Substitution effects on the photoinduced excited state dynamics of perylenemonoimides in solution and thin films

Perylene monoimide (PMI) derivatives are attracting significant attention due to their strong absorption in the visible range, thermal stability, and synthetic accessibility. These properties make them promising for application in various areas such as optoelectronic devices, photosensitizers, etc. In this work, the photophysical properties and excited state dynamics of four different PMI derivatives (PMIB, BrPMITB, PMITB, and APITB) were studied in solution and thin films utilizing steady-state and time-resolved spectroscopic techniques. Among the four PMI derivatives, APITB is designed as a donor-acceptor dyad, with thianthrene as a donor and PMI as an acceptor. The activation of the triplet state through the spin-orbit charge transfer intersystem crossing (SOCT-ISC) process in THF was observed upon substitution with the thianthrene group at the peri position of the PMI moiety. The SOCT-ISC process facilitates triplet formation in the APITB dyad within 423 ps. Meanwhile, other PMI derivatives showed fluorescence within the femtosecond timescale in THF. The PMI derivatives in thin films displayed different photo physical properties to those in THF. This discrepancy arises due to the effective intermolecular coupling between the PMI derivatives in thin films.

Fast Charge Separation in Distant Donor-Acceptor Dyads Driven by Relaxation of a Hot Excited State

A series of three perylenemonoimide-p-oligophenylene-dimethylaniline molecular dyads undergo photoinduced charge separation (CS) with anomalous distance dependence as a function of increasing donor-acceptor (DA) distances. A comprehensive experimental and computational investigation of the photodynamics in the donor-bridge-acceptor (DBA) chromophores reveals a clear demarcation concerning the nature of the CS accessed at shorter (bridgeless) and longer DA distances. At the shortest distance, a strong DA interaction and ground-state charge delocalization populate a hot excited state (ES) with prominent charge transfer (CT) character, via Franck-Condon vertical excitation. The presence of such a CT-polarized hot ES enables a subpicosecond CS in the bridgeless dyad. The incorporation of the p-oligophenylene bridge effectively decouples the donor and the acceptor units in the ground state and consequentially suppresses the CT polarization in the hot ES. Theoretically, this should render a slower CS at longer distances. However, the transient absorption measurement reveals a fast CS process at the longer distance, contrary to the anticipated exponential distance dependence of the CS rates. A closer look into the excited-state dynamics suggests that the hot ES undergoes ultrafast geometry relaxation (τ < 1 ps) to create a relaxed ES. As compared to a decoupled, twisted geometry in the hot ES, the geometry of the relaxed ES exhibits a more planar conformation of the p-oligophenylene bridges. Planarization of the bridge endorses an increased charge delocalization and a prominent CT character in the relaxed ES and forms the origin for the evident fast CS at the longest distance. Thus, the relaxation of the hot ES and the concomitantly enhanced charge delocalization adds a new caveat to the classic nature of distance-dependent CS in artificial DBA chromophores and recommends a cautious treatment of the attenuation factor (β) while discussing anomalous CS trends.

Crystalline Covalent Organic Frameworks with Tailored Linkages for Photocatalytic H2 Evolution

Crystalline covalent organic frameworks (COFs) are porous polymeric semiconductors with network topologies, which are built from the integration of selected organic blocks with covalent bond linkages. They have shown great promise for artificial photosynthesis, owing to broad light harvesting, high crystallinity, and high carrier mobility. This Minireview introduces state-of-the-art COF photocatalysts based on different linkages and discusses the origin of photocatalytic activities for hydrogen evolution. Three typical COF photocatalysts, with linkages including imine (-C=N-), β-ketoenamine (O=C-C=C-NH-), and vinylene (-C=C-), are discussed with a particular focus on the advancements in synthetic methodologies and structural design, as well as photoelectronic properties that are relevant to photocatalytic performance. The Minireview is expected to elucidate their structure-property relationships and the way to design photoactive COFs with enhanced performances.