The control of chemical structures of dual-functioning photosensitizers as both therapeutic and imaging agents

A series of BODIPY dimer derivatives were selected to investigate their molecular geometric structures, the spectral properties, the molecular frontier orbitals, spin–orbit coupling (SOC) matrix between the first singlet excited state (S1 state) and the first triplet excited state (T1 state) and the decay rate constant of the S1 state. BOP-3-3-BOP showed long absorption and emission spectra, significant fluorescence quantum yields (Φf) and respectable singlet oxygen quantum yields (ΦΔ) in comparison with other BODIPY dimers BOP-2-8-BOP, BOP-8-8-BOP and BOP-2-2-BOP. The C–C bond between the two units of BOP-3-3-BOP decreased in length while the ground state (S0 state) moved to the first excited state (S1 state) by absorbing energy, which closes to conjugated double C = C bond and extends π-conjugation between the two BODIPY units. The more planar between the two BODIPY units in the first excited state facilitates emitting the red fluorescent of BOP-3-3-BOP. The smaller singlet–triplet adiabatic energy gap and larger SOC value promoted the intersystem crossing (ISC) process and produced singlet oxygen with high efficiency for BOP-3-3-BOP. Furthermore, BOP-3-3-BOP has significant kf (S1→S0) and respectable kisc (S1→T1), which is expected to be used as both therapeutic and imaging agent. Our results on effectively regulating photophysical properties, ISC rates and the molecular excited state radiation decay rates by tuning different linkage position between two BODIPY units will be useful for molecular design of BODIPY dimers for fluorescence imaging guided photodynamic therapy.