The unique four-level photocycle characteristics of excited-state intramolecular proton transfer (ESIPT) materials enable population inversion and large spectral separation between absorption and emission through their respective enol and keto forms. This leads to minimal or no self-absorption losses, a favorable feature in acting as an optical gain medium. While conventional ESIPT materials with an enol–keto tautomerism process are widely known, zwitterionic ESIPT materials, particularly those with high photoluminescence, are scarce. Facilitated by the synthesis and characterization of a new family of 2-hydroxyphenyl benzothiazole (HBT) with fluorene substituents, HBT-Fl1 and HBT-Fl2, we herein report the first efficient zwitterionic ESIPT lasing material (HBT-Fl2). The zwitterionic ESIPT HBT-Fl2 not only shows a remarkably low solid-state amplified spontaneous emission (ASE) threshold of 5.3 μJ/cm2 with an ASE peak at 609 nm but also exhibits high ASE photostability. Coupled with its substantially large Stokes shift (≈236 nm ≈10,390 cm–1) and an extremely small overlap of excited-state absorption with ASE emission, comprehensive density functional theory (DFT) and time-dependent DFT studies reveal the zwitterionic characteristics of HBT-Fl2. In opposition to conventional ESIPT with π-delocalized tautomerism as observed in analogue HBT-Fl1 and parent HBT, HBT-Fl2 instead shows charge redistribution in the proton transfer through the fluorene conjugation. This structural motif provides a design tactic in the innovation of new zwitterionic ESIPT materials for efficient light amplification in red and longer-wavelength emission.

An unsymmetrical squaraine (SQ) derivative containing anthracene and phenyl hydrazine (ANPHSQ) units linked to the central SQ was synthesized and used as electron donors in bulk heterojunction photovoltaic mode organic photodetectors (OPD), containing PCBM as the electron acceptor. Although the ANPHSQ exhibited a strong narrow band in the near infrared peaking at ∼760 nm in solution, the OPD constructed with this dye exhibited a broad spectral response extending to 950 nm. The enhanced sensitivity in the long wavelength region could be attributed to formation of ANPHSQ aggregates within the devices. A photocurrent of 1.3 mA/cm2, almost 3–5 orders of magnitude larger than the dark current (∼50 nA/cm2), was observed in the presence of light at −1 V bias condition. Devices showed an increase in external quantum efficiency from ∼4 to ∼12% as the bias varied from 0 to −1 V. At a reverse bias of −1 V, the device with a ANPHSQ exhibited a maximum shot-noise-limited specific detectivity of 6 × 1011 cm Hz1/2 W–1 (Jones) @ 840 nm with an ultrafast photoresponse in the range of ∼15 ns. The study of relaxation dynamics of the ANPHSQ excited state using femtosecond pump–probe spectroscopy and time-correlated single-photon counting indicated efficient charge transfer between the excited state of ANPHSQ and PCBM.

The ubiquitous electron transfer heme protein, Cytochrome c (Cyt c) catalyzes the peroxidation of cardiolipin (CL) in the early stage of apoptosis, where Cyt c undergoes conformational changes leading to the partial unfolding of the protein. Here the interaction dynamics of Cyt c with liposomes having different charges [CL, – 2; POPG (2-Oleoyl-1-palmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt), −1; and POPC (2-Oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine), 0] leading to various degrees of partial unfolding is investigated with steady state optical spectroscopy and femtosecond time-resolved pump–probe spectroscopy. The signature of the partial unfolding of the protein was observed in the absorption, fluorescence, and CD spectra of Cyt c–liposome complexes with an increase of lipid/protein (L/P) ratio, and the protein was refolded by the addition of 0.1 M of NaCl. The femtosecond transient absorption spectra of the complexes were measured by selectively exciting the heme and tryptophan (Trp) at 385 and 280 nm, respectively. Though significant changes were not observed in the excited state relaxation dynamics of the heme in liposomes by exciting at 385 nm, the 280 nm excitation exhibited a systematic increase of the excited state relaxation dynamics leading to the increase of lifetime of Trp and global conformational relaxation dynamics with the increase of anionic charge of the lipids. This reveals the decrease of efficiency of fluorescence resonance energy transfer from Trp to heme due to the increase of distance between them upon increase of partial unfolding of the proteins by liposomes. Such observation exhibits the Trp as a marker amino acid to reflect the dynamics of partial unfolding of the protein rising from the change in the tertiary structure and axial ligand interaction of the heme proteins in liposomes. The relaxation dynamics of the complexes in the presence of salt are similar to that of the protein alone, reflecting that the refolding of the protein and the interactions are dominated by electrostatic interaction rather than the hydrophobic interaction.