Excited-State Dipole Moments of Dual Fluorescent 4-(Dialkylamino)benzonitriles. Influence of Alkyl Chain Length and Effective Solvent Polarity

Article abstract of DOI:10.1021/j100205a041

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 μ_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 / concentration ratio gained from time-correlated single-photon counting measurements in the picosecond time domain.A value of ca. 17 D was obtained for μ_S(CT) and of ca. 10 D for μ_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, μ_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 μ_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 Φ_ISC of the 4-aminobenzonitriles increases with the length of the N-alkyl substituent as well as with the effective solvent polarity.