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(Photo)isomerization dynamics of merocyanine dyes in solution
J. Photo&em.
Photobiol.
A:
Chem,
(Photo)isomerization solution Anthony
Harriman
Center for
Fast Kinetics
Research,
6.5 (1992)
dynamics
The
79
79-93
University
of merocyanine
of Texas at Austin,
Austin,
dyes in
7X
78712 (USA)
Abstract The dynamics for isomerization of the first excited singlet state and of an unstable ground state conformer have been measured for a merocyanine dye in a variety of solvents. It is shown that the rates of both isomerizations depend on the temperature, viscosity and polarity of the surrounding medium. These effects can be explained in terms of the largescale torsional motion and change in dipole moment that accompany isomerization. As the barrier to isomerization of the first excited singlet state increases, fluorescence intensifies but there is no accompanying increase in triplet state population. The rate of decay of the triplet state depends on polarity of the solvent, due to solvent-induced changes in the energy gap, but not on viscosity. These various findings are discussed in terms of a potential energy diagram
that involves intermediate
population
of a “twisted”
excited singlet state.
1. Introduction
Merocyanine dyes, such as merocyanine 540, are attracting considerable attention because of their preferential uptake by leukaemia cells and their high photocytotoxicity [l]. In attempting to understand the mechanism for this photocytotoxicity, it is appropriate to characterize the photophysical properties of the dye in simpler media [2]. This, however, is a d-cult problem because of the conformational and zwitterionic possibilities available to the dye in the ground and excited states. In particular, merocyanine dyes are known to undergo efficient photoisomerization upon promotion to the singlet excited state 123. In this paper we describe the photophysical properties of a relevant merocyanine dye, one that is known to associate with and subsequently destroy leukemia cells under visible light illumination [3], in fluid media with a view to understanding how to control the barrier to isomerization. The object of this study was to determine if the yield of the (reactive) triplet state could be increased by decreasing the significance of the (undesired) isomerization process. This necessitates establishing a detailed understanding of the mechanism for the isomerization process. Our results show that the magnitude of the barrier to isomerization, and therefore the rate, depends on both solvent polarity and viscosity due to (i) changes in dipole moment that accompany excitation; (ii) solvent-induced changes in the potential energy of the reacting species; (iii) the cunformational mobility of the molecule. 2. Experimental
details
The merocyanine dye 1 was synthesized by conventional methods [3] and purified by extensive column and thin-layer chromatography. All solvents were of commercial
lOlO-6030/92/$5.00
0 1992 - Elsevier Sequoia. AU rights reserved
80 spectroscopic grade or were redistilled before use. Solutions were prepared fresh and protected against room light. Photophysical measurements were made by the methods described elsewhere [2]. In all cases, dilute solutions were used under an atmosphere of air, 02, or N2, as appropriate. Fluorescence quantum yields were corrected for instrumental responses and all kinetic measurements were signal-averaged and carried out at 20 “C, unless stated otherwise.
3. Results
and
discussion
3.1. Polarity of the ground and first excited k&et states The absorption maximum of 1 is solvent dependent
and undergoes a blue shift as the solvent polarity increases. In any given solvent, the energy of the absorption maximum (Eat,.) correlates with the xB solvent parameter introduced by Brooker et al. [4] for use as a solvent polarity indicator (Fig. 1). (This parameter is equivalent to the more popular E&30) solvent parameter and is used in this instance because it refers to a merocyanine dye having a structure comparable to that of 1.) From the gradient of Fig. 1, it can be deduced that 1 is more polar than the standard merocyanine and, from dielectric measurements made in diethylether solution, the ground state dipole moment (CL) was found to be (9.7f 0.3) D. This value agrees well with dipole moments measured for closely related merocyanines [5, 63. The polarity must arise from the presence of zwitterionic resonance structures, of which there are two extreme forms, since the non-ionic form is expected to have
XB
(kcal/mol)
Fig. 1. Plot of the absorption (,ya) in various solvents.
maxima
(E.3
recorded
for
1 against those of a standard merocyanine
D,- 1 2D,+
I
-- d2n’ +
1 I>
Fig. 2. Effect of solvent polarity on the magnitude of the Stokes’ shift measured for 1. The Stokes’ shift is expressed as the difference between mean wavenumber of absorption and fluorescence maxima while D, and n refer respectively to the solvent static dielectric constant and refractive index.
a dipole moment of about 2 D. According to MM2 molecular mechanics calculations, an all-trans configuration would correspond to a dipole separation of 7.3 A and, therefore, to a dipole moment for the zwitterionic form of 35 D. In this case, the zwitterionic structures would account for approximately 28% of the total ground state population. Alternately, the root mean square dipole separation over all possible configurations is 6.4 A. This would correspond to an average dipole moment for the zwitterion of 31 D and would require that the zwitterionic forms constitute about 31% of the ground state population. On average, therefore, the ground state comprises approximately 30% zwitterionic and approximately 70% non-ionic forms. The magnitude of the Stokes’ shift was found to exhibit a modest solvent dependence (Fig. 2). The data are seen to follow the Lippert-Mataga model [7] for a dipole in a solvent cavity and, from the gradient of Fig. 2, the decrease in dipole moment upon excitation into the first excited singlet state is (3.9f 0.2) D, for a dipole separation of 7.3 A. This attenuated polarity may arise from contraction of the dipole, if the ground state is in the all-trans configuration (see below), or by a decreased population of zwitterionic forms within the excited singlet state. In either case, the excited singlet state is less polar than the corresponding ground state and, therefore, the energy gap should be amenable to modulation by changes in solvent polarity. Polar sohents should exert a more pronounced stabilizing effect on the polar ground state than on the less polar excited singlet state so that the vertical energy gap is expected to increase with increasing solvent polarity (Table 1). 3.2. 77ze radiative rate constant The radiative rate constant was determined in a range of solvents at 20 “C! using the Strickler-Berg expression [SJ. For these experiments, a small aliquot of a concentrated solution of 1 in methanol was diluted with a large volume of a second solvent. Absorption and fluorescence spectral properties were used to derive a value for the radiative rate constant (k,,,) (Table 1). As indicated by Fig. 3, there is a direct correspondence between krad and the square of the solvent refractive index (n2), as predicted by the Strickler-Berg equation 181. This correlation can be used to suggest that there are no specific interactions (e.g. hydrogen bonding) between ground state 1 and solvent
83 molecules. In all solvents studied, there is a good ( f 10%) agreement between measured measurements (krad= c#+.).
