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

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Molecular and biomolecular spectroscopy, 2020-10, Vol.239, p.118472, Article 118472
• Main Authors: Petrushenko, I.K., Petrushenko, K.B.
• Format: Article
• Language: English
• Subjects: Absorption; BODIPY; Charge transfer transitions; Fluorescence; RI-CC2; TD-CAM-B3LYP
• ISSN: 1386-1425
• DOI: 10.1016/j.saa.2020.118472

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 2.95 eV, oscillator strength (f) is 0.80) relative to furo- (Ev = 2.97 eV, f = 0.69) and thieno-derivatives (Ev = 2.95 eV, f = 0.65). However, the dramatic decrease of the fluorescence quantum yield is expected owing to positions of their LE-type (Ev = 2.95 eV, f = 0.80) and CT-type (Ev = 2.79 eV, 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. [Display omitted] •We performed a computer study of the absorption and fluorescence of b fused BODIPYs.•We explained effects of b fused aromatics in the BODIPY core on optical properties.•Fluorescence quantum yields of BODIPYs are linked with nature of b fused aromatics.