Excited-state dipole moments solvatochromic methods

McRae s equation, which uses the Onsager dielectric functions, has been used to interpret the solvatochromism of a number of coordination compounds. Meyer and one of us have also used Kirkwood s approach, which is simpler since it neglects dispersion forces. Lippert s method is used for species in which absorption and emission data are available for the same excited state when the difference between absorption and emission energies is known a number of terms in the equations will cancel, and it is then relatively simple to obtain the ground- and excited-state dipole moments. This method is not usually applicable for coordination complexes as, if emission is observed, the emitting state is usually different from the main absorption band, e.g., in many cases absorption is to a singlet state and emission is from the corresponding triplet. 

A statistical evaluation and comparison of solvatochromic methods used to determine excited-state dipole moments has been carried out by Koutek [167]. Solvent effects can be taken into consideration using the reaction field theory developed by Katritzky, Zemer, Szafran, and Karelson [176-179] and Siretskii, Kirillov, and Bakhshiev [180] have proposed an equation containing a cos where is the angle between the direction of the ground-state dipole moment and the excited-state dipole moment. The equation worked well for certain aromatic dyes but its general applicability has not been tested. 

Prabhumirashi and Kunte [181] have proposed a new procedure employing Bakhshiev s equation for solvatochromic frequency shifts for excited-state dipole moments and specific solute -solvent interaction energies based on absorption spectra only, without using emission spectra. Suppan [182] has expanded upon the solvatochromic shift method and discussed the effect of the medium on the energies of electronic states and Ghoneim and Suppan [183] discussed solvatochromic shifts of non-dipolar molecules in polar solvents. 

Ayachit and Tonannavar [184] have critically examined Suppan s method and developed two improvements intended to resolve the uncertainty concerning the direction of excited-state dipole moments. Kurbako et al. [18S] have developed a new solvent shift approach based on a theory of universal intermolecular interactions. Abe has proposed a method of estimating the angle between ground- and excited-state dipole moments based on improved solvatochromic... 

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