k rad and that derived from the fluorescence
3.3. Photophysical properties in methanol solution In dilute methanol solution at 20 “C, 1 fluoresces with a quantum yield (&) of 0.18. The observed fluorescence spectrum is in reasonable mirror-symmetry with the lowest energy absorption profile and there is a small Stokes’ shift (approximately 700 cm-‘). Such findings imply that the excited singlet state retains the planar geometry of the ground state. The excited singlet state lifetime (r,), measured with streak camera detection following excitation at 532 nm with a 30 ps laser pulse, is (380 f20) ps. The calculated k,,,, (4.9~ 10’ s-l) agrees well with the experimentally derived value (krad= c&/T~= 4.7 X I@ s-‘). Phosphorescence could not be detected in a methanol glass at 77 K and, under such conditions, & approached unity. The excited singlet state could also be observed via transient absorption spectroscopy following excitation with a 30 ps laser pulse at 532 nm. The observed transient differential absorption spectrum shows an absorption band centred around 440 nm together with strong ground state bleaching centred at about 590 nm (Fig. 4(a)). The transient absorption band decayed with a lifetime of (370f30) ps (Fig. 4(b)), as measured at 440 nm, whereas partial recovery of the ground state occurred with a lifetime of (380&30) p.s. At the completion of these absorption spectral changes there remained a low intensity signal which did not decay within 100 ns of excitation (Fig. 4(a)). The shorter lived transient is assigned to the planar excited singlet state of the dye, in view of the correspondence between fluorescence and transient differential absorption decay rates, whereas the residual signal could be observed more easily following excitation with a 10 ns laser pulse at 532 nm. The transient differential absorption spectra recorded under these latter conditions are displayed in Fig. 5. It is seen that two distinct transient species can be resolved by virtue of their different lifetimes. Thus, the strong ground state bleaching that is observed upon ns laser excitation recovers completely but via biphasic kinetics (Fig. 6). The initial step in the recovery process is markedly O2 sensitive and its lifetime (TV) decreases from (750f30) ps in Nz saturated methanol to (470 f 30) ns upon aeration, Consequently, this component is attributed to the triplet excited state. It absorbs modestly in the far red region, where it can be resolved from the other transient, and it can be formed via triplet energy transfer from anthracene. The slower step in the recovery process is O2 insensitive and does not absorb at wavelengths above about 650 nm. Monitoring at 625 nm, which is an isosbestic point for the triplet state, does not show a growth of the second transient comensurate with triplet decay. This suggests that the longer lived transient arises directly from the excited singlet state. Monitoring at 610 nm in aerated solution shows that it decays slowly with a lifetime (rise) of (12.2f0.2) ms to reform the ground state of 1. In agreement with studies made with related merocyanines [l, 23 this longlived transient is attributed to an unstable ground state isomer. Using triplet energy transfer techniques with anthracene as donor, the molar extinction coefficient for the triplet state was measured (Fig. 7) and used to derive the quantum yield for formation of the triplet excited state (4) upon direct excitation. The derived value (& = 0.003 f 0.001) indicates that triplet state formation is extremely inefficient. In contrast, the isomer is formed with high quantum yield (&, = 0.44 f 0.05). Production of the isomer represents a form of internal conversion, for which the minimum quantum yield is 0.80 and the rate constant is equal to or greater than 2.5 x lo9 s-l.
84
665.
470 *
574.
WAVELENGTH
-rIMYE (PICOSECONDS)
670.
762.
(nm)
TIME (PICOSECONDS)
Fig. 4. (a) Transient differential absorption spectra recorded after excitation of 1 in methanol with a 30 ps laser pulse at 532 nm; delay times were 0, 0.45, 1.0, 2.0, 3.5, 5.0 and 6.0 ns. Decay profiles recorded are displayed: (b) at 440; (c) at 590 nm.
The rate constants for non-radiative deactivation of the first excited singlet state were found to decrease with decreasing temperature, in methanol solution. From the linear Arrhenius plots between - 40 “C and +40 “C, activation energies for the excited and ground states respectively, were derived to be 12 and 65 kJ mol-‘. These energy barriers are associated with the large-scale amplitude motion that must accompany isomerization of the polymethine chain for which the simplest description, in terms of potential energy, is depicted in Fig. 8. Here, the two ground state isomers (labelled stable and unstable) are separated by an energy barrier &. ‘H nuclear magnetic resonance (NMR) studies carried out in CD30D at various temperatures showed no indication of the presence of interconverting isomers but were consistent with an all-trans configuration. This suggests that the unstable isomer has the cis configuration. The excited singlet state (S1) generated by excitation retains the planar (all-trans) configuration of the ground state and can undergo intersystem-crossing to populate a triplet state (77,) of
(k,,=(l/7,) - krad)and for decay of the unstable isomer (kh = l/~_)
85
-0.0604 500
550
600
650
WAVELENGTH
c 750
700
(nm)
Fig. 5. Transient differential absorption spectra recorded 100 ns (0) of 1 in aerated methanol with a 10 ns laser pulse at 532 nm. =E_Z
-pHm
___..___i..._..,......,......i . . . . . ... ; i i i ; i
i .. ...? : :
and 1 ms (A) after excitation
.._ i
$3.29 U c1
i-4.6s 50
microseconds
... :: r
.a4
: 2.
: :
' 1.
0
TIIlE
0
:: 3.
i :1
i: 0
4.
i 0
XEB
Fig. 6. Recovery of ground state 1, as measured at 590 nm, following excitation in deoxygenated methanol with a 10 ns laser pulse at 532 nm. geometry. In competition to intersystem-crossing, S1 can rearrange to form a “twisted” singlet state (S,) which possesses a geometry intermediate between the cis and trams forms. The Sz state may be electronically excited or it could be a hot ground state. Its decay results in formation of the ground state isomers. In this model, the barrier (E,) to isomerization of S1 controls partitioning between Tr (and fluorescence) and S2. Switching off isomerization, by increasing El, will increase the yield of T1 and vice versa. An inherent feature of the model is that the ratio of is fixed and is independent of El. fluorescence and triplet quantum yields (&/A) Detailed experiments were performed, therefore, with a view to evaluating the aptness of this simple model.
unknown
3.4. Solvent effects on deactivation of the excited sing& state The fluorescence lifetime (rs) was found to depend markedly on the nature of the solvent at 20 “C (Table 1) but, in each case, the fluorescence decay profile could be fit to a single exponential component that was wavelength independent. In order to evaluate any specific solvent effects, such as hydrogen bonding, the rotational diffusion time (TV) was measured in different solvents using time-dependent fluorescence anisotropy [9]. The derived values are collected in Table 1. Assuming that the molecule
86
--G
4
-150 I 500
550 600 650 700 WAVELENGTH (nm)
2
750 NUCLEAR
-
COORDINATE
Fig. 7. Transient differential absorption spectra recorded for the triplet excited state of 1 following sensitization by anthracence in deoxygenated methanol solution. Fig. 8. Simple potential energy diagram for photo- and thermal isomerizations of 1. corresponds to a prolate ellipsoid [lo] of major axis 15.2A and minor axis 7.5 A, as estimated by computer simulation, the modified Debye-Stokes-Einstein expression allows an acceptable prediction of 7rot only for stick boundary conditions [ll]. Within experimental error, the ratio (T&T) is a constant with a mean value of (295 f 40) ps in both protic and aprotic solvents: cP-l Trot=
%‘vhyd
k,s
i’-
0)
This finding, from which the hydrodynamic volume (v&) is calculated to be 1250 A3, implies that there are no specific solvent effects and, in addition, it appears that dielectric friction does not exert a significant perturbation [12]. Using the averaged hydrodynamic volume, the dielectric friction time constant (T,,~) was calculated for each of the solvents from the following expression (Table 1): 7DF=(4~TD/3)(iLZ/Vhyakg~{(E-
1)/(26+
I)*}
(2)
where TD refers to the Debye relaxation time of the solvent and E is the static dielectric constant. Using this term to correct the hydrodynamic volume for any dielectric friction: Trot= 37vhyd/kB T+
TDF
(3)
does not result in an appreciable change in the magnitude of v&., nor does it improve the quality of a plot of 7r0t versus bulk solvent shear viscosity (17). Using time-resolved fluorescence spectroscopy, 7s was measured in different solvents and the derived values are collected in Table 1. The radiative rate constants were determined for the same solvents and used to calculate a value for the non-radiative decay rate constant (km) in each solvent: k, = (I/T,)
- krad
(4)
