Unraveling the Photophysical Characteristics, Aromaticity, and Stability of π-Extended Acene-Quinodimethyl Thioamides†.

Poly-aromatic systems that contain quinodimethyl (QDM) units are appealing for several photonic and spintronic applications owing to the unique electronic structure, aromaticity, and spin state(s) of the QDM ring. Herein, we report the synthesis and characterization of novel QDM-based chromophores 1-3, which exhibit unique photo-excited behavior and aromaticity. Extending the aromatic core with a biphenyl/phenanthryl- and a pyrrolo-fragment led to reducing the optoelectronic bandgap and modulating the photophysics QDM 1-3. Yet, QDM 2 and 3 suffer from "aromaticity imbalance" and become relatively unstable compared to the parent compound QDM 1. Further assessment of local aromaticity using computational tools revealed that the pseudo-quinoidal ring B is the main driving force allowing to easily populate the excited triplet state of these chromophores. The present study provides complementary guidelines for designing novel non-classical poly-aromatic systems.

Photophysical Insights of Halogenated Dipyrrolonaphthyridine-Diones as Potential Photodynamic Therapy Agents†.

We report the synthesis and photophysical characterization of novel halogenated dipyrrolonaphthyridine-diones (X2 -DPNDs, X = Cl, Br, and I), as candidates for photodynamic therapy (PDT) application. Apart from the heavy atom-induced spin-orbit coupling (SOC) dynamics in the investigated X2 -DPNDs, it was found that the position of the halogen atom (relative to the nitrogen of the pyrrole ring) also influenced the triplet excited state behavior. Interestingly, the faster/efficiency sensitization of 3 O2 to 1 O2 using X2 -DPND correlates with the rate of triplet population, kISC >1.6 × 108 s-1 for I2 -DPND vs kISC >2.9 × 109 s-1 for Cl2 -DPND and Br2 -DPND (where τISC  = 343 ± 3 ps for I2 -DPND and τISC  = 5-6 ns for Cl2 -DPND and Br2 -DPND are the lowest time constants/values for ISC). Furthermore, the heavy atom-induced SOC in Cl2 -DPND and Br2 -DPND did not lead to a reduction of the corresponding fluorescence (ca 75% vs 67% for the parent DPND). The attractive photophysical characteristics of Cl2 /Br2 -DPND put them on the landscape as not only promising PDT agents but also as fluorescence probes. The present study is a stepping stone in the development of novel organic photosystems for synergistic photomedicinal applications.

Organopolymer with dual chromophores and fast charge-transfer properties for sustainable photocatalysis.

Photocatalytic polymers offer an alternative to prevailing organometallics and nanomaterials, and they may benefit from polymer-mediated catalytic and material enhancements. MPC-1, a polymer photoredox catalyst reported herein, exhibits enhanced catalytic activity arising from charge transfer states (CTSs) between its two chromophores. Oligomeric and polymeric MPC-1 preparations both promote efficient hydrodehalogenation of α-halocarbonyl compounds while exhibiting different solubility properties. The polymer is readily recovered by filtration. MPC-1-coated vessels enable batch and flow photocatalysis, even with opaque reaction mixtures, via "backside irradiation." Ultrafast transient absorption spectroscopy indicates a fast charge-transfer process within 20 ps of photoexcitation. Time-resolved photoluminescence measurements reveal an approximate 10 ns lifetime for bright valence states. Ultrafast measurements suggest a long CTS lifetime. Empirical catalytic activities of small-molecule models of MPC-1 subunits support the CTS hypothesis. Density functional theory (DFT) and time-dependent DFT calculations are in good agreement with experimental spectra, spectral peak assignment, and proposed underlying energetics.