Stimuli-responsive NLO properties of tetrathiafulvalene-fused donor–acceptor chromophores

Abstract

The second-order nonlinear optical (NLO) properties of two tetrathiafulvalene (TTF)-fused electron donor-acceptor dyads have been determined using the Electric Field Induced Second Harmonic generation (EFISH) technique and theoretically rationalized. Dyads TTF-dppz (1) and TTF-BTD (2) were obtained by direct fusion of a TTF electron donor unit either with a dipyrido[3,2-a:2',3'-c]phenazine (dppz) or a benzothiadiazole (BTD) electron acceptor moiety. Dyad 1 acts as a reversible acido-triggered NLO switch by protonation/deprotonation at two nitrogen atoms of the dppz acceptor moiety induced by sequential exposure to HCl and ammonia vapors. Dyad 2, on the other hand, displays redox-tunable NLO properties upon two consecutive oxidations to its radical cation 2+˙ and dication 22+ species. The resulting final dication 22+ exhibits an inversion of the sign of β, due to a completely inverted distribution of the frontier molecular orbitals with respect to those of its neutral species, leading to a scarcely polar species in the excited state, as indicated by the theoretical calculations.

Structural diradical character

“…22 To decide whether further efforts towards obtaining these data and towards optimizing the switching behavior are worthwile, we aim at a true first-principles prediction of the diradical character of the closed switch. In contrast to the analogous nitronyl nitroxide compound 23 (see Supporting Information (SI)), KS-DFT optimizations of the molecular structure for the closed switch in its singlet state give no consistent answer to whether it is predominantly a closed-shell or an open-shell structure. Consequently, no predictions of its diradical character and its NLO properties appear possible, unless a particular approximate exchange-correlation functional can be identified as sufficiently reliable for this purpose.…”

“…It would be particularly useful if the NLO properties of molecules or molecular materials could be switched by external stimuli. There has been considerable experimental [19][20][21][22][23] and theoretical 20,[24][25][26][27] work on switchable organic and organometallic NLO-active molecules, showing that a variety of stimuli such as pH, temperature, redox reactions, and light can be used for this purpose.…”

A novel acid-controlled second-order nonlinear optical switch based on dimethyldihydropyrene/cyclophanediene photoswitch

“…In recent years, numerous works have been devoted to researching the switching of the second-order NLO response upon exposure to many external stimuli, such as light, heat, pH, redox and electric field, which increase the attention on diverse applications, for example, environment monitors, sensors, and energy-efficient memories. 4,[7][8][9][10][11][12][13] In this context, NLO switches require that molecules have the ability to display large contrast in NLO response during switching between two or more chemical forms. Generally, at the molecular level, a conspicuous contrast in the second-order NLO property is mainly exhibited in the first hyperpolarizability, b, of different structural forms.…”

“…In recent years, numerous works have been devoted to researching the switching of the second-order NLO response upon exposure to many external stimuli, such as light, heat, pH, redox and electric field, which increase the attention on diverse applications, for example, environment monitors, sensors, and energy-efficient memories. 4,7–13…”

Structural diradical character

“…It would be particularly useful if the NLO properties of molecules or molecular materials could be switched by external stimuli. There has been considerable experimental and theoretical work on switchable organic and organometallic NLO‐active molecules, showing that a variety of stimuli such as pH, temperature, redox reactions, and light can be used for this purpose.…”