It was found that k, exhibited a pronounced solvent dependence, decreasing by a factor of approximately 45 on changing from water to heptanol. For compounds that can readily photoisomerize it is often observed that k, correlates with the reciprocal of the solvent viscosity, as predicted by Kramers’ theory [13] or a similar function [14]:
87
k,=(A/q") exp[-&/RTJ
(5)
where EA is the barrier height and A is the pre-exponential frequency factor. However, as pointed out by Eisenthal and co-workers [15], when the viscosity is varied by changing the solvent it may be necessary to compensate for variations in polarity if isomerization involves a change in dipole moment. In order to evaluate the importance of polarity effects on k,, an isoviscosity (7 = 5 cP) experiment was performed using linear alcohols at different temperatures [16, 173. The magnitude of km was determined in each case, using eqn. (4), and related to the solvent polarity parameter E,(30) (Table 2) [18]. Assuming that the activation energy EA depends linearly on solvent polarity, as measured by the E-,(30) parameter, the polarity-dependent non-radiative decay rate constant can be expressed in the form 1151:
k,=A
exp[p(ET(30)-3O)IRTJ exp[-E,/RT'j
(6)
The pre-exponential frequency factor Awas calculated from the temperature dependence studies carried out in methanol solution; A = 1.3 X 1013 s-l. The p term in eqn. (6) describes the dependence of E A on solvent polarity whereas E,, corresponds to the inherent (polarity-independent) activation energy. From the isoviscosity studies (Fig. 9), p= (0.108f0.005) and E, = (33 f4) kJ mol-‘. These derived values indicated that, at constant viscosity and temperature, the rate of isomerization increases with increasing solvent polarity. This is a substantial effect and serves to decrease the fluorescence lifetime and quantum yieId in polar TABLE
2
Non-radiative rate constants measured for the planar excited singlet state and the unstable (cis) isomer at a viscosity of 5 CP in linear alcohols Solvent
Temperature W)
Ethanol Propanol Butanol Pentanol Hexanol ‘Data
k=x 10-a
(kcal mol-‘)
ki,
(s-l)
W’)
54.4 51.4 50.6 49.7 48.7
1.04 2.31 3.90 4.35 5.70
0.064 9.9 46.0 122.0 324.0
ET(30)a
-41 -8 6 16 26
taken from ref. 16.
-134 IS
l--15.5 21
24
I-(30) - 301 (kcaI/mol) Fig. 9. Isoviscosity plots for photo- (e) alcohols at different temperatures.
27
and thermal (A) isomerizations of 1 in various linear
25
1 so
0.00
3.00
1nV Fig. 10. Effect of solvent viscosity on the polarity-correctedrate constant for non-radiative decay of the first excited singlet state. solvents. Clearly, the isomerization barrier height decreases with increasing solvent polarity, dropping from 33 to 12 kJ mol-’ upon changing from alkane to methanol solvents. As stated earlier, the planar excited singlet state is less polar than the ground state and, by comparison, will be destabilized by polar solvents. Thus, the potential energy of the excited state increases with increasing solvent polarity and, provided the crossover (or inflection) point on the potential energy plot remains independent of solvent, this will be reflected by a decrease in the energy barrier for isomerization. Using the above parameters, the polarity-corrected rate constants (k,‘) were calculated for 1 in different solvents at 20 “C: km0 =k,
exp[ - @(&(30)
-3o)jRq
As shown by Fig. 10, there is a poor correlation between km0 and the solvent shear viscosity, as expressed in terms of Kramers’ theory [13]. Although there is a general decrease in the rate of isomerization with increasing solvent viscosity, the data points are widespread and the apparent slope (cr= (1.2f 0.3)) is too steep [13, 143. Therefore, in keeping with many other studies carried out with relevant systems [19], this simple form of Kramers’ theory does not give a good description of the viscosity effects for photoisomerization of 1. More rigorous theoretical treatments are available for this type of process [19], but are not needed to test the validity of Fig. 8. Overall, these resuhs indicate that non-radiative deactivation of the first excited singlet state is controlled by both viscosity and polarity terms. Solvent induced changes in rs are reflected in comparable changes in & but the ratio (&/+J does not remain constant (Table 1). Instead, decreases in & are matched by increases in & (Fig. 11). These two properties appear to be antagonistic, not cooperative. This behaviour implies that the planar-excited singlet state is only weakly coupled to the triplet manifold. Inhibiting geometric rearrangement of the central double bonds, by imposing a high barrier to isomerization, favours fluorescence but inhibits intersystem-crossing to the triplet state. 3.5. Solvent effects on ground state isometization The long-lived isomer, which is formed from the excited singlet state, decays via first-order kinetics to reform the ground state. The lifetime of the isomer (7iso), as measured at 20 “C following excitation with a 10 ns laser pulse at 532 nm, is solvent dependent and representative values are collected in Table 3. Following the procedures outlined for deactivation of the excited singlet state, it was concluded that decay of
89 1.000*
0.600
L
l
--
50
0.600--
@f
0.400--
%a 0
0.200--
0
0.000T 0.000
0.001
0.002
0.003
0.004
9,
Fig. 11. Correlation between quantum yields for fluorescence and triplet state formation measured in different solvents. TABLE
3
Solvent effects on the properties of the isomer and triplet Solvent
rib (CR)
ETC
&sod
%0= Gus)
TIC (ELs)
Acetone CH&N CH,OH DMFa CzHsOH DMSO” C,H,OH= C,H,OH’ Pyridine EtAc” Formamide 2-PROH” MTHF= Water
0.304 0.345 0.551 0.924 1.078 1.996 2256 2.948 0.952 0.455 3.764 2.859 0.550 1.002
42.2 46.0 55.5 43.8 51.9 45.0 50.7 49.9 40.2 38.1 56.6 48.6 37.4 63.1
0.29
1.4 2.4 12.2 5.1 11.5 11.3 22.7 27.6 2.6 1.5 69.2 26.0 2.2 33.3
385 445 750 460 670 450 655 580 385 330 860 550 200 t
0.35 0.44 0.27 0.31 0.11 0.26 0.18 0.09 0.21 0.32 0.25 0.20 -
“DMFN,N-dimethylfo rmamide; DMSO dimethylsulphoxide; C,H70H propan-l-01; C&&OH butanl-d; EtAc ethyl acetate; 2-PROH propan-2-ol; MTHF 2-methyltetrahydrofuran. bBulk solvent viscosity at 20 “C from S.L. Murov, Handbook of Photochemisby, Marcel Dekker, New York, 1973. ‘I&(30) solvent polarity parameter taken from (b) above. df0.04. “f5%. ‘Not detected. the isomer is controlled by both solvent viscosity and polarity. From a temperature dependence study in methanol covering the range -40 “C to + 40 “C, the preexponential frequency factor A was derived to be 1.5 X 1013 s-l; this value is within experimental error of that found for the corresponding excited state process. An isoviscosity (5 cP) experiment, carried out with linear alcohols at different temperatures, gave rise to J3= ( - 0.090 f 0.005) and E, = (54f 5) k3 mol-’ (Fig. 9). Comparison with the corresponding excited state process shows that /3 is smaller and, more significantly,
90
it is of the opposite sign, whereas the polarity-corrected activation energy is significantly larger. The difference in J?Z.may arise because the ground state isomerization involves full cis-trans interconversion, whereas the excited state process involves transformation of a planar state to a twisted one. Thus, a less substantial structural change is involved in the latter process. The derived p value indicates that the rate of ground state (thermal) isomerization decreases with increasing solvent polarity, which is in sharp contrast to the behaviour of excited singlet state. The energy barrier for thermal isomerization increases with increasing solvent polarity in such a way that, following the assumptions made for the photoisomerization process, polar solvents must decrease the potential energy of the isomer. This hypothesis demands that the unstable isomer is quite polar. After correcting the derived k,,, values for variations in solvent polarity, according to eqn. (7), it is seen that the rate of isomerization decreases markedly with increasing solvent viscosity. The derived kho values follow Kramers’ theory over a reasonably wide viscosity range, giving a! = (0.86 f 0.06) (Fig. 12). Therefore, the use of bulk solvent shear viscosity appears to be justified for the slow thermal isomerization but not the rapid photoisomerization. Quantum yields for formation of the isomer (&,) were measured in each solvent at 20 “C following excitation with a 10 ns laser pulse at 532 nm. Some of the measured values are compiled in TabIe 3. It is seen that there is a clear correlation between A.0 and k,, indicating that non-radiative deactivation of the planar excited singlet state represents the rate-determining step in the formation of the isomer. In order to explain the observed trend in (&f/+3, however, it is necessary to invoke intermediate population of a twisted excited singlet state. The predominant route for formation of the triplet state involves intermediate population of this twisted state. This situation is depicted in Fig. 13 where T1 is coupled to SZ rather than coupled directly to S1. In this case, SZ is an electronically excited singlet state (not a hot
6, -2
-1
0
1
t-6 2
STABLE NUCLEAR
COORDINATE
G-1nI) Fig. 12. Effect of solvent viscosity on the polarity-correctedrate constant for isomerization of
the unstable
(cis)
Fig. 13. Potential singlet state.
