J. Phys. Chem. 1996, 100, 3507-3512
3507
Role of Nonfluorescent Twisted Intramolecular Charge Transfer State on the Photophysical Behavior of Aminophthalimide Dyes T. Soujanya,† R. W. Fessenden,‡ and A. Samanta*,†,‡ School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 134, India, and Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: September 11, 1995; In Final Form: NoVember 13, 1995X
Photophysical behavior of 4-aminophthalimide and its N,N-dimethyl derivative is studied in a series of solvents. A comparison of the fluorescence properties of the two compounds reveals that they behave quite differently despite their structural similarity. The efficient nonradiative decay of the dimethyl derivative in polar media is interpreted in terms of a nonemitting twisted state. Semiempirical calculations based on the AM1 method have been performed, and the results support the twisting mechanism. Calculations of the ground and excited state profiles as a function of torsion angle provide a clear picture of whether the twisting can influence the photophysical behavior of this class of dyes. The results provide insight into the design of efficient dye systems.
Introduction A large number of dye systems are currently being used as fluorescence probes of microenvironment in organized media such as cyclodextrins, micelles, membranes, polymers, surfaces, etc.1-8 A close look at these polarity sensitive fluorescence probes reveals that a majority of them are bichromophoric systems comprised of electron donor and acceptor groups. The sensitivity of the fluorescence maxima and quantum yields on the polarity of the media in these systems results from intramolecular charge transfer (ICT) character of the lowest singlet state. A new class of fluorescence probes, which shows remarkably higher sensitivity toward solvent polarity and are presently being preferred, consists of systems where the electron donor and the acceptor are linked by a single bond.5 In these systems, electronic excitation is believed to be associated with an intramolecular charge transfer coupled with rotational relaxation toward a twisted conformation.9,10 The phenomenon is termed twisted intramolecular charge transfer (TICT). (Dimethylamino)benzonitrile (DMABN) is a well-known example of this class of systems.9-11 Recently, Zachariasse et al. have put forward a different mechanism to account for dual fluorescence of DMABN and related systems.12-14 According to this mechanism, a change in hybridization of the amino nitrogen (from sp2 to sp3) takes place following electronic excitation, and the anomalous fluorescence of DMABN originates from a state in which the amino nitrogen is pyramidal. Even though both the mechanisms, rotational isomerization or rehybridization of the alkylamino group, can explain the polar nature of the second state involved in fluorescence, we would like to refer to this state as the TICT state because of wider acceptability of the former mechanism.5,10,15-17 The TICT state, being more polar than the ICT state, exhibits greater sensitivity to the polarity of the medium.10 While the TICT state in DMABN is fluorescent, it may not be so in other systems.18 Since the π-systems of the donor and the acceptor are decoupled from each other at the twisted conformation, the fluorescence from the TICT state is overlap-forbidden unless other effects are operative. This makes †
University of Hyderabad. Radiation Laboratory. * Corresponding author. All correspondence to University of Hyderabad. X Abstract published in AdVance ACS Abstracts, January 15, 1996.
the detection of these inVisible states rather difficult. It is important to note that, whether fluorescent or not, a low-lying TICT state can play a crucial role in dictating the fluorescence efficiency in flexible dye systems by providing an additional decay channel. In view of this important role of the TICT states, it is necessary to examine carefully whether there exists any possibility of the presence of such a state adjacent to the ICT state. This knowledge will be helpful in designing more efficient dye systems. In this particular paper, we have examined the fluorescence properties of two structurally similar dyes, 4-aminophthalimide (AP) and 4-(N,N-dimethylamino)phthalimide (DAP). The choice of these systems is based on
our earlier work in which we have shown that AP serves as an excellent probe for the microheterogeneous structure of cyclodextrins.19 During the course of our investigation on the photophysical behavior of aminophthalimide derivatives, we have observed significant difference in the radiative properties of structurally related species; the influence of solvent is also found to be quite different. In this paper we make an attempt to rationalize the photophysical behavior of two structurally similar species. From a detailed analysis of the photophysical properties and supportive theoretical calculations, we conclude that a nonemittive TICT state provides additional nonradiative pathway for DAP. Our results suggest that although rotational motion of the dimethylamino group in DAP provides an efficient nonradiative channel, rotation of the amino group in AP does not affect the overall photophysics of the system due to energetically unfavorable conditions. Experimental Details
‡
0022-3654/96/20100-3507$12.00/0
AP was purchased from Kodak and recrystallized several times from an ethanol-water mixture before use. DAP was © 1996 American Chemical Society
3508 J. Phys. Chem., Vol. 100, No. 9, 1996
Soujanya et al.
