THE EFFECT OF ACCEPTOR GROUP VARIATION ON THE SOLVATOCHROMISM OF DONOR-ACCEPTOR FLUOROPHORES

The absorption and emission characteristics of five hydroxytetrahydroc hrysenes substituted with acceptor groups (nitro, cyano, methylketone, 1 degrees amide and methyl ester) (THC-NO2, THC-CN, THC-COCH3, THC-CO NH2 and THC-CO2CH3, respectively) were investigated in an extensive se t of solvents. The order of absorption and fluorescence bathochromicit y are: THC-NO2 > THC-COCH3 > THC-CN greater than or equal to THC-CO2CH 3 > THC-CONH2 and THCNO2 much greater than> THC-COCH3 > THC-CO2CH3 > T HC-CN, THC-CONH2, respectively. The emission spectra of these compound s are sensitive to the solvent polarity(E(r)[30] scale) in the order: THC-NO2 > THC-COCH3 > THC-CO2CH3, > THC-CONH2 > THC-CN. The response o f the emission maxima of these compounds to the solvent polarity and h ydrogen-bond donor/acceptor properties (x/alpha/beta and acity/basity scales) was also determined. The emission energies of THC-NO2 were mo st sensitive to x, beta, acity, and basity of the solvent; those of t he amide were least sensitive to the solvent x, beta, and basity. The ground- and excited-state dipole moments were determined by semiempir ical molecular orbital calculations and the absorption/fluorescence so lvent-shift method, respectively. THC-NO2 had the largest ground- and excited-state moments. The ester and amide had the smallest ground- an d excited-state moments, respectively. In general, unsatisfactory resu lts were obtained for correlations of the emission and absorption ener gies, fluorescence solvatochromism and the ground- and excited-state d ipole moments with the Hammett substituent constants of the five accep tor groups. Acceptable correlations were obtained for the absorption a nd emission energies and the fluorescence solvatochromism with the sub stituent constants if the cyano compound was excluded.