Substituent effect on the dual fluorescence of benzanilides and N-methylbenzanilides in cyclohexane. Direct evidence for intramolecular charge transfer
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A series of benzoyl para- and meta-substituted benzanilides (BAs) and N-methylbenzanilides (MBAs) were synthesized and their absorption and fluorescence spectra in nonpolar solvent cyclohexane were investigated. Quantum mechanical calculations indicated that the ground state BAs existed preferentially in the trans configuration, whereas MBAs existed in the cis configuration, and benzoyl substitution hardly changed the ground-state structure in the same series. It was observed that all of the synthesized compounds displayed dual fluorescence in cyclohexane, i.e., a normal emission at ca. 330 nm and an abnormal long-wavelength fluorescence at around 500 nm. Although the normal emission of both BAs and MBAs did not show obvious variation, the long-wavelength emission shifted strongly to the red with increasing electron-withdrawing ability of the substituent at the benzoyl moiety. The emission energies of the long-wavelength fluorescence of BAs and MBAs in cyclohexane were found to vary linearly with the Hammett substituent coefficient sigma with almost the same slopes of -0.34 eV for both lines. In diethyl ether, similar linear correlation was also found for BAs with a slope of -0.36 eV that is close to that in cyclohexane. It was concluded that the long-wavelength emission of both BAs and MBAs in cyclohexane originated from a single intramolecular charge transfer state of similar configuration for both BAs and MBAs and of high extent of charge separation. The latter was supported by a linear slope of -0.67 for the correlation of long-wavelength emission energies of BAs against their reduction potentials. It was suggested that the reaction constant rho might reflect the extent of charge separation of the CT reaction and the rho value for a full charge separation was calculated to be -8.5 and +8.5 for a substituent at the electron donor and acceptor moiety, respectively.