Substituent Effect on the Dual Fluorescence of Benzanilides and N-Methylbenzanilides in Cyclohexane. Direct Evidence for Intramolecular Charge Transfer

Abstract

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 σ 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 F might reflect the extent of charge separation of the CT reaction and the F 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.

Intramolecular charge transfer with N-benzoylaminonaphthalenes. 1-Aminonaphthalene versus 2-aminonaphthalene as electron donors

“…11-20 The longwavelength fluorescence of 1-NBAs and 2-NBAs were found to shift to the red with increasing electron withdrawing ability of the substituent at the benzoyl moiety, and was therefore assigned to the intramolecular charge transfer (ICT) state with aminonaphthalene and benzoyl moiety being electron donor and acceptor, respectively, as has been done similarly with BAs. [16][17][18][19][20] The CT nature was further supported by the observation of a substantial red shift of the long-wavelength emission with increasing solvent polarity, for example, the ICT emission of 1-NBA shifted from 514 nm in pure cyclohexane to 522, 546 and 558 nm, respectively, when 1, 5 and 20%(v/v) of diethylether was introduced into cyclohexane. Importantly, linear free energy correlations between the CT emission energies and the Hammett constants 25 of the as given in eqn.…”

“…3 kcal mol Ϫ1 higher in total energy than trans-NBAs, revealing that NBAs exist preferentially in trans-conformation, as BAs did. 17, 20 The ground state structures of all trans-NBA derivatives were optimized with the AM1 method and the dihedral angles and the dipole moments were compiled in Table 1. It follows from Table 1 that the benzoyl substitution does not result in an obvious change in the ground-state structures within the same series, whereas the dihedral angle ЄC1-C2-N3-C4 of 1-NBAs is much higher than that in 2-NBAs, suggesting the steric effects of the substitution ortho to the amino group and/or of the peri-H at C-8 in 1-NBAs.…”

“…We recently showed that with benzoylsubstituted benzanilides (BAs, Scheme 1) the CT emission energies varied linearly with the Hammett constants of the substituents at the benzoyl moiety [eqn. (1)]: 20 In eqn. (1) E 0,0 is the (0,0)-transition energy, δE rep is the repulsion energy of the Franck-Condon ground state reached after…”

“…The linear slope indicated the charge transfer extent in the CT state while the intercept indexed the electron donating ability of the un-substituted aniline moiety. 20 We therefore decided to employ the CT reaction in BAs to probe the difference of 1-AN and 2-AN as electron donors by preparing N-benzoylaminonaphthalene derivatives with substituent at the benzoyl moiety and examining the substituent effects on the charge transfer fluorescence.…”

Enhancement of intramolecular charge transfer strength in diphenylamine substituted symmetric 1,3,4-oxadiazole derivatives

“…11 Hence, plenty efforts focus on studying organic materials with intramolecular charge transfer characteristic have been conducted. [12][13][14][15][16][17] Among all these materials investigated, organic small molecules have been considered as an important kind of intramolecular charge transfer materials because of their well-dened structures, easy adjustability and purication. [18][19][20] Commonly, the molecular structures of organic small molecules with intramolecular charge transfer characteristic contain electron donors (D) and electron acceptors (A), which are linked through p-bridge (D-p-A).…”