Pressure- and aggregation-induced modulation of linear and nonlinear optical properties in a push–pull chromophore: insights from computational modelling

We report a theoretical investigation of the structural and optical responses of a molecular crystal based on a push-pull chromophore subjected to increasing isotropic pressure ranging from 1 to 30 kbar. Geometry optimizations at the DFT level reveal pronounced changes in unit cell parameters, particularly along the stacking and charge-transfer directions, accompanied by significant volume compression, reaching 17% at the highest pressure. Pressure also alters key intramolecular torsional angles and intermolecular stacking geometries, with non-linear variations and discontinuities observed in the evolution of these parameters. Time-dependent DFT calculations on pressure-adapted geometries of molecular dimers show that these structural changes lead to abrupt shifts in excited-state energies, oscillator strengths, exciton localization, and charge-transfer character. The external pressure is also shown to strongly influence the second-harmonic generation (SHG) response of the dimers, which are considered representative of the stacking arrangements in thin films. As rationalized using a truncated sum-over-states (SOS) approach, the pressure-induced variation in the SHG response is closely linked to changes in the charge-transfer character and absorption strength of a small set of low-lying excited states. Overall, our calculations indicate that increasing the external pressure from 1 to 30 kbar leads to an 11% decrease in the static first hyperpolarizability of the dimer. The dynamic first hyperpolarizability computed at an incident wavelength of 800 nm evolves non-monotonically with pressure, exhibiting a maximum around 8 kbar due to resonance effects at the second harmonic and an overall reduction of 74% from 1 to 30 kbar. These results suggest that external pressure provides an effective means to modulate the nonlinear optical properties of 2D materials based on these push-pull chromophores.