Organic donor:acceptor semiconductor nanoparticles (NPs) formed through the miniemulsion method have been shown to be active photocatalysts. Here, we report photocatalytic hydrogen (H2) evolution under sacrificial conditions with Pt as a cocatalyst by NPs comprising only the nonfullerene acceptor Y6, stabilized by either sodium dodecyl sulfate (SDS) or the thiophene-containing surfactant 2-(3-thienyl)­ethyl­oxy­butyl­sulfonate sodium salt (TEBS). Typically, changes in the photocatalytic activity of donor:acceptor NPs are associated with differences in morphology due to the use of surfactants. However, as these NPs are single component, their photocatalytic activity has a significantly lower dependence on morphology than two-component donor:acceptor NPs. Results from ultrafast transient absorption spectroscopy show a minor difference between the photophysics of the TEBS- and SDS-stabilized Y6 NPs, with free charges present with either surfactant. The similar photophysics suggest that both TEBS- and SDS-stabilized Y6 NPs would be expected to have similar rates of H2 evolution. However, the results from photocatalysis show that Y6 NPs stabilized by TEBS have a H2 evolution rate 21 times higher than that of the SDS-stabilized NPs under broadband solar-like illumination (400–900 nm). Transmission electron microscopy images of the Y6 NPs show effective photodeposition of Pt on the surface of the TEBS-stabilized NPs. In contrast, photodeposition of Pt is inhibited when SDS is used. Furthermore, the ζ potential of the NPs is higher in magnitude when SDS is present. Hence, we hypothesize that SDS forms a dense, insulating layer on the NP surface which hinders the photodeposition of Pt and reduces the rate of H2 evolution. This insulating effect is absent for TEBS-stabilized Y6 NPs, allowing a high rate of H2 evolution. The TEBS-stabilized Y6 NPs have a H2 evolution rate higher than most single-component organic photocatalysts, signaling the potential use of the Y-series acceptors for H2 evolution in Z-scheme photocatalysis.

Nanoparticles of acenes exhibit highly efficient intermolecular singlet fission (SF). Recent reports indicate that altering the morphology of 6,13-bis-(triisopropylsilylethynyl)­pentacene (TIPS-Pn) nanoparticles has a profound influence on their SF dynamics. Here, we show that poly­(vinyl alcohol) (PVA) induces a phase transition in preformed TIPS-Pn nanoparticles. These nanoparticles are amorphous when initially formed but crystalline after addition of PVA. Surface characterization indicates that a diffuse PVA layer surrounds the nanoparticles. We propose that a periodic interaction between the hydroxyl groups of PVA and TIPS groups of TIPS-Pn on the nanoparticle surface induces a large-scale structural rearrangement to yield crystalline TIPS-Pn. Such reorganization in preformed organic nanoparticles is unprecedented, and we believe that this is the first report of such an effect induced by polymer adsorption. Transient absorption spectroscopic results reveal that SF within these nanoparticles is accelerated by an order of magnitude upon structural rearrangement.

Aqueous nanoparticle (NP) dispersions are commonly used as model systems for the spectroscopic study of singlet exciton fission (SF) in acenes such as 6,13-(triisopropylsilylethynyl)­pentacene (TIPS-Pn). However, the potential for particle size effects to complicate interpretation of results in such model systems is generally ignored. In this work, we study amorphous TIPS-Pn NP dispersions prepared by the re-precipitation method over a range of particle sizes. Time-resolved fluorescence and femtosecond transient absorption spectroscopies show that exciton dynamics in these systems depend significantly upon particle size. Kinetic analysis reveals that SF becomes slower at smaller NP sizes, while triplet exciton decay (through both correlated triplet pair relaxation and geminate triplet–triplet annihilation) accelerates. These significant size-dependent effects are ascribed to increased morphological disorder within smaller NPs, weakening the intermolecular couplings which control SF and triplet migration. A non-radiative singlet quenching channel separate from SF is also identified, which has not been previously reported for NPs of SF-capable chromophores. This non-radiative singlet decay becomes a significant relaxation pathway at small particle sizes, substantially reducing SF yields. Interestingly, exciton kinetics in the largest NPs considered here (81 nm diameter) approach those of bulk amorphous TIPS-Pn. This work demonstrates that particle size effects are significant for small NPs of SF chromophores and must be accounted for in order to accurately model bulk materials with such NP dispersions.

We report the use of blend nanoparticles (NPs) of the organic semiconductors PM6 and Y6 for the photocatalytic production of hydrogen under sacrificial conditions, with a 2% mass loading of Pt cocatalyst. When prepared using TEBS, a thiophene-containing surfactant, these blend NPs have a desirable intermixed morphology. Under ≈1-sun illumination from 400 to 900 nm, hydrogen is produced at a rate of 8000 ± 400 μmol h–1 g–1. Remarkably, this rate remains high at 5200 ± 300 μmol h–1 g–1 under 650 to 900 nm excitation, where Y6 is exclusively excited, generating free charges by hole transfer from Y6 to PM6. The rate drops to 2400 ± 200 μmol h–1 g–1 under 400 to 600 nm excitation, where PM6 is preferentially excited and free charges are generated through electron transfer. We also show that the external quantum efficiency is wavelength-independent. This work is the first study to show that free charge generation through hole transfer contributes significantly to hydrogen evolution in a donor:acceptor blend.

TIPS-pentacene is a small-molecule organic semiconductor that is widely used in optoelectronic devices. It has been studied intensely owing to its ability to undergo singlet fission. In this study, we aim to develop further understanding of the coupling between the electronic and nuclear degrees of freedom of TIPS-pentacene (TIPS-Pn). We measured and analyzed the 2D electronic spectra of TIPS-Pn in solutions. Using center line slope (CLS) analysis, we characterized the frequency-fluctuation correlation function of the 0-0 vibronic transition. Strong oscillations in the CLS values were observed for up to 5 ps with a frequency of 264 cm −1 , which are attributable to a large vibronic coupling with the TIPS-Pn ring-breathing vibrational mode. In addition, detailed analysis of the CLS values allowed us to retrieve two spectral diffusion lifetimes, which are attributed to the inertial and diffusive dynamics of solvent molecules. Amplitude beating analysis also uncovered couplings with another vibrational mode at 1173 cm −1 . The experimental results can be described using the displaced harmonic oscillator model. By comparing the CLS values of the simulated data with the experimental CLS values, we estimated a Huang-Rhys factor of 0.1 for the ringbreathing vibrational mode. The results demonstrated how CLS analysis can be a useful method for characterizing the strength of vibronic coupling.