A mononuclear Fe(ii) complex involving a tetrathiafulvalene-based ligand exhibits thermal spin-crossover (around 143 K) with pronounced hysteresis behaviour (48 K). The chromophoric and π-extended ligand allows Near-Infrared (NIR) sensitization for the light-induced excited spin-state trapping (LIESST) with T(LIESST) = 90 K.

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.