ground energy
state diagram
isomer_ indicating
formation
of the triplet
state from
a twisted
excited
91
ground state) possessing a twisted geometry but T1 has the (all-trans) planar geometry of the ground state. The model differs significantly from that displayed in Fig. 8 because of the mode of population of T r. Although it is probable that S1 and T1 are weakly coupled in such a way that direct intersystem-crossing occurs to some finite extent, the major route for population of T1 is via S2. Thus, minimizing the formation of Sz by increasing El has the (undesired) effect of reducing the population of T1 and increasing the (&/&) ratio. Partitioning of S2 between intersystem-crossing and formation of ground state isomers (internal conversion) may also be controlled by environmental effects since both processes involve structural changes. The Sz state, however, has not been detected by time-resolved fluorescence spectroscopy. 3.6. Deactivation of the trzpIet excited state The excited triplet state is formed in very low quantum yield in all solvents studied (Table 1). Using energy transfer techniques, with anthracene as donor, the lifetime of the triplet state (TJ was measured in deoxygenated solution at 20 “C (Table 3). At low triplet concentrations, the decay profile could be fit to a single exponential component. There was no indication that the triplet decayed to form the isomer and, instead, it returned quantitatively to the ground state. The derived decay rates are solvent dependent and, as shown in Fig. 14(a), there is a good correlation between In k, (k,=l/TJ and the Er(30) solvent polarity parameter without having to correct for differences in viscosity. Indeed, the polarity-corrected triplet decay rate constants (k,D) were found to be independent of the solvent viscosity (Fig. 14(b)). The absence of a viscosity effect (linear least squares analysis of Fig. 14(b) gave cy= (0.08 &0.04)) is taken as an indication that decay of the triplet does not involve a significant structural rearrangement of the polymethine bridge. According to the Fermi “golden rule”, k, should depend solely on the energy gap between ground and triplet states [20], in the absence of oxygen; the rate should
rJ 30
40
Q(30)
“~500~ -m. I 7.5m(7 n
0
606
50
(kcal/mol)
+r -
8.000
- 1.500
I
l
I’
n
-CL750
0.000
0.750
1.5w
lnr)
Fig_ 14. (a) Effect of solvent polarity on the rate constant for decay of the triplet excited state in deoxygenated solution; (b) effect of viscosity on the polarity-corrected rate constants for decay of the triplet.
decrease with increasing energy gap. For 1, the observed correlation between decay rate and solvent polarity (Fig. 14(a)) can be explained, therefore, in terms of solvent induced changes in the triplet energy gap. This is a small effect, however, since /3 has a value of only -0.031 (Fig. 14(a)). This value suggests that the dipole moment of the triplet is slightly less than that of the ground state. In this case, polar solvents would increase the triplet energy gap by decreasing the potential energy of the ground state and this effect would be reflected in a reduced k,. No low temperature phosphorescence could be detected and attempts to measure the triplet energy by optoacoustic studies gave unreliable results because of the inherently low &.
4. Conclusion This study has shown that, for merocyanine 1, the efficiency of isomerization can be modulated by environmental effects such as temperature, viscosity or solvent polarity. This behaviour arises because of the large-scale torsional motion and dipole moment change that accompany isomerization. Our results indicate that Auorescence and conversion to a twisted geometry are the only significant contributors to deactivation of the planar excited singlet state, as formed upon excitation. This situation means that the (reactive) triplet state is not strongly coupled to the planar excited state Sr, as in Fig. 8, but is accessed via the twisted excited singlet state S2, as in Fig. 13. Therefore, inhibiting formation of the twisted excited singlet state also inhibits formation of the triplet state while fluorescence is enhanced. Our attempts to increase the yield of the triplet state by eliminating isomerization have failed because of the extremely low rate of direct intersystem-crossing from the planar excited singlet state. The rate constant for this process must be less than 2.4X 10’ s-l, based on measurements made in glycerol at 5 “C where &= 0.96, r,= 1.60 ns, and krad= 6.0x 108 s-l. (We are trying to obtain a better estimate of this rate constant from low temperature flash photolysis studies.) It is clear now that, using Fig. 13 as a model, the yield of the triplet state can be improved only by modulating the partition of S2 between triplet and ground states or by increasing the rate of direct intersystem-crossing from Sr. The former situation could be achieved by introducing bulky substituents into the polymethine chain which inhibit full cis-trans isomerization but which do not prevent twisting of S1. The latter situation may be realized by introducing heavy atoms into the benzthiazole subunit. Such studies are in progress and will be reported elsewhere [21].
Acknowkdgement
This work was supported by the National Institutes of Health (CA53619-01). The CFKR is supported jointly by the Biotechnology Resources Program of the NIH (RR00886) and by The University of Texas at Austin.
