Ultrafast Triplet State Formation in a Methylated Fungi-Derived Pigment: Toward Rational Molecular Design for Sustainable Optoelectronics

Abstract

Organic triplet-generating materials have prolonged excited-state lifetimes that could enhance photocatalysis, light-emitting diodes, and versatile optoelectronics. Dimethylxylindein, a methylated derivative of a naturally sourced electronic material, xylindein, generates detectable triplet states upon photoexcitation in solution. Femtosecond transient absorption measurements of dimethylxylindein demonstrate that bluer excitation wavelengths enhance the intersystem crossing (ISC) in dichloromethane (DCM), which reduces the photostability compared to robust xylindein. No appreciable triplet states are generated in the more polar ethanol or thin films. The methoxy groups open two ISC pathways: one ultrafast (μ140 fs) and one slower (μ20 ps), the latter process enhanced by nonplanar structural motions that promote spin-orbit coupling over the chromophore ring-conjugated framework. Tunable femtosecond stimulated Raman spectroscopy (FSRS) and systematic quantum calculations characterize a light-induced charge-transfer state that becomes more stabilized in polar solvents like ethanol to quench ISC. In thin films, though the stabilized charge-transfer state on ultrafast timescales may promote photosensitivity, the triplet yield could be boosted via rational design strategies, including thionation and implementation of heterocyclic nitrogen, to broaden the sustainable optoelectronic applications of xylindein derivatives.