EXCITED-STATE DIPOLE-MOMENTS OF DUAL FLUORESCENT 4-(DIALKYLAMINO)BENZONITRILES - INFLUENCE OF ALKYL CHAIN-LENGTH AND EFFECTIVE SOLVENT POLARITY

Singlet excited-state dipole moments of a number of aminobenzonitriles have been determined in cyclohexane, benzene, and 1,4-dioxane, using time-resolved microwave conductivity (TRMC) and fluorescence spectroscopy techniques. For the 4-(dialkylamino)benzonitriles (methyl, ethyl, propyl, and decyl) intramolecular charge transfer (ICT) occurs in the excited singlet state even in the nonpolar solvent cyclohexane. The overall dipole moment mu(S) increases with the effective solvent polarity and with the length of the alkyl group, from 9.9 D in the case of 4-(dimethylamino)benzonitrile (DMABN) in cyclohexane to 17.2 D for 4-(didecylamino)benzonitrile (DDABN) in 1,4-dioxane. The individual dipole moments of the initially excited state (LE) and the charge transfer state (CT) were determined from the TRMC data using information on the [CT]/[LE] concentration ratio gained from time-correlated single-photon counting measurements in the picosecond time domain. A value of ca. 17 D was obtained for mu(S)(CT) and of ca. 10 D for mu(S)(LE). These values are to be compared with ground-state dipole moments, which have also been determined, of approximately 6.7 D. For 4-aminobenzonitrile (ABN), 4-(methylamino)benzonitrile (MABN), and 3-(diethylamino)benzonitrile (m-DEABN), none of which display evidence for ICT in the fluorescence measurements, mu(S) values of 8.3, 9.5, and 10 D independent of solvent are found and are assigned to the LE state alone. Possible mechanisms for the increase in charge separation in the 4-(dialkylamino)benzonitrile series, rotational isomerization and rehybridization of the amino group, are discussed. From long-lived components of the TRMC transients, a triplet-state dipole moment mu(T) Of 12 D has been estimated for DMABN in both cyclohexane and p-dioxane, indicating that ICT does not occur in the triplet state. The singlet-triplet intersystem crossing efficiency PHI(ISC) of the 4-aminobenzonitriles increases with the length of the N-alkyl substituent as well as with the effective solvent polarity.