Twist angle dependent mesomerism and spectroscopy in substituted anilines: Absorption and fluorescence spectra of indoline derivatives as planar model compounds

Efficient singlet-state deactivation of cyano-substituted indolines in protic solvents via CN--HO hydrogen bonds

The photophysical properties of indoline (I) and three of its derivatives, namely, N-methylindoline (MI), 5-cyanoindoline (CI), and 5-cyano-N-methylindoline (CMI), are studied in H-donating solvents of varying polarity. Based on measurements of fluorescence yield and lifetime, and of triplet yield and hydrated-electron formation, two distinct mechanisms of solvent-induced fluorescence quenching are evidenced. The first mechanism involves the cyano substituent and leads to an increase in the rate constant of internal conversion of one order of magnitude in ethanolic solution and of more than two orders of magnitude in water, as compared to solutions in n-hexane or acetonitrile. A similar trend had previously been observed in the case of 4-N,N-dimethylaminobenzonitrile (DMABN). The second mechanism reduces the fluorescence lifetimes of the non-cyanated derivatives in aqueous solution by one order of magnitude and is related to the formation of hydrated electrons. Neither of these mechanisms is influenced by methylation at the ring nitrogen. Quantum chemical calculations are performed on the ground and excited states of the hydrogen-bonded complexes between protic solvents and MI as well as CMI. Stable hydrogen-bonded configurations involving the CN substituent and a solvent OH group are found; these configurations are stable both in the ground and the first excited singlet states, whereas the corresponding complex at the ring amino nitrogen is stable in the ground state only. The CN--HO configuration is therefore a prime candidate for a mechanistic explanation of the observed quenching by the first mechanism. These findings may have useful applications for the design of fluorescence probes for water in biological systems.

Intramolecular Charge-Transfer Mechanism in Quinolidines: The Role of the Amino Twist Angle

Quantum-chemical calculations with the approximate coupled-cluster singles-and-doubles model CC2 have been carried out for 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6). For this molecule dual fluorescence was experimentally observed, raising the discussion about the importance of the amino twist angle for this process. The calculations suggest that both the ground state and the normal fluorescent state are significantly twisted by 30 degrees -40 degrees and that the molecule is flexible enough to move into an even stronger twisted conformation (60 degrees -70 degrees ) in its intramolecular charge-transfer (ICT) state which is responsible for the anomalous fluorescence band. Such a conformation both minimizes the total energy in the S1 state and maximizes the dipole moment. The barrier from the normal fluorescent state to the ICT state region is very small. Comparison to the situation in the 1-methyl-derivative NMC6 suggests that a large alkyl substituent makes the preferably planar normal fluorescent state energetically unfavorable compared to the ICT state and thus promotes the occurrence of dual fluorescence.

Solvent Dependence of the Spectra and Kinetics of Excited-State Charge Transfer in Three (Alkylamino)benzonitriles

Steady-state absorption and emission spectra and emission decay kinetics are reported for 4-aminobenzonitrile (ABN), 4-(1-azetidinyl)benzonitrile (P4C), 4-(1-pyrrolidinyl)benzonitrile (P5C), and 4-(1-piperidinyl)benzonitrile (P6C) in 24 room temperature solvents. In solvents of modest to high polarity, P4C, P5C, and P6C exhibit dual fluorescence and emission decays characteristic of the transformation from an initially prepared (LE) state to a more polar charge transfer (CT) state, whereas ABN does not undergo this reaction. The frequencies of the steady-state absorption and emission spectra of all of these solutes can be rationalized using a dielectric continuum description of the solvent and considering only the minima on the reactive surfaces, which are assumed to involve both an intramolecular (twisting) and a solvation coordinate. Characteristics of the gas-phase solutes deduced from this analysis are in good agreement with electronic structure calculations and indicate that differences in their spectra mainly reflect differences in the relative energies of the gas-phase LE and CT states. The relative yields of LE and CT emission are not described as satisfactorily by this model, and reasons for this failure are discussed. The kinetics of the LE --> CT reaction vary considerably with solute and solvent. In many solvents, the emission decays of P4C are reasonably described by a simple two-state kinetic scheme with time-independent rate constants. In P5C and P6C multiexponential decays are observed that reflect time-dependent shifts of the component spectra as well as time-dependent reaction rates. A simplified analysis of these complex dynamics provides estimates for both the free energy change Delta(r)G and (average) LE --> CT rate constant k(f) for a wide range of solute and solvent combinations. The driving force for reaction (-Delta(r)G) follows the order P6C > P5C > P4C and increases with increasing solvent polarity. The reaction rates are correlated to Delta(r)G and follow the opposite trend. The relationships observed between k(f) and Delta(r)G suggest that static solvent effects, i.e., barrier height changes, are the primary determinants of the solvent dependence in P4C. Correlations between barrier-corrected rates and solvation times suggest that dynamical solvent effects contribute substantially to the solvent dependence of the rates in P5C, and especially P6C.

