6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

Recent advances in time-dependent density functional theory (TDDFT) have led to computational methods that can predict properties of photoexcited molecules with satisfactory accuracy at comparably moderate cost. We apply these methods to study the photophysics and photochemistry of 4-(dimethyl)aminobenzonitrile (DMABN). DMABN is considered the paradigm of photoinduced intramolecular charge transfer (ICT), leading to dual fluorescence in polar solvents. By comparison of calculated emission energies, dipole moments, and vibrational frequencies with recent results from transient spectroscopy measurements, a definitive assignment of the electronic and geometric structure of the two lowest singlet excited states of DMABN is possible for the first time. We investigate the mechanism of the ICT reaction by means of minimum energy path calculations. The results confirm existing state-crossing models of dual fluorescence. Our study suggests that analytical TDDFT derivative methods will be useful to predict and classify emissive properties of other donor-acceptor systems as well.

Photoinduced intramolecular charge transfer in 4-(dimethyl)aminobenzonitrile - a theoretical perspective

The fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles is discussed. The magnitude of the energy gap between the two lowest excited states is shown to determine the occurrence or absence of ICT. The excited state behaviour of a series of six 4-aminobenzonitriles in which the amino nitrogen atom is part of a three- to eight-membered heterocyclic ring, P3C to P8C, is studied by using photostationary and time-resolved fluorescence measurements. The ICT rate constant strongly decreases with decreasing ring size. ICT does not occur with P3C and P4C in diethyl ether. This is attributed to the increase of the amino nitrogen inversion barrier with decreasing ring size. The configurational change of the amino nitrogen from pyramidal to planar is considered to be an important reaction coordinate in the ICT process. The photophysics of the 4-aminobenzonitriles is different from that of other systems such as donor/acceptor-substituted stilbenes and 9,9′-bianthryl, which are governed by the charge distribution and macroscopic Coulombic interaction in their CT states.

Intramolecular charge transfer in the excited state Kinetics and configurational changes

The PICT model for dual fluorescence of aminobenzonitriles.

The intramolecular charge transfer (ICT) reaction from the locally excited state (LE) to the charge transfer state (CT) in the singlet excited state is investigated for the dual fluorescent 4-(dial...

Intramolecular charge transfer in dual fluorescent 4-(dialkylamino)benzonitriles.

The energy of the charge transfer (CT) emission maximum of a series of dual fluorescent 4-aminobenzonitriles in diethyl ether and acetonitrile does not show a correlation with the redox potentials of the amino (D) and benzonitrile (A) subgroups. It is therefore concluded that these electron donor and acceptor subgroups cannot be treated in the same way as the A and D molecules in exciplexes 1(A−D+). This means that in the intramolecular charge transfer (ICT) state of the aminobenzonitriles, a considerable electronic coupling between the A− and D+ parts exists, in contradiction with the twisted intramolecular charge transfer (TICT) hypothesis. For 9,9′-bianthryl, 10-cyano-9,9′-bianthryl and 10,10′-dicyano-9,9′-bianthryl, photostationary and time-resolved measurements are presented, which show that the influence of the cyano substituents on the ICT reaction in the excited state can be understood by considering the redox potentials of the anthryl groups. The introduction of an additional cyano group in an aminobenzonitrile, however, leads to the disappearance of ICT and dual fluorescence, indicating that the photophysics of these molecules cannot be understood on the basis of the redox potentials of the D and A constituents.

Excited-state intramolecular charge transfer in donor acceptor-substituted aromatic hydrocarbons and in biaryls The significance of the redox potentials of the D/A subsystems

W. Kuehnle

Papers

Excited-state intramolecular charge transfer in donor acceptor-substituted aromatic hydrocarbons and in biaryls The significance of the redox potentials of the D/A subsystems