Structure of Organoboron Dyes and Multiphoton Absorption: Insights from Theory.

Computer simulations play an essential role in the interpretation of experimental multiphoton absorption spectra. In addition, models derived from theory allow for the establishment of "structure-property" relationships. This work contributes to these efforts and presents the results of an analysis of two- and three-photon absorptions for a set comprising 450 conjugated molecules performed at the CAM-B3LYP/aug-cc-pVDZ level. The molecular set is composed of organoboron dyes presenting various core topologies combined with a palette of conjugated linkers giving donor-acceptor architectures. The charge-transfer character of the investigated structures is manifested by the presence of the low-lying electronic excited state. The multiphoton excitation to the state in question is intense and significant from an application point of view. The analysis performed in this work clearly demonstrates that there is a strong correlation between the intensities of the two- and three-photon transitions to the lowest intramolecular charge-transfer state, hinting that developed design rules aiming at maximizing two-photon absorption efficiency will also be useful in designing three-photon absorbers. As part of this study, we also performed two-photon absorption calculations using the coupled-cluster RI-CC2 model with the aug-cc-pVDZ basis set for 450 molecules to guide the selection of the density functional approximation.