Absorption and fluorescence properties of non-symmetric benzo-, furo-, and thieno-fused structures at the b bonds in the BODIPY frame.

Abstract

The influence of furo-, thieno-, and benzo-fused structures at the b bonds in the BODIPY frame on the optical properties is investigated by TD-CAM-B3LYP and RI-CC2 calculations. The most important result is that substituents at the b bond of the BODIPY core affect strongly the S1-S2 gaps. In contrast to the S1 (local excited (LE-type)) state, energy of which is nearly the same for all the substituents at the b bond, energy of the S2 (charge transfer (CT-type)) state depends strongly on the nature of the substituent and decreases in the following order: furo-fused > thieno-fused > benzo-fused. In the last case the inversion of S1 and S2 levels occurs. No red shift of the main long-wave absorption transition and no substantial changes in its intensity can be predicted by the calculations for benzo[b]-derivative (vertical energy (Ev) is 2.95eV, oscillator strength (f) is 0.80) relative to furo- (Ev=2.97eV, f=0.69) and thieno-derivatives (Ev=2.95eV, f=0.65). However, the dramatic decrease of the fluorescence quantum yield is expected owing to positions of their LE-type (Ev=2.95eV, f=0.80) and CT-type (Ev=2.79eV, f=0.01) transitions. In the case of thieno-fused BODIPY, owing to the approach of the energy levels of the vertical S2 and S1 states, the energy equilibrium of the [1]CT-type state becomes lower than that of the [1]LE state, and Φf of the thieno-derivative may be substantially lower than Φf of the furo-derivative.