Experimental and theoretical studies of electronic transition dipole moments of methyl benzoate derivatives

Abstract

The absorption and fluorescence transition dipole moments ( M ˆ g e and M ˆ e g ) for methyl benzoate and its eight ortho and para substituted derivatives have been investigated using steady-state and time-resolved fluorescence techniques and semiempirical quantum-chemical calculations. It is found that for the ortho-substituted (–OH and –OCH 3) derivatives of methyl benzoate the experimentally established absorption transition dipole moments of the S 1←S 0 electronic transition are in fair agreement with semiempirical computational predictions. It is important to note here that for these compounds, M ˆ g e values do not depend on the polarity of the medium indicating that the radiative transition probability is not affected by the environment. On the other hand, the electron donating para substituents (–NH 2 and –N(CH 3) 2) of methyl benzoate change the total absorption spectrum. The folded structure of the long-wavelength absorption band in studied solvents points the presence of two electronic transitions. Their presence is confirmed by the quantum-chemical calculations. The experimentally obtained values of the absorption and emission transition dipole moments of these molecules (i.e., M ˆ g e / M ˆ e g ≠ 1 ) suggest that the change of the electronic and molecular structure take place in the excited state.