TABLE 1: Spectral Data of AP and DAP in Some Selected Solvents at Room Temperature
a
APc
solvent
dielectric constanta
ET(30)b
diethyl ether 1,4-dioxane tetrahydrofuran acetone acetonitrile tert-butyl alcohol 2-propanol methanol water
4.20 2.21 7.58 20.56 35.94 12.42 19.92 32.66 78.30
34.5 36.0 37.4 43.1 45.3 43.5 48.5 55.0 63.1
max λabs
(nm)
350 352 357 357 357 370 367 368 370
DAP max λfluo (nm)
max λabs (nm)
max λfluo (nm)
425 435 445 457 458 501 505 518 540
371 378 377 382 388 387 389 396 408
440 457 459 483 490 516 523 534 562
From ref 20. b Measured from the longest wavelength absorption maximum of betaine dye. c Some of these data are taken from ref 19.
synthesized by refluxing AP with acid-free dimethyl sulfate at 130 °C for 1 h. After 15 min of cooling, the reaction mixture was added to water and filtered. The solid thus obtained was purified by column chromatography using silica gel column and eluted with ethyl acetate and hexane mixture. The purity of the compound was checked by NMR spectra and elemental analysis. The ET(30) dye, 2,6-diphenyl-4-(2,4,6-triphenyl-Npyridino)phenoxide, was kindly provided by Dr. C. Reichardt of Philips University, Marburg, Germany. The solvents were purified by standard procedures. The purity of the solvents was checked by the ET(30) procedure.20 The absorption spectra were recorded using a JASCO 7800 UV-vis spectrophotometer. Fluorescence spectra were recorded with a Hitachi F-3010 spectrofluorimeter. The fluorescence lifetimes were obtained using the Photon Technology International LS-100 fluorescence spectrophotometer. The pulse width of the exciting flash was approximately 1.6 ns. A nonlinear least-squares fitting program was used. The goodness of the fit was estimated from χ2, a plot of the weighted residuals, and autocorrelation functions. The fluorescence quantum yields were measured by comparing the areas on excitation of optically matched solutions at 375 nm. Quinine sulfate was used as reference (φf ) 0.55 in 1 N H2SO4).21 Theoretical Calculations Our calculations are based on semiempirical AM1 (Austin Model 1) method22,23 which consists of a modified MNDO Hamiltonian24 and is parametrized for polar systems and transition states.25,26 The calculations were performed on a DEIL microVAX 3300 computer. The initial optimization of the ground state geometries of the molecules was achieved using the MMX molecular mechanics program. Subsequently, unrestricted geometry optimization at the semiempirical level was performed for various twist angles (every 10°) of the amino or the dimethylamino group. The gradient norms were monitored to test for successful convergence. The excited state energies were obtained for each optimized geometry using configuration interaction with 128 microstates with the MICROS option. The choice of a large number of microstates was to ensure true representation of the correlation effect. The microstates were generated by exciting one electron from each of the eight highest occupied orbitals to the eight lowest unoccupied orbitals. The effect of solvent on different electronic states has been estimated by first calculating the solvation energy using Onsager’s formulations and then adding this energy to the gas phase energy. The solvation energy of a fully relaxed state has been estimated using the relation27
∆Esf )
2µi2 - 1 a3 2 + 1
(
)
(1)
where µi represents the dipole about which the solvent relaxes fully, a is the Onsager cavity radius, and is the dielectric constant of the solvent. When the dipole moment of the solute changes from µi to µf so fast that the reorientation of the solvent molecules is not possible during this time scale (e.g., during the electronic excitation), then one needs to calculate the solvation energy using a different relation. For the partially solvated state the solvent stabilization is estimated using the following relation:28
