Intermolecular packing effects on the two-photon absorption of the H₄TCPE linker

Materials exhibiting efficient nonlinear optical properties, such as two-photon absorption (2PA), are of increasing interest for applications including biomedical imaging, optical data storage, and long-range communications. Metal-organic frameworks (MOFs) provide a versatile platform for enhancing such 2PA responses by allowing specific ordered arrangements of chromophores. In particular, organic molecules exhibiting aggregation-induced emission (AIE) have shown a significant increase in 2PA cross sections when incorporated into MOFs as linkers. This is possible not only because of the periodic ordered arrangement of photoactive molecules, but also due to subtle changes to the electronic properties of AIE-linkers and hindering their structural flexibility. The latter has been reported for tetrakis [4-(4-carboxyphenyl)phenyl]ethylene, known as H₄TCPE, a linker used for MOFs with strong and tunable 2PA response. It is known that confinement effects in MOFs and the restriction of intramolecular motion of H₄TCPE are key to its enhanced 2PA. However, the influence of specific intermolecular packing arrangements of H₄TCPE on the 2PA response remains insufficiently understood. In this work, we study computationally the 2PA response of H₄TCPE as a function of intra- and intermolecular packing. Using finite (non-periodic) monomeric and dimeric models, we first investigate how intramolecular flexibility affects the absorption spectra and 2PA cross sections and later analyze dimer configurations by varying the relative orientations to assess how intermolecular packing modulates the 2PA response. Finally, we compare the most favorable computed arrangements with experimentally reported H₄TCPE-based MOF structures and propose optimal conformations for an efficient response.