Effects of a Self‐Assembled Molecular Capsule on the Ultrafast Photodynamics of a Photochromic Salicylideneaniline Guest

Abstract

Selective and slow: The effects on the photophysics of a single salicylideneaniline molecule, induced by confinement in an all‐organic, weakly interacting molecular capsule of a matching size, are studied by time‐resolved spectroscopy. The capsule acts efficiently to select only one deactivation pathway after photoexcitation and keeps the photochromism in the solid state with a 300‐fold retardation—relative to the solution state—in the decay time of the photochromic product (see picture).

Comparison between Spontaneous and Photoinduced Ionization Mechanisms for p-Quaterphenyl in M-ZSM-5 (M = H+, Na+) Zeolites

“…47,48 Excitation pulses at 300 nm (7)(8) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 moved and/or shaken throughout the experience to ensure that a fresh region of the sample was probed by each laser pulse.…”

A Spectroscopic Study of Tautomeric Equilibrium of Salicylideneaniline in ZSM-5 Zeolites

“…The broad and low intense features found above 400 nm are very similar to the residual absorption observed upon photo-excitation of SA in solutions and in confining media [27]. This contribution was assigned to a trans-keto SA tautomer created upon the relaxation of excited state of the cis-keto form [5,26,27,28]. However, to the best of our knowledge, this species has never been stabilized in solution and the attribution is solely based on results of the time-resolved spectroscopic measurements.…”

“…The band is characteristic of the π-π* transition of enol SA form [5,11,12]. The keto-enol tautomeric equilibrium of SA is known to be shifted towards a keto form in polar solvents [11,12,27] and such a behavior has been confirmed by quantum-chemical calculations using the polarizable continuum model [34]. Based on UV-visible spectra reported for anils in solutions or in confined state, the band at 390 nm in the spectra of SA@M-ZSM-5 samples can be ascribed to the π-π* transition of cis-keto SA tautomer or its zwitterionic form [3,12,13,24,35].…”

“…Photo- and thermal excitation of the molecule lead to an excited state intramolecular proton transfer producing an excited cis-keto form, (cis-keto)*, with a possible zwitterionic structure [3,9,10,13,23,24,25]. This form then returns to the ground state enol SA conformer via either (cis-keto)*→0.277778em cis-keto deactivation or (cis-keto)*→0.277778em trans-keto isomerization followed by a conversion to the cis-keto tautomer in the ground state [5,25,26,27,28]. Results of quantum-chemical calculations generally confirm this scenario [25,29,30].…”

“…However, the host–guest interactions in these materials may lead to the co-existence of different conformers already in the ground state that complicates the investigation of the systems upon photo-excitation. To overcome this problem, the photodynamics of SA was studied after the incorporation of the molecule in photo-inert molecular capsules with weak host–guest interactions [27]. Another approach consisted in introducing SA into Al-free zeolites, where the molecule can exist in a quasi-isolated state because of an inert environment of pure silica structure [42].…”