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16 17 18
19 20 21
L. G. S. Brooker, A. C. Craig, D. W. Heseltine, P. W. Jenkins and L. L_ Lincoln, J. Am. Chern. Sot., 87 (1965) 2443. P. R. Dragsten and W. W. Webb, Biochemistry, 17 (1978) 5228. L. M. Kushner and C. P. Smyth, .T. Am. Chem. Sot., 71 (1949) 1401. B. S. Brunschwig, S. Ehrenson and N. Sutin, J. Phys. Chem., 91 (1987) 4714. S. J. Strickler and R. A. Berg, J. Chem. Phys., 37 (1962) 814. E. L. Quitevis and M.-L. Horng, J. Phys. Chem., 94 (1990) 5684. F. Perrin, _K Phys. Radium, 5 (1934) 497. C.-M. Hu and R. J. Zwanzig, 1. Chem. Phys., 60 (1974) 4354. T.-W. Nee and R. J. Zwanzig, I: Chem. Phys., 52 (1970) 6353. H. A. Kramers, Physica (Utrecht), 7 (1940) 284. S. P. Velsko and G. R. Fleming, I. Chem. Phys., 76 (1982) 3553. J. M. Hicks, M. T. Vandersall, E. V. Sitzmann and K. B. Eisenthal, Chem. Phys. L&t_, 135 (1987) 413. J. D. Simon and S.-G. Su, J. Phys. Chem., 94 (1990) 3656. T.-L. Chang and W. L. Borst, J. Chem. Phys., 93 (1990) 4724. H. Ratajczak and W. J. Orville-Thomas (eds.), Molecular Inferactions, Wiley, New York, 1982. Y.-P. Sun and J. Saltiel, X Phys. Chem, 93 (1989) 8310. J. B. Birks, Photophysics of Aromatic Molecules, Wiley, London, 1970. A. Harriman and T. E. Sosebee, J. Chem. Phys., to be submitted.
Photobiol.
A:
Chem,
(Photo)isomerization solution Anthony
Harriman
Center for
Fast Kinetics
Research,
6.5 (1992)
dynamics
The
79
79-93
University
of merocyanine
of Texas at Austin,
Austin,
dyes in
7X
78712 (USA)
Abstract The dynamics for isomerization of the first excited singlet state and of an unstable ground state conformer have been measured for a merocyanine dye in a variety of solvents. It is shown that the rates of both isomerizations depend on the temperature, viscosity and polarity of the surrounding medium. These effects can be explained in terms of the largescale torsional motion and change in dipole moment that accompany isomerization. As the barrier to isomerization of the first excited singlet state increases, fluorescence intensifies but there is no accompanying increase in triplet state population. The rate of decay of the triplet state depends on polarity of the solvent, due to solvent-induced changes in the energy gap, but not on viscosity. These various findings are discussed in terms of a potential energy diagram
that involves intermediate
population
of a “twisted”
excited singlet state.
1. Introduction
Merocyanine dyes, such as merocyanine 540, are attracting considerable attention because of their preferential uptake by leukaemia cells and their high photocytotoxicity [l]. In attempting to understand the mechanism for this photocytotoxicity, it is appropriate to characterize the photophysical properties of the dye in simpler media [2]. This, however, is a d-cult problem because of the conformational and zwitterionic possibilities available to the dye in the ground and excited states. In particular, merocyanine dyes are known to undergo efficient photoisomerization upon promotion to the singlet excited state 123. In this paper we describe the photophysical properties of a relevant merocyanine dye, one that is known to associate with and subsequently destroy leukemia cells under visible light illumination [3], in fluid media with a view to understanding how to control the barrier to isomerization. The object of this study was to determine if the yield of the (reactive) triplet state could be increased by decreasing the significance of the (undesired) isomerization process. This necessitates establishing a detailed understanding of the mechanism for the isomerization process. Our results show that the magnitude of the barrier to isomerization, and therefore the rate, depends on both solvent polarity and viscosity due to (i) changes in dipole moment that accompany excitation; (ii) solvent-induced changes in the potential energy of the reacting species; (iii) the cunformational mobility of the molecule. 2. Experimental
details
The merocyanine dye 1 was synthesized by conventional methods [3] and purified by extensive column and thin-layer chromatography. All solvents were of commercial
lOlO-6030/92/$5.00
0 1992 - Elsevier Sequoia. AU rights reserved
80 spectroscopic grade or were redistilled before use. Solutions were prepared fresh and protected against room light. Photophysical measurements were made by the methods described elsewhere [2]. In all cases, dilute solutions were used under an atmosphere of air, 02, or N2, as appropriate. Fluorescence quantum yields were corrected for instrumental responses and all kinetic measurements were signal-averaged and carried out at 20 “C, unless stated otherwise.
3. Results
and
discussion
3.1. Polarity of the ground and first excited k&et states The absorption maximum of 1 is solvent dependent
and undergoes a blue shift as the solvent polarity increases. In any given solvent, the energy of the absorption maximum (Eat,.) correlates with the xB solvent parameter introduced by Brooker et al. [4] for use as a solvent polarity indicator (Fig. 1). (This parameter is equivalent to the more popular E&30) solvent parameter and is used in this instance because it refers to a merocyanine dye having a structure comparable to that of 1.) From the gradient of Fig. 1, it can be deduced that 1 is more polar than the standard merocyanine and, from dielectric measurements made in diethylether solution, the ground state dipole moment (CL) was found to be (9.7f 0.3) D. This value agrees well with dipole moments measured for closely related merocyanines [5, 63. The polarity must arise from the presence of zwitterionic resonance structures, of which there are two extreme forms, since the non-ionic form is expected to have
XB
(kcal/mol)
Fig. 1. Plot of the absorption (,ya) in various solvents.
maxima
(E.3
recorded
for
1 against those of a standard merocyanine
D,- 1 2D,+
I
-- d2n’ +
1 I>
Fig. 2. Effect of solvent polarity on the magnitude of the Stokes’ shift measured for 1. The Stokes’ shift is expressed as the difference between mean wavenumber of absorption and fluorescence maxima while D, and n refer respectively to the solvent static dielectric constant and refractive index.
a dipole moment of about 2 D. According to MM2 molecular mechanics calculations, an all-trans configuration would correspond to a dipole separation of 7.3 A and, therefore, to a dipole moment for the zwitterionic form of 35 D. In this case, the zwitterionic structures would account for approximately 28% of the total ground state population. Alternately, the root mean square dipole separation over all possible configurations is 6.4 A. This would correspond to an average dipole moment for the zwitterion of 31 D and would require that the zwitterionic forms constitute about 31% of the ground state population. On average, therefore, the ground state comprises approximately 30% zwitterionic and approximately 70% non-ionic forms. The magnitude of the Stokes’ shift was found to exhibit a modest solvent dependence (Fig. 2). The data are seen to follow the Lippert-Mataga model [7] for a dipole in a solvent cavity and, from the gradient of Fig. 2, the decrease in dipole moment upon excitation into the first excited singlet state is (3.9f 0.2) D, for a dipole separation of 7.3 A. This attenuated polarity may arise from contraction of the dipole, if the ground state is in the all-trans configuration (see below), or by a decreased population of zwitterionic forms within the excited singlet state. In either case, the excited singlet state is less polar than the corresponding ground state and, therefore, the energy gap should be amenable to modulation by changes in solvent polarity. Polar sohents should exert a more pronounced stabilizing effect on the polar ground state than on the less polar excited singlet state so that the vertical energy gap is expected to increase with increasing solvent polarity (Table 1). 3.2. 77ze radiative rate constant The radiative rate constant was determined in a range of solvents at 20 “C! using the Strickler-Berg expression [SJ. For these experiments, a small aliquot of a concentrated solution of 1 in methanol was diluted with a large volume of a second solvent. Absorption and fluorescence spectral properties were used to derive a value for the radiative rate constant (k,,,) (Table 1). As indicated by Fig. 3, there is a direct correspondence between krad and the square of the solvent refractive index (n2), as predicted by the Strickler-Berg equation 181. This correlation can be used to suggest that there are no specific interactions (e.g. hydrogen bonding) between ground state 1 and solvent
83 molecules. In all solvents studied, there is a good ( f 10%) agreement between measured measurements (krad= c#+.).