∆Esp ) -
(
)
(
)
2µf2 n2 - 1 -1 2 n2 - 1 b µ ‚µ b f i 2 + 1 2n2 + 1 a3 a3 2n2 + 1
(2)
where n is the refractive index of the solvent. Results Spectral Data. The absorption and fluorescence spectra of DAP are characterized by broad structureless bands whose locations are primarily determined by the polarity of the media. The spectral features of DAP are found to be quite similar to those of AP whose photophysical behavior has been briefly reported by us in connection with a study of the guest-host complexation in cyclodextrins.19 A compilation of the absorption and fluorescence spectral data of the two compounds has been made in Table 1. The features that are common to the systems are the following: (i) An increase of polarity is associated with a red shift of the band maximum; absorption spectral shifts are considerably smaller than the shifts for fluorescence, which is suggestive of an excited state that is significantly more polar than the ground state. (ii) Specific interaction in hydrogen-bond-donating solvents. This is evident from larger Stokes shifts in alcoholic solvents and in water. The fact that acetonitrile is more polar than tert-butyl alcohol (as measured either by the microscopic solvent polarity parameter, ET(30), or bulk parameter, ), and yet the spectra are redshifted in the latter solvent, points to hydrogen-bonding interaction in water and in alcoholic solvents. A comparison of the position of the band maxima of the two compounds in a given solvent reveals that the spectral maxima for DAP are redshifted relative to those of AP. This behavior is consistent with the inductive effect of the alkyl groups. The correlation of the emission maxima of DAP with a microscopic solvent polarity function, ET(30), is shown in Figure 1. Change of Dipole Moment on Excitation. The spectral nature clearly points to the charge transfer character of the bands. To quantify the extent of charge separation in the ground and excited states, we have analyzed the solvatochromic absorption and fluorescence data in terms of solvent polarity functions. While doing so, we have excluded the hydrogen bond forming solvents. Although a number of expressions are available which
Photophysical Behavior of Aminophthalimide Dyes
J. Phys. Chem., Vol. 100, No. 9, 1996 3509 TABLE 2: Fluorescence Quantum Yields and Lifetimes of AP and DAP in Some Nonhydrogen-Bond-Forming Solvents at Room Temperature AP solvent
ET(30)
φf
τ (ns)
φf
τf (ns)
36.0 37.4 40.7 43.1 45.3
0.73 0.70 0.76 0.68 0.63
15.0 12.4
0.62 0.52 0.50 0.17 0.12
14.9 13.8
νja - νjf )
[
]
2(µe - µg)2 - 1 n2 - 1 + constant - 2 3 2 + 1 2n + 1 hca
compound
sovlent
ET(30)
kRSB, 107 s-1
kR , 107 s-1
knr, 107 s-1
1,4-dioxane tetrahydrofuran acetonitrile 1,4-dioxane tetrahydrofuran acetonitrile
36.0 37.4 45.3 36.0 37.4 45.3
3.9 4.8 2.7 6.4 7.3 4.6
4.9 5.6 4.5 4.2 3.8 3.1
1.8 2.4 2.6 2.5 3.5 22.6
between the ground and excited state dipole moment is found to be 4.1 D based on a cavity radius of 3.37 Å. It is quite evident from these ∆µ data that the nature of the fluorescent states in two dyes is very similar. Fluorescence Quantum Yields and Lifetimes. The fluorescence quantum yields (φf) of the two compounds are measured in a number of solvents, and a summary of these data is presented in Table 2. It is interesting to note that although the fluorescence quantum yield of AP remains more or less constant on increase of polarity, for the structurally similar compound DAP the fluorescence efficiency is found to decrease considerably on increase in solvent polarity. The fluorescence quantum yield of DAP in acetonitrile is found to be only onefifth of that in 1,4-dioxane. The fluorescence lifetimes (τf) are also measured in some selected solvents, and as can be seen from Table 2, the variation of lifetimes parallels the variation of φf. To find out the influence of solvent polarity on the radiative and nonradiative rates, we have calculated the radiative rate constants (krSB) using Strickler-Berg formula:33
