Preparation of Xanthene-TEMPO Dyads: Synthesis and Study of the Radical Enhanced Intersystem Crossing.

We prepared a rhodamine-TEMPO chromophore-radical dyad (RB-TEMPO) to study the radical enhanced intersystem crossing (REISC). The visible light-harvesting chromophore rhodamine is connected with the TEMPO (a nitroxide radical) via a C-N bond. The UV-vis absorption spectrum indicates negligible electron interaction between the two units at the ground state. Interestingly, the fluorescence of the rhodamine moiety is strongly quenched in RB-TEMPO, and the fluorescence lifetime of the rhodamine moiety is shortened to 0.29 ns, from the lifetime of 3.17 ns. We attribute this quenching effect to the intramolecular electron spin-spin interaction between the nitroxide radical and the photoexcited rhodamine chromophore. Nanosecond transient absorption spectra confirm the REISC in RB-TEMPO, indicated by the detection of the rhodamine chromophore triplet excited state; the lifetime was determined as 128 ns, which is shorter than the native rhodamine triplet state lifetime (0.58 µs). The zero-field splitting (ZFS) parameters of the triplet state of the chromophore were determined with the pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectra. RB-TEMPO was used as a photoinitiator for the photopolymerization of pentaerythritol triacrylate (PETA). These studies are useful for the design of heavy atom-free triplet photosensitizers, the study of the ISC, and the electron spin dynamics of the radical-chromophore systems upon photoexcitation.

Mechanistic Investigation of Sensitized Europium Luminescence: Excited State Dynamics and Luminescence Lifetime Thermometry.

Fluorescent nanothermometers based on thermal-dependent lifetime have a significant advantage in biological imaging owing to their immunity toward scattering, absorption, and autofluorescence. In this study, we present the first example of a water-soluble europium complex ([L1Eu]-) that exhibits high sensitivity (1.2% K-1 at 298 K) based on a temperature-dependent lifetime in the millisecond time range. This complex and its analogues show considerable potential for organelle imaging. The mechanism behind this thermal-sensitive behavior has been extensively investigated using transient absorption spectroscopy and variable temperature time-resolved luminescence methods. A highly efficient ligand sensitization process and a thermally activated back energy transfer process have been demonstrated. This study bridges the gap in small molecule thermometers with lifetimes longer than 1 ms and provides guidance in ligand design for metal coordination complex thermometers.

Photoelectrochemical Cells Utilizing Tunable Corroles.

Organic dyes with their wide range of molecular structures and spectroscopic features show great promise for solar energy applications. Corroles, structural analogues to porphyrins, are highly fluorescent molecules with tunable properties. We have synthesized a series of structurally similar corroles chelating gallium and phosphorus, along with a ß-chlorinated phosphorus corrole, and determined their photophysical and electrochemical properties. The electrochemical potentials to oxidize the corroles range from 0.78 V vs NHE for the gallium corrole to 1.42 V for the ß-octachlorinated phosphorus corrole. We are interested in developing photosensitizers for water oxidation on a metal oxide-based photoanode, so the corroles were modified to contain a meso-phenyl-COOH substituent for binding to metal oxide surfaces. The ability of these corrole dyes to act as photosensitizers was assessed by comparing the corroles in a model dye sensitized solar cell design. Transient absorption spectroscopy was utilized to analyze recombination dynamics and determine the kinetics of iodide oxidation. The most efficient photoelectrochemical cell was achieved for the phosphorus corrole P-2 with electrochemical properties and kinetics suitable for both photoinduced electron injection into TiO2 and oxidation of iodide. This structure-function study highlights the wide window for tuning corrole electrochemical potentials while still maintaining desirable photophysical properties, important variables when designing dyes for applications in photoelectrochemical water-oxidation cells.