Beyond-Kasha Photochemistry in a Heteroleptic Platinum–Dithiolene Complex

Kasha’s rule implies that photochemical reactions occur in the lowest excited state, regardless of excitation wavelength. Only a few chromophores have been reported to exhibit efficient non-Kasha responses. While these are rare, their exploitation could revolutionize multiresponsive materials, improve solar energy utilization, and advance light-driven chemical reactions. Studying non-Kasha dynamics enhances the understanding of excited-state processes and could broaden the range of usable chromophores in real applications. This article focuses on the anionic heteroleptic dithiolene complex [Pt((R)-α-MBAdto)(quinoxdt)]⁻ ((R)-α-MBAdto = (R)-(+)α-methylbenzyl-dithiooxamidate; quinoxdt = [1,4]dithiino[2,3-b]quinoxaline-2,3-bis(thiolate)) in acetonitrile, which undergoes long-lived conformational changes exclusively upon excitation of higher excited states. In its tight ion-pair adduct with HCl, these changes drive HCl detachment within 70 ps, trigger a dramatic blue-shift in the S₁–S₀ gap, and lead to aggregate formation. Although these processes ultimately occur in the lowest excited state, they rely on non-Kasha isomerization, representing a “beyond-Kasha” process. Such systems pave the way for innovative multiresponsive materials and non-Kasha excitation-dependent photochemical applications.