Prolonging the lifetime of charge-separated states (CSS) is of paramount importance in artificial photosynthetic donor−acceptor (DA) constructs to build the next generation of light-energy-harvesting devices. This becomes especially important when the DA constructs are closely spaced and highly interacting. In the present study, we demonstrate extending the lifetime of the CSS in highly interacting DA constructs by making use of the triplet excited state of the electron donor and with the help of excitation wavelength selectivity. To demonstrate this, π-conjugated phenothiazine sulfone-based push−pull systems, PTS2−PTS6 have been newly designed and synthesized via the Pdcatalyzed Sonogashira cross-coupling followed by [2 + 2] cycloaddition−retroelectrocyclization reactions. Modulation of the spectral and photophysical properties of phenothiazine sulfones (PTZSO 2 ) and terminal phenothiazines (PTZ) was possible by incorporating powerful electron acceptors, 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (exTCBD). The quadrupolar PTS2 displayed solvatochromism, aggregation-induced emission, and mechanochromic behaviors. From the energy calculations, excitation wavelength-dependent charge stabilization was envisioned in PTS2−PTS6, and the subsequent pump−probe spectroscopic studies revealed charge stabilization when the systems were excited at the locally excited peak positions, while such effect was minimal when the samples were excited at wavelengths corresponding to the CT transitions. This work reveals the impact of wavelength selectivity to induce charge separation from the triplet excited state in ultimately prolonging the lifetime of CCS in highly interacting push−pull systems.

Pyridine-based derivatives Py-1, Py-2, and Py-3 were synthesized by the Suzuki cross-coupling reaction, and the effect of different substituents at the ortho position of the pyridine ring was investigated. In the molecular framework of push−pull compounds, electron acceptor (A) pyridine was substituted with an electron donor (D) dimethoxy triphenylamine unit. The effect of passing from a dipolar (D-A, Py-1) to a quadrupolar (D-A-D, Py-3) structure as well as the impact of functionalizing the pyridine with a bromine in the dipolar structure (D-A-Br, Py-2) was investigated. All the pyridine derivatives were found to be highly emissive both in solution and in the solid state, with fluorescence properties markedly sensitive to the environment and responsive to external stimuli. The spectral and photophysical properties in solution were investigated through conventional steady-state and advanced time-resolved spectroscopies with nanosecond and femtosecond temporal resolution. The expected efficient intersystem crossing for the heavy-atom-containing molecule was surprisingly not highlighted by our experiments, but an increased push−pull character was enabled by functionalizing the pyridine acceptor with the bromine. The ultrafast spectroscopic study revealed a peculiar excited state deactivation mechanism for the Br derivative among the others, involving photoinduced intramolecular charge transfer accompanied by twisting of the molecular structure (population of a TICT excited state). Similarly, a significant color change in the solid-state emission was observed for Py-2 upon mechanical grinding, suggesting a mechanochromic luminescence behavior in this case. Our findings suggest introducing heavy atoms in organic fluorophores as a new design strategy to obtain mechanoluminescent materials.

To solve the toxicity issues related to lead-based halide perovskite solar cells, the lead-free double halide perovskite Cs 2 AgBiBr 6 is proposed. However, reduced rate of charge transfer in double perovskites affects optoelectronic performance. We designed a series of pyridine-based small molecules with four different arms attached to the pyridine core as hole-selective materials by using interface engineering. We quantified how arm modulation affects the structure−property−device performance relationship. Electrical, structural, and spectroscopic investigations show that the N 3 ,N 3 ,N 6 ,N 6 -tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine arm's robust association with the pyridine core results in an efficient hole extraction for PyDAnCBZ due to higher spin density close to the pyridine core. The solar cells fabricated using Cs 2 AgBiBr 6 as a light harvester and PyDAnCBZ as the hole selective layer measured an unprecedented 2.9% power conversion efficiency. Our computed road map suggests achieving ∼5% efficiency through fine-tuning of Cs 2 AgBiBr 6 . Our findings reveal the principles for designing small molecules for electro-optical applications as well as a synergistic route to develop inorganic lead-free perovskite materials for solar applications.

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