Tailoring the Schiff base photoswitching – a non-adiabatic molecular dynamics study of substituent effect on excited state proton transfer

Abstract

Small molecular systems exhibiting Excited State Intramolecular Proton Transfer (ESIPT) attract considerable attention due to their possible role as ultrafast, efficient, and photostable molecular photoswitches. Here, by means of static potential energy profile scan and on-the-fly non-adiabatic dynamics simulations we study the photodeactivation process of a minimal-chromophore aromatic Schiff base, salicylidene methylamine (SMA), and its two derivatives 6-cyano-salicylidene methylamine (6-CN-SMA) and 3-hydroxy-salicylidene methylamine (3-OH-SMA). We show that the dominant character of the lowest excited singlet state - ππ* vs. nπ* - plays a crucial role in the system's photophysics and controls the ESIPT efficiency. We also show that the relative alignment of the ππ* and nπ* states may be controlled through chemical substitutions made to the aromatic ring of the Schiff-base molecule. We believe that our findings will improve the rational-design strategies employed for the ESIPT systems, especially in the context of their possible photoswitching.

Aggregation‐Induced Emission‐Based Sensing Platform for Selective Detection of Zn2+: Experimental and Theoretical Investigations

“…Finally, the absorption and emission spectra of Hbdhn measured in the 1 : 9 mixture of DMSO and polar protic solvents (methanol and ethanol), which can strongly facilitate the proton transfer, show no significant changes compared to the spectra in pure DMSO (Figures S34 and S35). This seems to support AIEE‐based (rather than intramolecular proton transfer) origin of the spectral features for Hbdhn in the presence of water.…”

“…Herein, we present the synthesis and full experimental and theoretical characterization of a new Schiff base chemosensor 1‐(benzo[1,3]dioxol‐4‐ylmethylene hydrazonomethyl)‐naphthalen‐2‐ol ( Hbdhn ). Although synthetically the system seems similar to the previously reported one, differing in the receptor part, it might demonstrate different functional properties and signaling mechanism due to the lack of one of the hydrogen bonds . The results show that the proposed system exhibits not only highly effective and selective turn‐on response towards Zn 2+ comparing to other water‐insoluble Zn 2+ sensors available in the literature (see Table S1), but also desired AIEE features.…”

“…Although synthetically the system seems similar to the previously reported one, [14d] differing in the receptor part, it might demonstrate different functional properties and signaling mechanism due to the lack of one of the hydrogen bonds. [31] The results show that the proposed system exhibits not only highly effective and selective turn-on response towards Zn 2 + comparing to other water-insoluble Zn 2 + sensors available in the literature (see Table S1), but also desired AIEE features. Note that the AIEE effect is rather not common for water-soluble Zn 2 + probes.…”

“…Note that although the latter pair (S 1 -1e III , S 1 -2e III ) corresponds to non-populated (higher-energy, > 2.8 kcal/mol with respect to S 1 -1e twist ) minima on the S 1 PES of Hbdhn, S 1 -1e II and S 1 -2e II (see insets in Figure 8a) have small, but non-negligible populations existing in equilibrium with their planar fluorescent analogues S 1 -1 e and S 1 -2 e. All these results indicate that (i) a lack of fluorescence signal for Hbdhn is mostly due to an intramolecular rotation leading to orthogonally twisted π-systems in the S 1 with visible involvement of LP(N) orbitals through LUMO, and (ii) strong emission of the ligand can be achieved by enforcing the system planarity and restraining photoinduced electron transfer (PET) [10e,18] from the lone-pair orbitals of the hydrazine bridge nitrogen atoms to the extended π-electron system by involving them in an efficient electronic interaction and consequently lowering energy of the quenching LP(N) below that of the fluorophore (Figures 10 and S31). Even though in the literature excited-state intramolecular proton transfer (ESIPT) is considered to be a main photo deactivation route for α-hydroxy Schiff base systems, [31] based on the calculations performed such a mechanism seems to be less plausible for Hbdhn as the keto conformers are higher in energy on the S 1 PES ( Figure S28), vide infra.…”

Recent Advances and Perspectives on Nonadiabatic Mixed Quantum–Classical Dynamics

“…This is especially important when the dynamics is determined by charge transfer states, such as in excited-state proton transfer 323 and organic photovoltaics applications. 324 Applications of TSH using LR-TDA/TDDFT excited states include studies of the deactivation of organic molecules with biological applications, [325][326][327][328][329] excited state proton transfer, [330][331] atmospheric chemistry, 332 and organometallic complexes. [333][334][335] A recent study from…”