Circularly Polarized Luminescence from Axially Chiral BODIPY DYEmers: An Experimental and Computational Study

With our new home‐built circularly polarized luminescence (CPL) instrument, we measured fluorescence and CPL spectra of the enantiomeric pairs of two quasi‐isomeric BODIPY DYEmers 1 and 2, endowed with axial chirality. The electronic circular dichroism (ECD) and CPL spectra of these atropisomeric dimers are dominated by the exciton coupling between the main π–π* transitions (550–560 nm) of the two BODIPY rings. Compound 1 has strong ECD and CPL spectra (glum=4×10−3) well reproduced by TD‐DFT and SCS‐CC2 (spin‐component scaled second‐order approximate coupled‐cluster) calculations using DFT‐optimized ground‐ and excited‐state structures. Compound 2 has weaker ECD and CPL spectra (glum=4×10−4), partly due to the mutual cancellation of electric–electric and electric–magnetic exciton couplings, and partly to its conformational freedom. This compound is computationally very challenging. Starting from the optimized excited‐state geometries, we predicted the wrong sign for the CPL band of 2 using TD‐DFT with the most recommended hybrid and range‐separated functionals, whereas SCS‐CC2 or a DFT functional with full exact exchange provided the correct sign. Chiral light excitement: Fluorescence and circularly polarized luminescence (CPL) spectra of BODIPY DYEmers 1 and 2 were measured with a new home‐built CPL instrument (see figure). Compound 1 has strong electronic circular dichroism (ECD) and CPL spectra (glum=4×10−3) well reproduced by M06‐2X and SCS‐CC2 calculations. Compound 2 has weaker ECD and CPL spectra (glum=4×10−4), due to the mutual cancellation of μ–μ and m–μ exciton couplings, and to its conformational freedom. The correct CPL sign could be obtained only with SCS‐CC2 or M06‐HF calculations.