Femto- to micro-second photobehavior of photosensitizer drug trapped within a cyclodextrin dimer

Abstract

The interactions of 5,10,15,20-tetrakis(4-sulfonatophenyl)-porphyrin (TSPP, a singlet oxygen photosensitiser molecule) with a hexa-2,4-diynediyl bridged β-cyclodextrin dimer (CD–CD) in aqueous solutions of pH 7 were studied using steady-state UV-visible absorption/emission and femto- to millisecond time-resolved spectroscopy. TSPP forms 1 : 1 complexes with CD–CD (K_e = 1.9 × 10⁸ M⁻¹ at 293 K). The value of K_e indicates a high affinity of TSPP to form complexes with CD–CD. The chemical nano-cavity has a notable effect on the fluorescence lifetimes of the Q_x state (9.3 ns in water and 10.8 ns in CD–CD). The rotational times (410 ps for TSPP in water and 0.03 ns (12%) and 1.1 ns (88%) for the TSPP:CD–CD complexes) indicate the robustness of the formed entities, and fast depolarization of emission, most probably involving the porphyrin skeleton and phenyl ring motions. The ultrafast femtosecond component (60–100 fs) of TSPP is moderately affected by the confining environment, which instead strongly influences the ps component (1–2 ps in water and 5 ps within CD–CD) assigned to the vibrational relaxation of the Q_x state. Moreover, a 50 ps component emerges in the emission transients in the 640–720 nm range, and which is assigned to a thermalization of the hot Q_x state. The effect of O₂ on the triplet state of the encapsulated TSPP was also studied and discussed in light of the shielding effect of the CD–CD cavity. We observed comparable quantum yield (0.62 and 0.69) of the generated singlet molecular oxygen of TSPP without and with CD–CD. We believe that our results on the molecular interaction between TSPP and CD–CD from femtosecond to millisecond regime at both ground and electronically first excited states give relevant information for improving our understanding of this kind of caged drugs, and thus for a better design of drug:nanocarrier complexes. A particular implication for the use of CD–CD as a drug carrier is the high affinity of this host for complex formation with TSPP, while the yield of singlet oxygen generation is still high.