibit a weak but noticeable dependence on solvent polarity. The activation energy tends to decrease as E ~ ( 3 0 )increases. Linear regression of a plot of E, vs ET(30) yields a slope of -0.10 f 0.05 with an intercept of 11 f 3 kcal mol-'. For the alcohols studied, E, does not vary much with ET(30) within experimental error: the values range from 5.6 f 0.8 kcal mol-' for methanol to 6.3 f 1.9 kcal mol-' for hexanol. The average value of E,, ( Ea)avg, is 6.0 f 0.3 kcal mol-' for the alcohols. In contrast, the value of E, for water is =2 kcal mol-l less than that of the alcohols, which is a factor of 2 beyond the experimental error. The values of E, in the alcohol-water mixtures are between those of water and the alcohols. As in the case of the alcohols, the values of E, for mixtures do not vary much with ET(30), and they fall within experimental error of the average (Ea)avg = 5.1 f 0.3 kcal mol-'. Clearly, water lowers the value of E, in the mixtures by -1 kcal mol-' from the value of E, in neat alcohols. TIC" Model. In the TICT model, internal conversion occurs via the TICT state.8-'0 This state is characterized by electron transfer from the amino groups to the xanthene ring and by
rotation about the xanthene-amine bond. The formation of the TICT state depends on solvent polarity and on the donor-aceptor properties of the amino group and the xanthene ring. In the TICT model, the energy of the TICT state mainly determines the nonradiative rate. Increasing the solvent polarity decreases the energy of the TICT state and increases the nonradiative rate. In the TICT model, solvent effects are taken into account by a polaritydependent E,. In one version of this model, E, is assumed to be a linear function of ET(30),29The dependence of the photophysics of the TICT compound (dimethylamino)benzonitrile on solvent polarity can be explained by using this assumption. If internal conversion for PyB involves the formation of the TICT state, In k,, should increase and E, should decrease with ET(30). These trends are not observed for PyB in alcohols and alcohol-water mixtures. Although E, decreases with ET(30) (Figure 4), we cannot reconcile the nonradiative rate of PyB in water with the rate in alcohol-water mixtures by using this model. The value of ET(30) decreases from 63.20kcal mol-' in water to 63.10 kcal mol-' in 91.5% water-8.5% ethanol and to 62.86 kcal mol-' in 87% water-1 3% methanol. According to this model, k, should be greater in pure water than in the leas polar alcohol-water mixtures. However, k,, increases from (4.1 f 0.2) X lo8 s-l in water to (6.0 f 0.5) X lo8 s-l in 91.5% water-8.5% ethanol. Similarly, k, increases from (4.1 f 0.2) X lo8s-' in water to (5.3 f 0.7) X lo8 s-' in 87% water-13% methanol. Twu-State I(inetic Me!d". The linearity of In k,vs ET(30) for PyB in alcohols and alcohol-water mixtures shows that solvent polarity is a strong determinant in the nonradiative rate of PyB. However, the different slopes and activation energies indicate that specific solutesolvent interactions also influence the nonradiative rate. For RBH+ in alcohols, In k,, and E, increase linearly with ET(30).'8J9This behavior was analyzed in terms of a two-state kinetic mechanism. The linear increase in In k, with E ~ ( 3 0 ) observed for PyB in alcohols suggests that we use this mechanism here. In this mechanism, photophysical kinetic parameters are assumed to be polarity-dependent. Briefly, a fluorescent planar state A*, which can only decay radiatively to the ground singlet state So,is in rapid equilibrium with a nonemissive nonplanar state B*, which can only decay to So via internal conversion. The nonradiative rate constant in this mechanism is give by knr = ki&A*B* (3) where K,.*. is the equilibrium constant between A* and B* and ki, is the rate constant for internal conversion from B*. Equation 3 is obtained by applying the steady-state approximation to B* and assuming ki,