k rad and that derived from the fluorescence
3.3. Photophysical properties in methanol solution In dilute methanol solution at 20 “C, 1 fluoresces with a quantum yield (&) of 0.18. The observed fluorescence spectrum is in reasonable mirror-symmetry with the lowest energy absorption profile and there is a small Stokes’ shift (approximately 700 cm-‘). Such findings imply that the excited singlet state retains the planar geometry of the ground state. The excited singlet state lifetime (r,), measured with streak camera detection following excitation at 532 nm with a 30 ps laser pulse, is (380 f20) ps. The calculated k,,,, (4.9~ 10’ s-l) agrees well with the experimentally derived value (krad= c&/T~= 4.7 X I@ s-‘). Phosphorescence could not be detected in a methanol glass at 77 K and, under such conditions, & approached unity. The excited singlet state could also be observed via transient absorption spectroscopy following excitation with a 30 ps laser pulse at 532 nm. The observed transient differential absorption spectrum shows an absorption band centred around 440 nm together with strong ground state bleaching centred at about 590 nm (Fig. 4(a)). The transient absorption band decayed with a lifetime of (370f30) ps (Fig. 4(b)), as measured at 440 nm, whereas partial recovery of the ground state occurred with a lifetime of (380&30) p.s. At the completion of these absorption spectral changes there remained a low intensity signal which did not decay within 100 ns of excitation (Fig. 4(a)). The shorter lived transient is assigned to the planar excited singlet state of the dye, in view of the correspondence between fluorescence and transient differential absorption decay rates, whereas the residual signal could be observed more easily following excitation with a 10 ns laser pulse at 532 nm. The transient differential absorption spectra recorded under these latter conditions are displayed in Fig. 5. It is seen that two distinct transient species can be resolved by virtue of their different lifetimes. Thus, the strong ground state bleaching that is observed upon ns laser excitation recovers completely but via biphasic kinetics (Fig. 6). The initial step in the recovery process is markedly O2 sensitive and its lifetime (TV) decreases from (750f30) ps in Nz saturated methanol to (470 f 30) ns upon aeration, Consequently, this component is attributed to the triplet excited state. It absorbs modestly in the far red region, where it can be resolved from the other transient, and it can be formed via triplet energy transfer from anthracene. The slower step in the recovery process is O2 insensitive and does not absorb at wavelengths above about 650 nm. Monitoring at 625 nm, which is an isosbestic point for the triplet state, does not show a growth of the second transient comensurate with triplet decay. This suggests that the longer lived transient arises directly from the excited singlet state. Monitoring at 610 nm in aerated solution shows that it decays slowly with a lifetime (rise) of (12.2f0.2) ms to reform the ground state of 1. In agreement with studies made with related merocyanines [l, 23 this longlived transient is attributed to an unstable ground state isomer. Using triplet energy transfer techniques with anthracene as donor, the molar extinction coefficient for the triplet state was measured (Fig. 7) and used to derive the quantum yield for formation of the triplet excited state (4) upon direct excitation. The derived value (& = 0.003 f 0.001) indicates that triplet state formation is extremely inefficient. In contrast, the isomer is formed with high quantum yield (&, = 0.44 f 0.05). Production of the isomer represents a form of internal conversion, for which the minimum quantum yield is 0.80 and the rate constant is equal to or greater than 2.5 x lo9 s-l.
84
665.
470 *
574.
WAVELENGTH
-rIMYE (PICOSECONDS)
670.
762.
(nm)
TIME (PICOSECONDS)
Fig. 4. (a) Transient differential absorption spectra recorded after excitation of 1 in methanol with a 30 ps laser pulse at 532 nm; delay times were 0, 0.45, 1.0, 2.0, 3.5, 5.0 and 6.0 ns. Decay profiles recorded are displayed: (b) at 440; (c) at 590 nm.
The rate constants for non-radiative deactivation of the first excited singlet state were found to decrease with decreasing temperature, in methanol solution. From the linear Arrhenius plots between - 40 “C and +40 “C, activation energies for the excited and ground states respectively, were derived to be 12 and 65 kJ mol-‘. These energy barriers are associated with the large-scale amplitude motion that must accompany isomerization of the polymethine chain for which the simplest description, in terms of potential energy, is depicted in Fig. 8. Here, the two ground state isomers (labelled stable and unstable) are separated by an energy barrier &. ‘H nuclear magnetic resonance (NMR) studies carried out in CD30D at various temperatures showed no indication of the presence of interconverting isomers but were consistent with an all-trans configuration. This suggests that the unstable isomer has the cis configuration. The excited singlet state (S1) generated by excitation retains the planar (all-trans) configuration of the ground state and can undergo intersystem-crossing to populate a triplet state (77,) of
(k,,=(l/7,) - krad)and for decay of the unstable isomer (kh = l/~_)
85
-0.0604 500
550
600
650
WAVELENGTH
c 750
700
(nm)
Fig. 5. Transient differential absorption spectra recorded 100 ns (0) of 1 in aerated methanol with a 10 ns laser pulse at 532 nm. =E_Z
-pHm
___..___i..._..,......,......i . . . . . ... ; i i i ; i
i .. ...? : :
and 1 ms (A) after excitation
.._ i
$3.29 U c1
i-4.6s 50
microseconds
... :: r
.a4
: 2.
: :
' 1.
0
TIIlE
0
:: 3.
i :1
i: 0
4.
i 0
XEB
Fig. 6. Recovery of ground state 1, as measured at 590 nm, following excitation in deoxygenated methanol with a 10 ns laser pulse at 532 nm. geometry. In competition to intersystem-crossing, S1 can rearrange to form a “twisted” singlet state (S,) which possesses a geometry intermediate between the cis and trams forms. The Sz state may be electronically excited or it could be a hot ground state. Its decay results in formation of the ground state isomers. In this model, the barrier (E,) to isomerization of S1 controls partitioning between Tr (and fluorescence) and S2. Switching off isomerization, by increasing El, will increase the yield of T1 and vice versa. An inherent feature of the model is that the ratio of is fixed and is independent of El. fluorescence and triplet quantum yields (&/A) Detailed experiments were performed, therefore, with a view to evaluating the aptness of this simple model.
