Ultrafast formation of exciplex species in dicyanoanthracene ZSM-5 revealed by transient emission and vibrational spectroscopy

The photo-physical properties of dicyanoanthracene (DCA) molecules adsorbed on the external surface of ZSM-5 zeolite, forming DCA@ZSM-5 composites, have been investigated by picosecond transient emission, femtosecond transient absorption infrared vibrational spectroscopy, steady-state UV-vis, and quantum chemistry calculations. Following the photoexcitation at 420 nm of DCA@ZSM-5, the formation of the localized, LE, excited S-1 state of DCA emitting below 500 nm is observed. LE is rapidly and quasi-exclusively converted into two distinct exciplex species, EX1 and EX2, detected by their emission above 520 nm, with a lifetime of 5 ns and 20 ns, respectively. The different transient species can be identified by the frequency of the CN stretching vibration that is a marker of the charge delocalisation and that is peaking respectively at 2162 (LE), 2174 (EX1) and 2187 (EX2) cm(-1). DFT and TD-DFT calculations further support the assignment. The results show that the external surface of zeolite is an appropriate playground for the development of novel photoactive host-guest materials.

Automated summary

The photo-physical properties of dicyanoanthracene (DCA) molecules adsorbed on the external surface of ZSM-5 zeolite, forming DCA@ZSM-5 composites, were investigated. It was found that following the photoexcitation of DCA@ZSM-5 at 420 nm, the formation of localized excited states (LE) emitting below 500 nm was observed. The LE states were rapidly converted into two distinct exciplex species, EX1 and EX2, detected by their emission above 520 nm, with different lifetimes and CN stretching vibration frequencies.