Energy and Electron Transfer in Enhanced Two-Photon-Absorbing Systems with Triplet Cores

Abstract

Enhanced two-photon-absorbing (2PA) systems with triplet cores are currently under scrutiny for several biomedical applications, including photodynamic therapy (PDT) and two-photon microscopy of oxygen. The performance of so far developed molecules, however, is substantially below expected. In this study we take a detailed look at the processes occurring in these systems and propose ways to improve their performance. We focus on the interchromophore distance tuning as a means for optimization of two-photon sensors for oxygen. In these constructs, energy transfer from several 2PA chromophores is used to enhance the effective 2PA cross section of phosphorescent metalloporphyrins. Previous studies have indicated that intramolecular electron transfer (ET) can act as an effective quencher of phosphorescence, decreasing the overall sensor efficiency. We studied the interplay between 2PA, energy transfer, electron transfer, and phosphorescence emission using Rhodamine B−Pt tetrabenzoporphyrin (RhB−PtTBP) adducts as model compounds. 2PA cross sections (σ₂) of tetrabenzoporphyrins (TBPs) are in the range of several tens of GM units (near 800 nm), making TBPs superior 2PA chromophores compared to regular porphyrins (σ₂ values typically 1−2 GM). Relatively large 2PA cross sections of rhodamines (about 200 GM in 800−850 nm range) and their high photostabilities make them good candidates as 2PA antennae. Fluorescence of Rhodamine B (λ_fl = 590 nm, φ_fl = 0.5 in EtOH) overlaps with the Q-band of phosphorescent PtTBP (λ_abs = 615 nm, ε = 98 000 M^-1 cm^-1, φ_p ∼ 0.1), suggesting that a significant amplification of the 2PA-induced phosphorescence via fluorescence resonance energy transfer (FRET) might occur. However, most of the excitation energy in RhB−PtTBP assemblies is consumed in several intramolecular ET processes. By installing rigid nonconducting decaproline spacers (Pro₁₀) between RhB and PtTBP, the intramolecular ETs were suppressed, while the chromophores were kept within the Förster r₀ distance in order to maintain high FRET efficiency. The resulting assemblies exhibit linear amplification of their 2PA-induced phosphorescence upon increase in the number of 2PA antenna chromophores and show high oxygen sensitivity. We also have found that PtTBPs possess unexpectedly strong forbidden S₀ → T₁ bands (λ_max = 762 nm, ε = 120 M^-1 cm^-1). The latter may overlap with the laser spectrum and lead to unwanted linear excitation.