kRSB ) 2.88 × 10-9n2〈νjf-3〉av-1∫(/νj) dνj
(3)
In the above equation, µg and µe refer to the ground and excited state dipole moments, respectively, and a is the Onsager cavity radius. The solution of this equation requires the knowledge of Onsager cavity radius, which is usually equated to the van der Waals radius of the molecule. Due to lack of crystal structure data on these systems, we have estimated a from AM1 calculated geometrical parameters. The distance between the amino nitrogen (N-1) and the carbonyl oxygen (O-11), which corresponds to the maximum distance across which charge separation occurs, is found to be 6.70 Å for AP. Therefore, the Onsager cavity radius was taken as half of this distance, i.e., 3.35 Å. That we are estimating the Onsager radius from the N1-O11 distance instead of using the longest possible distance is based on a recent paper by us32 on a series of derivatives of the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group in which we have clearly shown that the length of the alkyl group attached to the amino group is unimportant in dictating the observed dipole moment. The least-squares fit analysis of the data based on the Lippert-Mataga equation is shown in Figure 2. The slope of the line yielded a change of dipole moment of 3.6 D for AP. In case of DAP, the difference
3.9
TABLE 3: Radiative and Nonradiative Rate Constants of AP and DAP in Selected Solvents
DAP
correlate the solvatochromic shifts with various solvent polarity functions, the most widely used expression was based on treatments of Lippert and Mataga which utilizes the bulk solvent properties dielectric constant () and refractive index (n) to represent the solvent polarity functions. According to the Lippert-Mataga equation,29-31 the energy difference of the absorption and fluorescence maxima, νja and νjf, respectively, is related to and n of the solvent as follows:
14.0
From ref 20.
AP
Figure 2. Plot of Stokes shift (νja - νjf) versus the solvent polarity function ∆f for AP (0) and DAP (4). Hydrogen-bond-forming solvents are not considered for this plot.
DAP
1,4-dioxane tetrahydrofuran dichloromethane acetone acetonitrile a
Figure 1. Correlation of fluorescence maxima of DAP with ET(30) in some selected aprotic solvents. Protic solvents are excluded from the plot as they form a hydrogen bond with the probe (see discussion).
a
(4)
where
〈νjf-3〉av ) [∫νj3f(νj) dνj]/[f(νj) dνj] The calculated radiative rate constants kRSB(d1/τR) of the two compounds in three selected solvents are presented in Table 3. The radiative rate constants, independently estimated using the relation kR ) φf/τf, are also shown in the same table. It can be seen from the table that although there is some variation in the radiative rate constants calculated by the two methods, kRSB or kR values of both the compounds remain more or less constant in media of different polarities. The nonradiative rate constants (knr) are estimated using the familiar relation knr ) (1 - φf)/τf and presented in the same table. Although no significant influence of polarity of knr is observable for AP (it remains constant in the range (1.8-2.6) × 107 s-1), knr increases by an order of magnitude for DAP on changing the solvent from dioxane to acetonitrile. Theoretical Calculation. In order to understand the photophysical behavior of the two compounds, AM1 calculations have been carried out. We addressed ourselves to the following aspects while performing the calculations: (i) optimized ge-
3510 J. Phys. Chem., Vol. 100, No. 9, 1996
Figure 3. Variation of ground state energy of AP as a function of the twist angle of the amino group in the gas phase (0) and in acetonitrile (4).
