| Abstract
| - Enhanced two-photon-absorbing (2PA) systems with triplet cores are currently under scrutiny for severalbiomedical applications, including photodynamic therapy (PDT) and two-photon microscopy of oxygen. Theperformance of so far developed molecules, however, is substantially below expected. In this study we takea 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 foroxygen. In these constructs, energy transfer from several 2PA chromophores is used to enhance the effective2PA cross section of phosphorescent metalloporphyrins. Previous studies have indicated that intramolecularelectron transfer (ET) can act as an effective quencher of phosphorescence, decreasing the overall sensorefficiency. We studied the interplay between 2PA, energy transfer, electron transfer, and phosphorescenceemission using Rhodamine B−Pt tetrabenzoporphyrin (RhB−PtTBP) adducts as model compounds. 2PAcross sections (σ2) of tetrabenzoporphyrins (TBPs) are in the range of several tens of GM units (near 800nm), making TBPs superior 2PA chromophores compared to regular porphyrins (σ2 values typically 1−2GM). Relatively large 2PA cross sections of rhodamines (about 200 GM in 800−850 nm range) and theirhigh photostabilities make them good candidates as 2PA antennae. Fluorescence of Rhodamine B (λfl = 590nm, φfl = 0.5 in EtOH) overlaps with the Q-band of phosphorescent PtTBP (λabs = 615 nm, ε = 98 000 M-1cm-1, φp ∼ 0.1), suggesting that a significant amplification of the 2PA-induced phosphorescence viafluorescence 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 nonconductingdecaproline spacers (Pro10) between RhB and PtTBP, the intramolecular ETs were suppressed, while thechromophores were kept within the Förster r0 distance in order to maintain high FRET efficiency. The resultingassemblies exhibit linear amplification of their 2PA-induced phosphorescence upon increase in the numberof 2PA antenna chromophores and show high oxygen sensitivity. We also have found that PtTBPs possessunexpectedly strong forbidden S0 → T1 bands (λmax = 762 nm, ε = 120 M-1 cm-1). The latter may overlapwith the laser spectrum and lead to unwanted linear excitation.
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