Abstract
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 (ZnPc). 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.
Highlights
Sixteen peripheral fluorenyl groups [23], respectively, evidenced that 2-fluorenyl units were able to enhance the luminescence quantum yield of the central tetraphenylporphyrin (TPP) core when appended by methoxy bridges to the meso-phenyl rings of a central H2 TPP core (Figure 1)
The similarity between the absorption and emission spectra of ZnTOFPc1 and ZnTOFPc2 confirm that the introduction of n-butyl chains has almost no effect on their electronic structure, apart from perhaps slightly decreasing their fluorescence quantum yield
Given that the fluorescence of these phthalocyanines originates from their lowest singlet state (Q(0,0) band) after minimal structural reorganization and given that the transition to this singlet state results in a strong absorption a low energy, the linear optical properties of interest are directly linked to the highestoccupied occupiedmolecular molecular orbital orbital (HOMO)–LUMO gap of the various phthalocyanines
Summary
Molecules 2020, 25, 239 studying new porphyrin- or phthalocyanine-based molecular assemblies that might efficiently undergo two-photon excitation [10,11,12,13] while presenting a significant luminescence, constitutes an appealing goal for researchers interested in developing new photosensitizers for theranostic applications [4,5,14,15,16] Along these lines, some of us [17,18] and others [19], independently reported that fluorenyl units, when appended at the meso positions of porphyrins, were suited to enhance the fluorescence quantum yields of this macrocyclic core, most likely by limiting the rate of non-radiative decay via internal conversion [20]. Sixteen peripheral fluorenyl groups [23], respectively, evidenced that 2-fluorenyl units were able to enhance the luminescence quantum yield of the central tetraphenylporphyrin (TPP) core when appended by methoxy bridges to the meso-phenyl rings of a central H2 TPP core (Figure 1).
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