unknown
3.4. Solvent effects on deactivation of the excited sing& state The fluorescence lifetime (rs) was found to depend markedly on the nature of the solvent at 20 “C (Table 1) but, in each case, the fluorescence decay profile could be fit to a single exponential component that was wavelength independent. In order to evaluate any specific solvent effects, such as hydrogen bonding, the rotational diffusion time (TV) was measured in different solvents using time-dependent fluorescence anisotropy [9]. The derived values are collected in Table 1. Assuming that the molecule
86
--G
4
-150 I 500
550 600 650 700 WAVELENGTH (nm)
2
750 NUCLEAR
-
COORDINATE
Fig. 7. Transient differential absorption spectra recorded for the triplet excited state of 1 following sensitization by anthracence in deoxygenated methanol solution. Fig. 8. Simple potential energy diagram for photo- and thermal isomerizations of 1. corresponds to a prolate ellipsoid [lo] of major axis 15.2A and minor axis 7.5 A, as estimated by computer simulation, the modified Debye-Stokes-Einstein expression allows an acceptable prediction of 7rot only for stick boundary conditions [ll]. Within experimental error, the ratio (T&T) is a constant with a mean value of (295 f 40) ps in both protic and aprotic solvents: cP-l Trot=
%‘vhyd
k,s
i’-
0)
This finding, from which the hydrodynamic volume (v&) is calculated to be 1250 A3, implies that there are no specific solvent effects and, in addition, it appears that dielectric friction does not exert a significant perturbation [12]. Using the averaged hydrodynamic volume, the dielectric friction time constant (T,,~) was calculated for each of the solvents from the following expression (Table 1): 7DF=(4~TD/3)(iLZ/Vhyakg~{(E-
1)/(26+
I)*}
(2)
where TD refers to the Debye relaxation time of the solvent and E is the static dielectric constant. Using this term to correct the hydrodynamic volume for any dielectric friction: Trot= 37vhyd/kB T+
TDF
(3)
does not result in an appreciable change in the magnitude of v&., nor does it improve the quality of a plot of 7r0t versus bulk solvent shear viscosity (17). Using time-resolved fluorescence spectroscopy, 7s was measured in different solvents and the derived values are collected in Table 1. The radiative rate constants were determined for the same solvents and used to calculate a value for the non-radiative decay rate constant (km) in each solvent: k, = (I/T,)
- krad
(4)
It was found that k, exhibited a pronounced solvent dependence, decreasing by a factor of approximately 45 on changing from water to heptanol. For compounds that can readily photoisomerize it is often observed that k, correlates with the reciprocal of the solvent viscosity, as predicted by Kramers’ theory [13] or a similar function [14]:
87
k,=(A/q") exp[-&/RTJ
(5)
where EA is the barrier height and A is the pre-exponential frequency factor. However, as pointed out by Eisenthal and co-workers [15], when the viscosity is varied by changing the solvent it may be necessary to compensate for variations in polarity if isomerization involves a change in dipole moment. In order to evaluate the importance of polarity effects on k,, an isoviscosity (7 = 5 cP) experiment was performed using linear alcohols at different temperatures [16, 173. The magnitude of km was determined in each case, using eqn. (4), and related to the solvent polarity parameter E,(30) (Table 2) [18]. Assuming that the activation energy EA depends linearly on solvent polarity, as measured by the E-,(30) parameter, the polarity-dependent non-radiative decay rate constant can be expressed in the form 1151:
k,=A
exp[p(ET(30)-3O)IRTJ exp[-E,/RT'j
(6)
The pre-exponential frequency factor Awas calculated from the temperature dependence studies carried out in methanol solution; A = 1.3 X 1013 s-l. The p term in eqn. (6) describes the dependence of E A on solvent polarity whereas E,, corresponds to the inherent (polarity-independent) activation energy. From the isoviscosity studies (Fig. 9), p= (0.108f0.005) and E, = (33 f4) kJ mol-‘. These derived values indicated that, at constant viscosity and temperature, the rate of isomerization increases with increasing solvent polarity. This is a substantial effect and serves to decrease the fluorescence lifetime and quantum yieId in polar TABLE
2
Non-radiative rate constants measured for the planar excited singlet state and the unstable (cis) isomer at a viscosity of 5 CP in linear alcohols Solvent
Temperature W)
Ethanol Propanol Butanol Pentanol Hexanol ‘Data
k=x 10-a
(kcal mol-‘)
ki,
(s-l)
W’)
54.4 51.4 50.6 49.7 48.7
1.04 2.31 3.90 4.35 5.70
0.064 9.9 46.0 122.0 324.0
ET(30)a
-41 -8 6 16 26
taken from ref. 16.
-134 IS
l--15.5 21
24
I-(30) - 301 (kcaI/mol) Fig. 9. Isoviscosity plots for photo- (e) alcohols at different temperatures.
27
and thermal (A) isomerizations of 1 in various linear
25
1 so
0.00
3.00
1nV Fig. 10. Effect of solvent viscosity on the polarity-correctedrate constant for non-radiative decay of the first excited singlet state. solvents. Clearly, the isomerization barrier height decreases with increasing solvent polarity, dropping from 33 to 12 kJ mol-’ upon changing from alkane to methanol solvents. As stated earlier, the planar excited singlet state is less polar than the ground state and, by comparison, will be destabilized by polar solvents. Thus, the potential energy of the excited state increases with increasing solvent polarity and, provided the crossover (or inflection) point on the potential energy plot remains independent of solvent, this will be reflected by a decrease in the energy barrier for isomerization. Using the above parameters, the polarity-corrected rate constants (k,‘) were calculated for 1 in different solvents at 20 “C: km0 =k,
exp[ - @(&(30)
-3o)jRq
As shown by Fig. 10, there is a poor correlation between km0 and the solvent shear viscosity, as expressed in terms of Kramers’ theory [13]. Although there is a general decrease in the rate of isomerization with increasing solvent viscosity, the data points are widespread and the apparent slope (cr= (1.2f 0.3)) is too steep [13, 143. Therefore, in keeping with many other studies carried out with relevant systems [19], this simple form of Kramers’ theory does not give a good description of the viscosity effects for photoisomerization of 1. More rigorous theoretical treatments are available for this type of process [19], but are not needed to test the validity of Fig. 8. Overall, these resuhs indicate that non-radiative deactivation of the first excited singlet state is controlled by both viscosity and polarity terms. Solvent induced changes in rs are reflected in comparable changes in & but the ratio (&/+J does not remain constant (Table 1). Instead, decreases in & are matched by increases in & (Fig. 11). These two properties appear to be antagonistic, not cooperative. This behaviour implies that the planar-excited singlet state is only weakly coupled to the triplet manifold. Inhibiting geometric rearrangement of the central double bonds, by imposing a high barrier to isomerization, favours fluorescence but inhibits intersystem-crossing to the triplet state. 3.5. Solvent effects on ground state isometization The long-lived isomer, which is formed from the excited singlet state, decays via first-order kinetics to reform the ground state. The lifetime of the isomer (7iso), as measured at 20 “C following excitation with a 10 ns laser pulse at 532 nm, is solvent dependent and representative values are collected in Table 3. Following the procedures outlined for deactivation of the excited singlet state, it was concluded that decay of
89 1.000*
0.600
L
l
--
50
0.600--
@f
0.400--
%a 0
0.200--
0
0.000T 0.000
0.001
0.002
0.003
0.004
9,
Fig. 11. Correlation between quantum yields for fluorescence and triplet state formation measured in different solvents. TABLE
3
Solvent effects on the properties of the isomer and triplet Solvent
rib (CR)
ETC
&sod
%0= Gus)
TIC (ELs)
Acetone CH&N CH,OH DMFa CzHsOH DMSO” C,H,OH= C,H,OH’ Pyridine EtAc” Formamide 2-PROH” MTHF= Water
0.304 0.345 0.551 0.924 1.078 1.996 2256 2.948 0.952 0.455 3.764 2.859 0.550 1.002
42.2 46.0 55.5 43.8 51.9 45.0 50.7 49.9 40.2 38.1 56.6 48.6 37.4 63.1
0.29
1.4 2.4 12.2 5.1 11.5 11.3 22.7 27.6 2.6 1.5 69.2 26.0 2.2 33.3
385 445 750 460 670 450 655 580 385 330 860 550 200 t
0.35 0.44 0.27 0.31 0.11 0.26 0.18 0.09 0.21 0.32 0.25 0.20 -
