Analysis of the solvatochromism of 9,9'-biaryl compounds using a pure solvent dipolarity scale

The UV/vis absorption and fluorescence spectra of 9,9'-bisanthracenyl (BA), 9,9'- bisacridinyl (BAC), 9,9'-bicarbazyl (BC), and 9-(9'-anthracenyl)carbazole (C9A) were precisely recorded in solvents that change their solvent dipolarity scale (SdP) values from 0.000 (in 2-methylbutane at 293 K) to 1.294 (in 1-chlorobutane at 77 K). An analysis based (i) on the solvatochromic behavior of the four 9,9'-biaryl compounds in terms of the dipolarity of the solvents used, quantified by means of their SdP values, and (ii) on the behavior of the emission band profiles, allows the conclusion that, whereas the apolar 9,9'-biaryl compounds BA, BAC, and BC need solvents with SdP values larger than 0.74 to give a twisted intramolecular charge-transfer (TICT) process after excitation, for dipolar C9A the TICT mechanism is predominant already at solvent SdP values smaller than 0.23, i.e., near a solvent dipolarity of 0. The accuracy and simplicity achieved by application of the SdP scale in analyzing the solvatochromic behavior of the four 9,9'-biaryl compounds studied make evident the advantages of this solvent scale in comparison to other empirical solvent scales, which not only include the solvent's dipolarity but also consist of a more limited numerical range of values only.