Time-dependent density functional theory (TDDFT) study of the excited charge-transfer state formation of a series of aromatic donor-acceptor systems

Singlet excitation energy calculations for a series of acceptor para-substituted N,N-dimethyl-anilines that are dual (4-(N,N-dimethylamino)benzonitrile, 4DMAB-CN, 4-(N,N-dimethylamino)benzaldhyde, 4DMAB-CHO, 1-methyl-7-cyano-2,3,4,5-tetrahydro-1H-1-benzazepine, NMC7) and nondual (4-aminobenzonitrile, 4AB-CN, 3-(N,N-dimethylamino)benzonitrile, 3DMAB-CN, and 4-nitro(N,N-dimethyl) aniline, 4DMAB-NO(2)) fluorescent have been performed using time-dependent density functional theory (TDDFT). The B3LYP and MPW1PW91 functionals with a 6-311+G(2d,p) (Bg) basis set have been used to compute excitation energies. Ground-state geometries were optimized using density functional theory (DFT) with both B3LYP and MPW1PW91 functionals combined with a 6-31G(d) basis set. For most of the molecules presented in this study, potential energy surfaces have been computed according to the coordinates related to the three following mechanisms proposed in the literature: twisting, wagging, and planar intramolecular charge transfer (ICT). Comparison of the three models for the different molecules leads to the conclusion that only the twisting ICT model is able to explain the low frequency, strongly solvent-dependent energy band present in the fluorescence spectra. According to this model, the 4AB-CN molecule is calculated to be nondual fluorescent in agreement with the experimental spectra. The single band observed in the fluorescence spectra of TMAB-CN (4-(N,N-dimethylamino)-3,5-(dimethyl)benzonitrile) is due to a large stabilization of the charge-transfer excited state along the twisting coordinate. The nondual fluorescence of the 4DMAB-NO(2) molecule is explained by the same mechanism. In the case of 3DMAB-CN, the single observed emission, which is solvent-dependent, has been assigned to the lowest charge-transfer excited state. The dual fluorescence of 4DMAB-CN and 4DMAB-CHO is explained within the twisting ICT model by a double mechanism (already proposed by Serrano et al.: Serrano-Andrés, L.; Merchán, M.; Roos, B. J.; Lindh, R. J. Am. Chem. Soc. 1995, 117, 3189) that involves the presence of two low-lying states close enough in energy. The observation of dual fluorescence in NMC7, that has been one of the origins of the planar ICT model put forward by Zachariasse et al. (Zachariasse, K.; van der Haar, T.; Hebecker, A.; Leinhos, U.; Kühnle, W. Pure Appl. Chem. 1993, 65, 1745), could be fully understood by a double mechanism within the twisting ICT model. Within the set of investigated molecules, our calculations confirm that the twisting ICT model is the only mechanism acceptable to explain the dual and nondual fluorescence phenomenon. Our calculations are in complete agreement with experimental data.