Wavelength-dependent photochemistry of a salicylimine derivative studied with cryogenic and ultrafast spectroscopy approaches.

Salicylimines are versatile compounds in which an excited-state intramolecular proton transfer and torsional motions may set in upon photoexcitation. Here, we study N-(α-phenylethyl)salicylimine (PESA) to elucidate how the photochemical reaction pathways depend on the excitation wavelength and to what extent the relative photoproduct distribution can be steered towards a desired species. DFT structure and potential energy calculations disclose that the most stable ground-state conformer is an enol species and that the photodynamics may proceed differently depending on the excited state that is reached. With matrix isolation infrared spectroscopy, the predominance of the enol conformer of PESA is confirmed. Illumination of the cryogenic sample with different wavelengths shifts the ratio of enol and keto products, and by sequential irradiation a selective re- and depopulation is possible. Femtosecond transient absorption spectroscopy further reveals that also at room temperature, the outcome of the photoreaction depends on excitation wavelength, and in combination with the calculations, it can be rationalized that the decisive step occurs within the first hundred femtoseconds. Since the ultrafast dynamics mostly match those of similar salicylimines, our findings might also apply to those systems and provide additional insight into their reported sensitivity on excitation energy.

Steering the Ultrafast Opening and Closure Dynamics of a Photochromic Coordination Cage by Guest Molecules.

Photochemical studies on supramolecular hosts that can encapsulate small guest molecules commonly focus on three aspects: photoswitching the cage to release or trap the guest, the effect of the confining environment on the guest, and light-induced exciton or charge transfer within the cage structure. Here, we exploit ultrafast spectroscopy to address how the guest alters the photoswitching characteristics of the cage. For this, the impacts of three disparate guest compounds on ring-opening or ring-closure of a dithienylethene (DTE) ligand in a photoswitchable DTE-based coordination cage are juxtaposed. The guest modulates both outcome and timescale of the cage's photodynamics, by an interplay of structural strain, heavy-atom effect, and enhancement of charge-transfer processes exercised by the guest on the photo-excited cage. The approach might prove beneficial for attuning the applicability of photoswitchable nanocontainers and desired guest compounds.