Photophysics of α-azinyl-substituted 4,4-difluoro-8-(4-R-phenyl)-4-bora-3a,4a-diaza-s-indacenes

[Display omitted] •Several meso-(p-R-phenyl)- and α-azinyl-substituted BODIPYs have been synthesized.•Their fluorescence properties in C2Cl4 and benzene have been studied.•The nonradiative rate was correlated to the Hammett constant of the para-substituent. A series of 15 compounds of the general formula 8-(4-R1-phenyl)-3-R2-5-R3-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, where R1 = NMe2, Me, Br, CN, or NO2, and R2, R3 = H, acridin-9-yl, or 6-phenyl-[1,2,5]oxadiazolo[3,4-b]pyrazyne-5-yl, have been synthesized and investigated for their fluorescence properties in tetrachloroethylene and benzene solutions. For tetrachloroethylene solutions of 8-(4-dimethylaminophenyl)-substituted compounds, bright fluorescence from a locally excited state has been observed. The same compounds in benzene show intramolecular charge transfer with an increased Stokes shift and greatly reduced radiative rate and fluorescence quantum yield. The rest of the compounds are characterized only by locally-excited fluorescence both in tetrachloroethylene and benzene. Their nonradiative rate constant knr obeys the equation ln knr = a + bnacr + cσp, where σp is the Hammett constant of the para-substituent, nacr is the number of acridine substituents; a, b, c are fitting parameters. The linearity of the formula implies an additive effect of the substituents on the activation energy of the nonradiative decay.