Investigating XC-functionals towards describing experimentally relevant excited-state properties of NIR-BODIPY derivatives

The predictive and analytical power of time-dependent density functional theory (TD-DFT) has been instrumental in the design and mechanistic understanding of numerous organic chromophores. Yet, the widely popular boron-dipyrromethene (BODIPY) dye class suffers from notorious TD-DFT accuracy issues, undermining the serviceability of the technique. Highly correlated wave function approaches are much better at reproducing photophysical properties but become computationally unviable when making the push towards larger near-infrared (NIR) active structures. In an effort to find the protocol most capable of helping experimentalists design and analyze novel NIR BODIPYs, we have benchmarked 11 global or range-separated hybrid exchange-correlation functionals (XCFs) with different amounts of Hartree-Fock exchange. By relating both transition energies and oscillator strengths, first through a set of resolution-of-the-identity second-order coupled cluster (riCC2) calculations and then directly to experimental data, it is revealed that M06-2X and M06-HF behave most consistently for singlet and triplet excitations. To optimize accuracy across states, we recommend a hybrid approach where singlets are obtained through full TD-DFT and triplets are treated using the Tamm-Dancoff approximation.