Intramolecular donor-acceptor systems. 3. a third type of emitting singlet state for n-alkyl-6-n-arylamino-2-naphthalenesulfonates. Solvent modulation of substituent effects on charge-transfer emissions

Plots of the fluorescence maxima (as emission energies) for N-methyl-2-N-(X-phenylamino)-6-naphthalenesulfonates [1, R=CH3; X=4-Br, F, Cl, H, CH3) OCH3, 3-OCH3, 3-CH3, 3,5-(CH3)2] against the solvent polarity parameter, ET(30), for dioxane-water mixtures yield correlations which suggest emissions from S1,np and S1,ct states. Comparison of the extrapolated S1,Ct emissions (in hydrocarbon solvent) for 1 with those for the compounds lacking the N-methyl group leads to the conclusion that the nonbonding electrons of the nitrogen are not conjugated to the positively charged phenyl in the S1,ct state, which therefore is S1,ct(U). Correlating emission energies/2.301RT with the substituent constant σ+ yields a p value of -25±2, the highest ever noted for a solution process. The emission from the compound with H on nitrogen is assigned as arising from S1,ct(C) with new data confirming the original p value of -10.6 (C=conjugated; U=unconjugated). Other cases of isomeric charge-transfer states are cited. The p values correlating the energies of emissions from the S1,ct(C) and S1,ct(U) states exhibit a reasonably linear dependence on solvent polarity (as measured by the empirical parameter ET(30)), the dependence being different for the two states. The more localized the charge in the emitting state, the greater the variation in the p value with solvent. The correlation coefficient of -38±3 derived from the mass spectrometric data vs. σ+ is assumed to reflect the interaction of a substituent with a unit charge; other p values may now be interpreted in a quantitative way.