Determination of ground and excited state dipole moments of dipolar laser dyes by solvatochromic shift method

• Solvent effect on absorption and fluorescence spectra of three coumarins studied.
• Bathochromic and hypsochromic shifts observed with increasing solvent polarity.
• Onsager radii from ab initio calculations were used to determine dipole moments.
• Excited state dipole moments are larger than their ground state counterparts in solvents studied.
• Experimental dipole moments and those from ab initio calculations are compared.

The absorption and fluorescence spectra of three medium sized dipolar laser dyes: coumarin 478 (C478), coumarin 519 (C519) and coumarin 523 (C523) have been recorded and studied comprehensively in various solvents at room temperature. The absorption and fluorescence spectra of C478, C519 and C523 show a bathochromic and hypsochromic shifts with increasing solvent polarity indicate that the transitions involved are π → π∗ and n → π∗. Onsager radii determined from ab initio calculations were used in the determination of dipole moments. The ground and excited state dipole moments were evaluated by using solvatochromic correlations. It is observed that the dipole moment values of excited states (μe) are higher than corresponding ground state values (μg) for the solvents studied. The ground and excited state dipole moments of these probes computed from ab initio calculations and those determined experimentally are compared and the results are discussed.

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