Phthalocyanine-Cored Fluorophores with Fluorene-Containing Peripheral Two-Photon Antennae as Photosensitizers for Singlet Oxygen Generation

A series of free base and Zn(II) phthalocyanines featuring fluorenyl antennae linked by methoxy or oxo bridges to the phthalocyanine core (Pc) were synthesized and characterized. Selected linear and nonlinear (two-photon absorption) optical properties of these new compounds were subsequently studied. As previously observed for related porphyrin dendrimers bearing 2-fluorenyl peripheral dendrons, an efficient energy transfer occurs from the peripheral antennae to the central phthalocyanine core following excitation in the fluorenyl-based π-π* absorption band of these chromophores. Once excited, these compounds relax to the ground state, mostly by emitting intense red light or by undergoing intersystem crossing. As a result, the tetrafunctionalized Zn(II) phthalocyanines are fluorescent, but can also efficiently photosensitize molecular oxygen in tetrahydrofurane (THF), forming singlet oxygen with nearly comparable yields to bare Zn(II) phthalocyanine ( ). In comparison with the latter complex, the positive role of the fluorenyl-containing antennae on one- and two-photon brightness (2PA) is presently demonstrated when appended in peripheral (β) position to the phthalocyanine core. Furthermore, when compared to known porphyrin analogues, the interest in replacing the porphyrin by a phthalocyanine as the central core to obtain more fluorescent two-photon oxygen photosensitizers is clearly established. As such, this contribution paves the way for the future development of innovative biphotonic photosensitizers usable in theranostics.