Effect of nitro groups on the photo physical properties of benzimidazolone: A solvatochromic study

Electronic absorption and fluorescence spectra of mono, di, and tri-nitro benzimidazolones are measured at room temperature (298 K) in nine solvents with different polarities and the observed shifts are compared with benzimidazolone. Ground and excited state electric dipole moments are determined using the solvatochromic method based on the bulk solvent properties, F1(ɛ, n) and F2(ɛ, n). A reasonable agreement is observed between the experimental and ab initio   dipole moments. Change in dipole moment is also determined using the solvatochromic method based on the microscopic solvent polarity parameter, (ETN), which considers the polarization changes due to hydrogen bonding in different solvents. It has been observed that the correlation of the solvatochromic Stokes shifts with the parameter (ETN), is superior to that derived using bulk solvent polarity functions for all the benzimidazolones reported in the present study. Calculated difference between excited state and ground state dipole moments seems to be a good measure of the effect of nitro group when correlated with (ETN).

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► First nitro group decreases the dipole moment of benzimidazolone.
► Enhanced dipole moment of dinitro compound explained in terms of quinone-imine forms.
► Trinitro shows lower dipole moment due to competing ICT species in excited state.
► Nitro groups have profound influence on magnitude and direction of dipole moment.