“DMFN,N-dimethylfo rmamide; DMSO dimethylsulphoxide; C,H70H propan-l-01; C&&OH butanl-d; EtAc ethyl acetate; 2-PROH propan-2-ol; MTHF 2-methyltetrahydrofuran. bBulk solvent viscosity at 20 “C from S.L. Murov, Handbook of Photochemisby, Marcel Dekker, New York, 1973. ‘I&(30) solvent polarity parameter taken from (b) above. df0.04. “f5%. ‘Not detected. the isomer is controlled by both solvent viscosity and polarity. From a temperature dependence study in methanol covering the range -40 “C to + 40 “C, the preexponential frequency factor A was derived to be 1.5 X 1013 s-l; this value is within experimental error of that found for the corresponding excited state process. An isoviscosity (5 cP) experiment, carried out with linear alcohols at different temperatures, gave rise to J3= ( - 0.090 f 0.005) and E, = (54f 5) k3 mol-’ (Fig. 9). Comparison with the corresponding excited state process shows that /3 is smaller and, more significantly,
90
it is of the opposite sign, whereas the polarity-corrected activation energy is significantly larger. The difference in J?Z.may arise because the ground state isomerization involves full cis-trans interconversion, whereas the excited state process involves transformation of a planar state to a twisted one. Thus, a less substantial structural change is involved in the latter process. The derived p value indicates that the rate of ground state (thermal) isomerization decreases with increasing solvent polarity, which is in sharp contrast to the behaviour of excited singlet state. The energy barrier for thermal isomerization increases with increasing solvent polarity in such a way that, following the assumptions made for the photoisomerization process, polar solvents must decrease the potential energy of the isomer. This hypothesis demands that the unstable isomer is quite polar. After correcting the derived k,,, values for variations in solvent polarity, according to eqn. (7), it is seen that the rate of isomerization decreases markedly with increasing solvent viscosity. The derived kho values follow Kramers’ theory over a reasonably wide viscosity range, giving a! = (0.86 f 0.06) (Fig. 12). Therefore, the use of bulk solvent shear viscosity appears to be justified for the slow thermal isomerization but not the rapid photoisomerization. Quantum yields for formation of the isomer (&,) were measured in each solvent at 20 “C following excitation with a 10 ns laser pulse at 532 nm. Some of the measured values are compiled in TabIe 3. It is seen that there is a clear correlation between A.0 and k,, indicating that non-radiative deactivation of the planar excited singlet state represents the rate-determining step in the formation of the isomer. In order to explain the observed trend in (&f/+3, however, it is necessary to invoke intermediate population of a twisted excited singlet state. The predominant route for formation of the triplet state involves intermediate population of this twisted state. This situation is depicted in Fig. 13 where T1 is coupled to SZ rather than coupled directly to S1. In this case, SZ is an electronically excited singlet state (not a hot
6, -2
-1
0
1
t-6 2
STABLE NUCLEAR
COORDINATE
G-1nI) Fig. 12. Effect of solvent viscosity on the polarity-correctedrate constant for isomerization of
the unstable
(cis)
Fig. 13. Potential singlet state.
ground energy
state diagram
isomer_ indicating
formation
of the triplet
state from
a twisted
excited
91
ground state) possessing a twisted geometry but T1 has the (all-trans) planar geometry of the ground state. The model differs significantly from that displayed in Fig. 8 because of the mode of population of T r. Although it is probable that S1 and T1 are weakly coupled in such a way that direct intersystem-crossing occurs to some finite extent, the major route for population of T1 is via S2. Thus, minimizing the formation of Sz by increasing El has the (undesired) effect of reducing the population of T1 and increasing the (&/&) ratio. Partitioning of S2 between intersystem-crossing and formation of ground state isomers (internal conversion) may also be controlled by environmental effects since both processes involve structural changes. The Sz state, however, has not been detected by time-resolved fluorescence spectroscopy. 3.6. Deactivation of the trzpIet excited state The excited triplet state is formed in very low quantum yield in all solvents studied (Table 1). Using energy transfer techniques, with anthracene as donor, the lifetime of the triplet state (TJ was measured in deoxygenated solution at 20 “C (Table 3). At low triplet concentrations, the decay profile could be fit to a single exponential component. There was no indication that the triplet decayed to form the isomer and, instead, it returned quantitatively to the ground state. The derived decay rates are solvent dependent and, as shown in Fig. 14(a), there is a good correlation between In k, (k,=l/TJ and the Er(30) solvent polarity parameter without having to correct for differences in viscosity. Indeed, the polarity-corrected triplet decay rate constants (k,D) were found to be independent of the solvent viscosity (Fig. 14(b)). The absence of a viscosity effect (linear least squares analysis of Fig. 14(b) gave cy= (0.08 &0.04)) is taken as an indication that decay of the triplet does not involve a significant structural rearrangement of the polymethine bridge. According to the Fermi “golden rule”, k, should depend solely on the energy gap between ground and triplet states [20], in the absence of oxygen; the rate should
rJ 30
40
Q(30)
“~500~ -m. I 7.5m(7 n
0
606
50
(kcal/mol)
+r -
8.000
- 1.500
I
l
I’
n
-CL750
0.000
0.750
1.5w
lnr)
Fig_ 14. (a) Effect of solvent polarity on the rate constant for decay of the triplet excited state in deoxygenated solution; (b) effect of viscosity on the polarity-corrected rate constants for decay of the triplet.
decrease with increasing energy gap. For 1, the observed correlation between decay rate and solvent polarity (Fig. 14(a)) can be explained, therefore, in terms of solvent induced changes in the triplet energy gap. This is a small effect, however, since /3 has a value of only -0.031 (Fig. 14(a)). This value suggests that the dipole moment of the triplet is slightly less than that of the ground state. In this case, polar solvents would increase the triplet energy gap by decreasing the potential energy of the ground state and this effect would be reflected in a reduced k,. No low temperature phosphorescence could be detected and attempts to measure the triplet energy by optoacoustic studies gave unreliable results because of the inherently low &.
4. Conclusion This study has shown that, for merocyanine 1, the efficiency of isomerization can be modulated by environmental effects such as temperature, viscosity or solvent polarity. This behaviour arises because of the large-scale torsional motion and dipole moment change that accompany isomerization. Our results indicate that Auorescence and conversion to a twisted geometry are the only significant contributors to deactivation of the planar excited singlet state, as formed upon excitation. This situation means that the (reactive) triplet state is not strongly coupled to the planar excited state Sr, as in Fig. 8, but is accessed via the twisted excited singlet state S2, as in Fig. 13. Therefore, inhibiting formation of the twisted excited singlet state also inhibits formation of the triplet state while fluorescence is enhanced. Our attempts to increase the yield of the triplet state by eliminating isomerization have failed because of the extremely low rate of direct intersystem-crossing from the planar excited singlet state. The rate constant for this process must be less than 2.4X 10’ s-l, based on measurements made in glycerol at 5 “C where &= 0.96, r,= 1.60 ns, and krad= 6.0x 108 s-l. (We are trying to obtain a better estimate of this rate constant from low temperature flash photolysis studies.) It is clear now that, using Fig. 13 as a model, the yield of the triplet state can be improved only by modulating the partition of S2 between triplet and ground states or by increasing the rate of direct intersystem-crossing from Sr. The former situation could be achieved by introducing bulky substituents into the polymethine chain which inhibit full cis-trans isomerization but which do not prevent twisting of S1. The latter situation may be realized by introducing heavy atoms into the benzthiazole subunit. Such studies are in progress and will be reported elsewhere [21].
Acknowkdgement
This work was supported by the National Institutes of Health (CA53619-01). The CFKR is supported jointly by the Biotechnology Resources Program of the NIH (RR00886) and by The University of Texas at Austin.
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