Quantum chemical investigation of electronic structure of boron di-pyrromethene fluorophores with multi-reference (MCSCF) methods

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) centered chromophores attracted increasing attention in relation to their use in various nano scale molecular devices operating and/or signaling via interaction with light. The nature of the excited states populated, during the course of the excitation and deactivation processes has to be thoroughly investigated and understood in order to provide rational design strategies that yield enhanced applicational parameters. In this study; comparisons of BODIPY and tetra methyl BODIPY with DFT and TD-DFT calculations are reported with MCSCF technique. Vertical and adiabatic transitions between low lying states are inspected to understand the nature of the excitation process. Presumably, because of strong fluorescent property of the BODIPY core, no conical intersection the between ground singlet and the first excited singlet state could be located. Manual modification of BODIPY core coordinates yielded a non-optimized conical intersection at an inaccessibly high relative energy. With the detailed information on ground and excited states of BODIPY, realization of the excitation properties of a new class of orthogonal bis-BODIPYs that are promising as potential photosensitizers in photodynamic action was achieved. Guided by MCSCF calculations key design principles for singlet oxygen production by bis-chromophores are developed. Estimated intersystem crossing (ISC) capabilities of these bis-chromophores by quantum chemical modeling well agreed with the available experimental data. Our results show that details of the excited state wave functions of orthogonal bis-chromophores can help assessing their ISC ability and the related singlet oxygen generation properties. Improvement in fine tuning the excitation properties of bis-chromophores in order to absorb in longer wave lengths of the spectrum requires further combined experimental and theoretical studies.Key Words : BODIPY, Excited state, Chromophore, Fluorescence, Conical intersection, MCSCF, PDT, Singlet oxygen