Mechanisms of large red-shift and intensity enhancement of absorption bands by π-bond breaking in a benzoporphyrin sensitizer for photodynamic therapy

Abstract

A free-base monobenzoporphyrin photosensitizer called verteporfin for photodynamic therapy (PDT) has a benzopyrrole ring with a partial π-bond breaking. The π-bond breaking significantly red-shifts the absorption band (so-called Qy-band) to 688 nm and enhances the absorption intensity. In this paper, we theoretically study the mechanisms of the red-shift and intensity enhancement in verteporfin. Density functional analysis at the B3LYP-6-31G(d,p) level of theory reveals that the π-bond breaking increases the energies of the highest occupied molecular orbital (HOMO) and second lowest unoccupied molecular orbital (LUMO+1) by 0.28 and 0.11 eV, respectively, and decreases the energy of the lowest unoccupied molecular orbital (LUMO) by 0.08 eV, in contrast to the unmoving second lowest occupied molecular orbital (HOMO-1). In the consequence of the higher energy shift of the HOMO and lower energy shift of the LUMO, the HOMO–LUMO gap is significantly narrowed. Therefore, the absorption red-shift in verteporfin is attributed to the HOMO–LUMO gap narrowing. On the other hand, the molecular-orbital shifts split the four electronically excited states, (HOMO, LUMO), (HOMO, LUMO+1), (HOMO-1, LUMO), and (HOMO-1, LUMO+1). The splitting weakens the configuration interactions between the excited states, which enhances the transition probability of the Qy-band.