Krystyna Rotkiewicz

Theoretical model for the double fluorescence of p-cyano-N,N-dimethylaniline in polar solvents

Abstract The energies of the low lying electronic states of p -cyano-N,N-dimethylaniline (CDMA) in its planar and in 90° twisted conformation have been calculated using the modified INDO method. Solvent effects have been taken into account. The results agree reasonably well with the experimental data and support the hypothesis of the dual fluorescence of CDMA as being related to the intramolecular rotation of the N(CH 3 ) 2 group, with a decisive role of the solvent polarity.

Considerations on the dipole moment of molecules forming the twisted intramolecular charge transfer state

Abstract Several derivatives of dual-fluorescing dimethylaminobenzonitrile (DMABN) are compared regarding their ground and excited-state dipole moments extracted from electro-optical emission and solvatochromic measurements. The comparison includes bridged model compounds as well as compounds with different degrees of steric hindrance to planarity, as well as ester and carbazolyl derivatives. The results are discussed on the basis of the twisted intramolecular charge transfer (TICT) model, the most widely accepted model. They show that (i) the dipole moment of the A (TICT) state (long-wavelength band) shows only little solvent sensitivity (polarizability), (ii) this observed small dependence of the dipole moment on solvent polarity can be considered as the result of overlap of two fluorescence bands or of solvent-induced changes in the mean deviation from the ideal TICT geometry (“atomic polarizability”), (iii) the dipole moment of the B state (short-wavelength band) is considerably smaller than that of the A band, (iv) the order of the dipole moment values obtained from solvatochromy is the same as from theoretical calculations: μ gp ep ( 1 L b ) ep ( 1 L a ) eT , (v) ester derivatives show a reduced dipole moment with respect to the corresponding nitriles and (vi) sterically crowded compounds seem to possess a reduced dipole moment. The possible reasons are discussed. The agreement and the differences between the two methods are discussed and compared with results from dielectric loss measurements and from quantum-chemical calculations.

Dual fluorescence of donor-acceptor molecules and the Twisted Intramolecular Charge Transfer (TICT) states

Abstract The hypothesis assigning the dual fluorescence to the formation of a twisted internal charge transfer (TICT) state seems to be proved for p-cyano- and p-(9-anthryl)-dialkylanilines. It appears to be applicable also to numerous other molecules.

Intramolecular electron-transfer excited state in 6-cyanobenzquinuclidine

6-cyanobenzquinuclidine has a rigid structure with the lone-pair orbital of the amino-group nitrogen atom and the π-orbitals of the aromatic ring mutually perpendicular. It is a model for the previously postulated twisted internal charge-transfer excited states. The fluorescent singlet state was identified as a strongly polar state with a full charge separation, observed in absorption as the 1(n, π*) excited state. The results strongly support the twisted internal charge-transfer state hypothesis.

Intramolecular charge transfer state and unusual fluorescence from an upper excited singlet of a nonplanar derivative of p-cyano-N, N-dimethylaniline

Abstract The TICT (twisted internal charge transfer state) fluorescence of 4-cyano-2,6,N,N-tetramethylaniline (CTMA) was found in the vapour phase. The dipole moments of excited Franck-Condon and Franck-Condon twisted ground states were estimated from the solvent shifts of absorption and fluorescence. The near-equality of excited Franck-Condon and relaxed TICT state dipole moments is discussed. The fluorescence from an upper excited singlet state is found and its mechanism is explained.

Picosecond chemical relaxation of excited p-cyano-n, n-dialkylanilines

Abstract New evidence indicate that the dual fluorescence of p -dimethylamino-benzonitrile (I) originates in a chemical relaxation B * → A * , where B * is a state hidden in the first absorption band A * is a highly polar rotamer with NMe 2 group perpendicular to the ring. New derivatives are synthesised, II–V, including those of enforced coplanar (V) or twisted conformation of NMe 2 group (III, IV). All three transition moments, the main responsible for absorption, b and a , are parallel. The relaxation B * → A * is in CH 3 CN slower than the reorientation of the solvent molecules. A * seems to be related to a “Rydberg” state similar to those of aliphatic amines.

Picosecond isomerisation kinetics of excited p-dimethylaminobenzonitriles studied by oxygen quenching of fluorescence

Abstract Fluorescence quenching with O 2 under high pressure was used for the study of reaction kinetics in the 10 picosecond region. Quenching of the dual fluorescence of 2-methyl-4-dimethylaminobenzonitrile (II) in acetonitrile was analysed. The process B * → A * occurs with a lifetime of 11 ps, while the solvent reorientation correlation time is only 1 ps, which supports the isomerisation mechanism which involves twisting the amino group. The lifetime of A * is 4.5 ns.

Nonradiative deactivation and triplet states in donor-aryl-acceptor compounds (dialkylaminobenzonitriles)

Abstract Intersystem crossing rates and triplet state properties of TICT (twisted intramolecular charge transfer) model compounds (dimethylaminobenzonitrile — DMABN — and its methyl substituted derivatives, 3-cyanoindoline, 3-cyanobenzquinuclidine — CBQ — and benzonitrile) were studied by various experimental and theoretical methods (emission spectroscopy, transient absorption techniques, EPR spectroscopy and semiempirical quantum chemical calculations) with the aim to obtain a quantitative deactivation pattern of this class of compounds in solvents of different polarity. The lowest triplet state, populated after excitation, is generally, a non-charge-transfer state, i.e. the 3 A 1 of planar DMABN, irrespective of solvent polarity and compound flexibility, with the only exception of the rigidly perpendicular CBQ, which relaxes to the 3 A 1 state of the benzonitrile moiety. In non-polar solvents intersystem crossing increases with increasing charge transfer character of the excited singlet; in ethanol, the intersystem crossing rate is about the same for all compounds studied, with the exception of benzonitrile, and in general significantly lower than in a non-polar environment. From the mechanistic point of view, intersystem crossing from 1 TICT can be described as a thermal charge recombination process in the normal Marcus region. With increasing solvent polarity, the decreasing energy gap between these two states then entails a decreasing intersystem crossing rate and an increasing 1 TICT lifetime, as experimentally observed. In hydroxylic solvents, internal conversion to the ground state is the second dominant deactivation pathway of 1 TICT states; it shows a significant deuterium isotope effect.

Gas-phase dipole moments of molecules in their fluorescent state

Abstract The effect of an external electric field on the fluorescence of molecules in the gas phase as well as in solutions is studied and the permanent dipole moments of 9-cyanoanthracene and 2,6-N,N-tetramethyl-4-cyanoaniline (TMCA) in their fluorescent states have been determined from such investigations. By comparison of the gas-phase and liquid-phase dipole moments, it could be shown that the Onsager model is adequate to describe the effective electric field, and the polarizability density related to the molecular volume could be determined. The values of the gas-phase and of the solution effective dipole moments of TMCA oppose the solute-solvent exciplex model and favour an emitting internal charge-transfer state.

Emissive Properties and Intramolecular Charge Transfer of Pyrazoloquinoline Derivatives

The fluorescence properties of several pyrazoloquinoline derivatives were performed by stationary as well as by time-resolved spectrosopy. Non-donor-substituted compounds show a high quantum yield; transition dipole moments of absorption and fluorescence as well as experimental and calculated lifetimes are in excellent agreement. The donor-substituted compound DMA-DPPQ exhibits a charge transfer fluorescence in polar solvents. Additionally, dual fluorescence appears in polar protic solvents. The nature of the charge transfer state is discussed with respect to the the TICT model.