ometries of the ground and excited states; (ii) electronic effect of the substitution of the amino hydrogens by methyl groups; (iii) effect of the rotation of the amino or dimethylamino group on the energetics; (iv) charge distribution in the ground and excited states (to provide theoretical estimates of µg and µe); and (v) solvent effect on the state ordering and the barrier height. As mentioned in an earlier section, the AM1 method is parametrized appropriately for polar systems and transition states. Some recent publications also show that the method is capable of predicting most of the ground and excited state properties fairly well.25,26 Before applying this method to our systems, we have carried out extensive calculations on DMABN (the most well-studied system with TICT state) to test whether the available experimental data can be understood in terms of AM1 calculation. We have verified that AM1 predictions are in accordance with the experimental data.34 Although it is possible to obtain a fairly good estimate of a number of ground and excited state properties of the molecules such as dipole moments, geometries, transition energies, etc. from the calculation, one should take note of some of the limitations of the method. It is not possible to obtain a precise estimate of the barrier height of the intramolecular relaxation process from semiempirical calculations in which only the lowest excited state is considered. Thus, the calculated barrier heights do not represent actual barriers involved in the process, but only indicate the trend. The second limitation of the method is associated with the validity of the solvation model in which the solvent is taken as a continuous dielectric. Further, an incorrect choice of the Onsager radius could lead to overestimation or underestimation of the solvation energies. The variation of the ground state energies as a function of the twist of the amino or the dimethylamino group is depicted in Figures 3 and 4, respectively. The calculations suggest planar conformations for both the species in the ground state. With increase in the twist angle the total energy of the system is increased. The energy difference between the twisted form (twist angle of 90°) and the planar form is found to be 0.38 eV for AP and 0.30 eV for DAP in the gas phase. The effects of solvent on the ground state energy profiles are also shown in the same figures. The solvent stabilization of different conformers by a given solvent is found to be different due to the difference in dipole moments. With increase in the twist angle the dipole moment is decreased gradually. This results in greater solvent stabilization of the planar form relative to that of the twisted form. Consequently, for both compounds, the energy difference between the planar and the twisted form is increased (relative to that in the gas phase) on solvation. The variation of excited state energies as a function of the torsion angle is shown in Figures 5 and 6. It can be seen from Figure 5 that the gas phase excited state energies increase continuously for
Soujanya et al.
Figure 4. Variation of ground state energy of DAP as a function of the twist angle of the dimethylamino group in the gas phase (0) and in acetonitrile (4).
Figure 5. Variation of energy of the lowest singlet excited state of AP as a function of the twist angle in the gas phase (0), in 1,4-dioxane (b), and in acetonitrile (4).
Figure 6. Variation of energy of the lowest singlet excited state of DAP as a function of the twist angle in the gas phase (b), in 1,4dioxane (0), and in acetonitrile (4).
