Chemical control of photoinduced charges under confinement in zeolites

The organized internal porous void of dehydrated zeolites provides a suitable environment to promote long-lived photoinduced charge separation. Herein we have conducted time-resolved UV-visible absorption spectroscopy experiments from nanosecond to day time scale following nanosecond UV (266 nm) pulsed laser irradiation of trans-stilbene (t-St) occluded in channels of nonacidic M-FER, M-MFI, and M-MOR zeolites with various pore diameters, with differing framework aluminum content, and with different extraframework cations (M = Na ⁺, K ⁺, Rb ⁺, and Cs ⁺). The cation radical of trans-stilbene (t-St ^•+) and trapped electron (AlO ₄ ^•-) have been generated directly by means of laser-induced electron transfer within the channels of medium pore zeolites. We have highlighted that the general back electron transfer processes include direct charge recombination (CR), hole transfer (HT), and finally electron-hole recombination to re-form the occluded t-St ground state without any isomerization or oligomerization. It was demonstrated once again that zeolites can be active participants as electron acceptors and electron donors. The decays of t-St ^•+ are the combination of two processes: direct CR and hole transfer. The charge-separated species as t-St ^•+· AlO ₄ ^•- and t-St-AlO ₄ ^•+·AlO ₄ ^•- moieties are stabilized for approximately 10 h in aluminated medium pore zeolites with small extraframework cation such as Na ⁺. The most remarkable feature of the transient t-St-AlO ₄ ^•+ entity formation in M-MFI and M-MOR is the persistent intense color due to the prominent absorption bands in the visible range. The very slow CR rates are explained both by the long distance between the separated charges and by the large difference in free energy between the electron acceptor and electron donor (driving force -δG ⁰), which increases with Al content in the order Cs ⁺ < Rb ⁺ < K ⁺ < Na ⁺. The CR rates are markedly slowed by shifting them deep into the inverted region of the Marcus parabola where -δG ⁰ is larger than the reorganization energy coefficient (λ), which is particularly small under high confinement. The close match between t-St molecular size and zeolite channel diameter is critical to generating long-lived charge separations (hours).