Substituent effects on the emission quantum efficiency in pyrene-based luminogens featured two-photon absorption: A theoretical and experimental study

Pyrene-based luminogens have attracted significant attention for their potential applications in organic electronics, nonlinear optics and bioimaging. The attainment of a large two-photon absorption (TPA) cross section (σtpa) and a high emission quantum efficiency (Φf) are crucial goals in material design. However, the intrinsic modulation guidelines have rarely been revealed. In this study, we investigated the substituent effects on the excited-state properties in pyrene-based donor-π-acceptor (D-π-A) dyes. In the dimethyl sulfoxide (DMSO) solution, we performed first-principles calculations on σtpa, Φf and the TPA brightness (σtpa × Φf). The results demonstrate that both the H-parent compound and F-substituted derivative exhibit high Φf values. Nevertheless, they have low σtpa values due to the weak push-pull effect, resulting in small σtpa × Φf values in solution. Conversely, the CHO-substituted derivative has high σtpa and Φf values, leading to a large σtpa × Φf. However, the NMe2-substituted derivative possesses high σtpa but relatively low Φf owing to the small energy gap. Therefore, it has a modest σtpa × Φf in solution. In the solid state, we conducted experimental measurements to determine Φf. It is found that the solid powders of the H-, F- and CHO-substituted derivatives show considerably lower Φf values compared to the DMSO solution. In contrast, the NMe2-substituted derivative displays a higher Φf. The discrepant Φf values can be attributed to the different patterns in which the dimers pack themselves in the aggregated state. Our study not only provides a deep understanding for the photophysical properties of pyrene-based luminogens, but also lays the groundwork for the rational design of efficient two-photon fluorescent emitters.