AP with increase in twist angles. For DAP, however, the trend is quite different in the sense that the energy passes through a maximum. Another noticeable difference between the gas phase excited state profiles of the two compounds is that the excited state energy of AP in the twisted conformation is higher than that in the planar conformation, whereas for DAP the twisted conformation is relatively more stable than the planar conformation. The excited state profiles are also calculated in 1,4-dioxane and acetonitrile and shown in the same figures. In acetonitrile, the TICT states of AP and DAP are found to be stable than the ICT states by 0.27 and 1.59 eV, respectively. The calculated barriers to twisting in the excited state are found to be 0.55 and 0.06 eV for AP and DAP, respectively, in acetonitrile. Discussion The spectral similarity of the two compounds is a reflection of the structural similarity of AP and DAP. However, it is not
Photophysical Behavior of Aminophthalimide Dyes quite obvious why do the fluorescence quantum yield and lifetime of the two compounds vary in a different manner. The decrease in φf and τf of DAP in polar solvents is in contrast to the behavior of AP whose φf and τf remain more or less constant. The large difference in nonradiative rate constants of the two compounds in acetonitrile cannot be explained simply in terms of a more polar excited state of DAP because the difference in the excited dipole moments of the two compounds is too small to account for the observed trend. Since these systems are flexible enough to undergo conformational changes in the excited state, we make an attempt to explain the photophysical behavior in terms of two excited species instead of one and invoke the following scheme: hν
P 98 P* P* f P + hνf P* u PT* PT* f PT + hνf′ PT* f PT + ∆ PT* f PTT PT f P where P is the light absorbing species (and the only species present in the ground state); P*, PT, and PTT represent the Franck-Condon excited state, twisted form of P (twist angle of 90°), and the triplet state of the twisted form, respectively. In the present case, since one emission is observed instead of two depicted in the scheme, either P* or PT* is nonfluorescent. We would like to stress that for an interpretation of the experimental results the involvement of two excited states is required. The second state could be either a TICT state, proposed by Grabowski,9 or a state with pyramidal amino nitrogen, proposed by Zachariasse.12-14 Although both these states may have a role in determining the photophysical properties of the systems, we interpret our results in terms of TICT state because, as mentioned earlier, the twisting mechanism is currently favored over the other mechanism.5,16,17 The assumption that a single species is present in the ground state is supported by the results of AM1 calculation. The calculated barrier to twisting is found to be too large for both the compounds in the ground state (0.57 and 0.56 eV for AP and DAP, respectively, in acetonitrile) to expect the twisted form to be present in any significant amount and contribute to absorption. To find out whether the emission of these dyes originates from P* (ICT state) or PT* (TICT state), the dipole moments of the fluorescent states have been measured. Analysis of solvatochromic data yields ∆µ of 3.7 and 4.1 D for AP and DAP, respectively. Since changes in dipole moment on excitation are too small for both the compounds, it is most likely that emissions originate from the ICT state. This conclusion is also supported by the results of theoretical calculations. It can be seen from Table 4 that the experimentally determined ∆µ values match very closely with the same calculated theoretically for the ICT state. The theoretically calculated dipole moments of the TICT states of AP and DAP, 11.4 and 14.0 D, respectively, are much higher than the measured moments for the emitting states. The nature of the emitting state has also been ascertained from a comparison of the experimental and theoretical transition
J. Phys. Chem., Vol. 100, No. 9, 1996 3511 TABLE 4: Ground and Excited State Dipole Moments of AP and DAP change of dipole moment of dipole moment of the dipole moment the emitting state ground state AM1 AM1 Lippert AM1a Lippertb compound AP DAP
5.29 5.65
3.04 4.17
3.7 4.1
8.33 9.82
8.99 9.75
a Assuming that emission occurs from the planar conformation. Calculated on the basis of experimentally determined ∆µ and theoretically estimated µg values.
