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We report a significant Stokes shift enhancement in near-infrared fluorescing cyanines as a result of C4′-substitution with cyclic or acyclic amines. Based on a combined experimental and density functional study, a simple strategy for optimizing the Stokes shift is proposed. By tuning the relative energies of cyanine-like and bis-dipolar conformers, differing in the rotational angle of the amine substituent, it is possible to develop molecules that undergo conformational change upon excitation, resulting in a predictable Stokes shift. 

The significance of solvent structural factors in the excited-state proton transfer (ESPT) reactions of Schiff bases with alcohols is reported here. We use the super photobase FR0 -SB and a series of primary, secondary, and tertiary alcohol solvents to illustrate the steric issues associated with solvent to photobase proton transfer. Steady-state and time-resolved fluorescence data show that ESPT occurs readily for primary alcohols, with a probability proportional to the relative –OH concentration. For secondary alcohols, ESPT is greatly diminished, consistent with the barrier heights obtained using quantum chemistry calculations. ESPT is not observed in the tertiary alcohol. We explain ESPT using a model involving an intermediate hydrogen-bonded complex where the proton is “shared” by the Schiff base and the alcohol. The formation of this complex depends on the ability of the alcohol solvent to achieve spatial proximity to and alignment with the FR0 -SB* imine lone pair stabilized by the solvent environment. 

Abstract Visibly transparent luminescent solar concentrators (TLSC) can optimize both power production and visible transparency by selectively harvesting the invisible portion of the solar spectrum. Since the primary applications of TLSCs include building envelopes, greenhouses, automobiles, signage, and mobile electronics, maintaining aesthetics and functionalities is as important as achieving high power conversion efficiencies (PCEs) in practical deployment. In this work, massive‐downshifting phosphorescent nanoclusters and fluorescent organic molecules are combined into a TLSC system as ultraviolet (UV) and near‐infrared (NIR) selective‐harvesting luminophores, respectively, demonstrating UV and NIR dual‐band selective‐harvesting TLSCs with PCE over 3%, average visible transmittance (AVT) exceeding 75% and color metrics suitable for the window industry. With distinct wavelength‐selectivity and effective utilization of the invisible portion of the solar spectrum, this work reports the highest light utilization efficiency (PCE × AVT) of 2.6 for a TLSC system, the highest PCE of any transparent photovoltaic (TPV) devices with AVT greater than 70%, and outperforms the practical limit for non‐wavelength‐selective TPV. 

Abstract Transparent luminescent solar concentrators (TLSCs) selectively harvest ultraviolet and near‐infrared photons. Due to the absence of electrodes, busbars, and collection grids over the solar harvesting area, the device structure enables these devices to achieve the highest levels of transparency and aesthetics. Recently, CO_i8DFIC has been developed as a nonfullerene acceptor in organic photovoltaics with unprecedented performance. In this work, nonfullerene acceptors are introduced into TLSCs as the luminophores. The impact of CO_i8DFIC concentration on power conversion efficiency (PCE), aesthetic quality, and scalability is systematically studied. After device optimization, the CO_i8DFIC TLSCs are shown to achieve a PCE over 1.2% while the average visible transmittance exceeds 74% and color rendering index exceeds 80. This work reports the highest TLSC device efficiency at the highest visibly transparency and highlights that the photoluminescent properties of these emerging low bandgap organic molecules providing an encouraging path to higher TLSC performance.