Solvatochromic Effect in a Benzimidazole-Based Betaine: Determination of the Dipole Moments and Second-Order Hyperpolarizability

Abstract

Results of measurements of the solvent-dependent shift of the low-energy absorption band solvatochromic effect in a benzimidazole-based betaine are reported in the paper. Measurements of absorption spectra in several solvents of different polarities were performed. The solvatochromic shift of the low-lying absorption band was found to exceed 3000 cm -1 ; the results obtained were then employed to calculate the ground-and excited-state dipole moment of the molecule. The spectroscopic measurements were supplemented by measurements of the ground-state dipole moment. A remarkable change in the charge distribution was found to occur upon the electronic excitation: both the experimental results and the calculations indicate that the ground-state dipole moment is close to 13 D, whereas in the excited state it amounts to ca. 3 D. The second-order hyperpolarizability of the molecule was calculated from the measurements; its off-resonance value amounts to ca. 20 x 10 -40 m4 /V (4.8 x 10 -30 esu), depending on the solvent used.

Solvatochromism of a Zwitterionic Benzimidazole-Based Pyridinium Betaine Dye: UV−Vis Spectroscopic Measurements and Quantum-Chemical Calculations

“…Transition Energies and Oscillator Strengths. As was already mentioned in our earlier paper, the UV−vis absorption spectrum of IB1 calculated for an isolated molecule contains medium- or low-intensity transitions below 22 000 cm -1 . The results of our calculations demonstrate that one can safely neglect the S 0 → S 2 transition since its strength is insignificant irrespective of the molecular conformation (cf.…”

“…around 31 000 cm -1 , where the dominant electronic transition is the local one. In the low-energy region, where a weak CT absorption band is observed, the calculations performed for an isolated molecule, reported in our previous paper, underestimated the energy of the excited states and overestimated the intensity of the S 0 → S 1 transition. The results given in this paper clearly demonstrate that a reasonable agreement can be achieved only after taking into account the solute−solvent interactions and the effect of the conformational changes.…”

“…In our previous paper, we reported on the determination of the ground- and excited-state dipole moments, as well as the second-order hyperpolarizability of IB1. However, we observed a difference between the experimental results and quantum-chemical calculations performed for the free IB1 molecule, possibly resulting from solute−solvent interactions.…”