b
energies. The emission maximum of DAP is observed at 2.52 eV with the onset of the spectrum at 2.91 eV in acetonitrile. Thus, the origin of the emitting state could lie anywhere in this range. Theoretically calculated emission energy of DAP for the ICT state is found to be 2.76 eV in acetonitrile. Since the experimental transition energy is very close to the theoretically estimated energy of the ICT state, one can safely conclude that emission in these dyes originate from P*. A similar observation is noticed for AP. We now examine how the twist hypothesis accounts for the observed variation of φf, τf, and knr of DAP with polarity. The excited state energy profiles of AP (Figure 5) show that the TICT state is higher in energy than the ICT state by 0.7 eV in the gas phase. In a polar solvent such as in acetonitrile the situation is reversed where the former state is stable by 0.27 eV. However, the twisting process still involves a barrier of 0.56 eV in acetonitrile. Since the available thermal energy at room temperature is much lower than the barrier associated with the twisting process, one does not expect the TICT state to play any role in the photophysical properties of AP. We would like to emphasize here that consideration of the state energies of the ICT and TICT states alone does not preclude P* f PT* transformation; it is the barrier height which should be considered before any conclusion can be drawn regarding the feasibility of a process. It is encouraging to see that despite the semiempirical nature, the calculations provide a realistic picture of the twisting process. A similar analysis of the excited state energy profiles of DAP shows the following: (i) in the gas phase the TICT state is more stable than the ICT state by 0.02 eV, and the barrier associated with twisting is 0.13 eV; (ii) in acetonitrile the TICT state is stable by as much as 1.58 eV, and the barrier involved in twisting is only 0.06 eV. Thus, in acetonitrile both these conditions are highly favorable for twisting. The TICT state in DAP is therefore expected to influence its photophysical properties. In the case of DAP, the polarity dependence of φf (or τf) and knr arises from the polarity dependence of the P* f PT* process. It has been shown by Eisenthal and co-workers that the barrier height of the ICT f TICT process is polarity dependent due to the polar nature of the activated complex.35-37 As the polarity is increased, the barrier height is reduced. This enhances the rate of the twisting process, and as a consequence the radiative efficiency and lifetime of the emitting ICT state are decreased. One might raise the question, why is the TICT state in DAP nonfluorescent? One possible explanation could be that the energy gap between the TICT state and the corresponding Franck-Condon ground state is very low. The calculated energy profiles suggest that this gap is as low as 0.22 eV in acetonitrile. Although the actual energy gap, determined by the extent of solvation of the initial and the final states based on the lifetime of the TICT state, could be different from the one predicted above by assuming complete solvation, the internal conversion to the ground state will still be an efficient nonradiative pathway due to low-energy gap. Second, we find from the theoretical calculations that in polar solvents, where
3512 J. Phys. Chem., Vol. 100, No. 9, 1996 the TICT state will be sufficiently populated, a triplet state is almost isoenergetic (gap of only 0.05 eV) with the TICT state. We therefore believe that intersystem crossing to the triplet state could also be responsible for the nonfluorescent nature of the TICT state. The other alternative is the small transition matrix element between the TICT state and the ground state because of the twist angle between the donor and the acceptor making the transition overlap forbidden. Conclusion Since TICT states can influence the fluorescence efficiency of dyes, a priori determination of the location of these states and the barrier associated with the twisting process by simple theoretical calculations help designing new efficient fluorescing systems. We have shown in this paper how an invisible (nonfluorescent) TICT state can be detected by a combination of photophysical studies and an analysis of the theoretical results. Apart from providing a detailed understanding of the photophysical behavior of the two related dyes, the paper shows in general that the fluorescence yield of a dye is not affected by the rotary decay mechanism when the strength of the electron donor group is reduced. The results also highlight the potential of the AM1 method in predicting excited state properties of this class of systems. Acknowledgment. This work was supported by grants received from Council of Scientific and Industrial Research, Department of Science and Technology, Government of India, and the Office of Basic Energy Sciences of the Department of Energy, USA. This is Contribution No. NDRL-3804 from Notre Dame Radiation Laboratory. T.S. thanks the Department of Atomic Energy, Government of India, for the award of Dr. K. S. Krishnan Fellowship. We are thankful to Prof. C. Reichardt for generous gift of betaine dye, Dr. T. P. Radhakrishnan for assistance with the theoretical calculations, and Ms. S. Mukherjee and Dr. A. Chattopadhyay for lifetime measurements. References and Notes (1) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: Orlando, 1987. (2) Kalyanasundaram, K. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991; p 39. (3) Kamat, P. V.; Fox, M. A. In Lasers in Polymer Science and Technology: Applications, Vol. II; Fouassier, J. P., Rabek, J. F., Eds.; CRC Press: Boca Raton, FL, 1990; p 185.
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