Dual fluorescence and intramolecular charge transfer (ICT) are observed with aminobenzonitriles when two excited state levels (Sl and S2(CT) in DMABN) have an energy gap sufficiently small for vibronic coupling: a solvent-induced pseudo-Jan-Teller effect. It is argued that the N-inversion of the amino group acts as a promoting mode. These conclusions are based on a comparison of absorption spectra and photostationary and time-resolved fluorescence data. Dual fluorescence is also observed with MMD, in which the dimethylamino group is twisted towards a perpendicular configuration with respect to the phenyl ring.

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Abstract. 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 '(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, lO-cyan0-9,9'-bianthryl and 10,10'-dicyano-9,9'-bianthryl, photostationary and timeresolved 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.

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The fluorescence emission intensity between the Na(+), and the K(+) complex of Na(+),K(+)-ATPase, labeled with fluorescein 5'-isothiocyanate (FITC), differs by 30 to 40%. Experimental studies are carried out to elucidate the physical reasons which account this intensity difference. The dissociation constant of protolysis of the covalently bound FITC and its fluorescence decay times are determined in media of different ionic compositions and are compared with the corresponding properties of a synthetic model compound. The fluorophore bound to the protein is characterized by two decay times in the nanosecond range; the model compound, by a single one. The static fluorescence intensity changes are discussed on the basis of these results.

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