Photophysics of graphene quantum dot assemblies with axially coordinated cobaloxime catalysts

Abstract

We report a study of chromophore-catalyst assemblies composed of light harvesting hexabenzocoronene (HBC) chromophores axially coordinated to two cobaloxime complexes. The chromophore-catalyst assemblies were prepared using bottom-up synthetic methodology and characterized using solid-state NMR, IR, and x-ray absorption spectroscopy. Detailed steady-state and time-resolved laser spectroscopy was utilized to identify the photophysical properties of the assemblies, coupled with time-dependent DFT calculations to characterize the relevant excited states. The HBC chromophores tend to assemble into aggregates that exhibit high exciton diffusion length (D = 18.5 molecule2/ps), indicating that over 50 chromophores can be sampled within their excited state lifetime. We find that the axial coordination of cobaloximes leads to a significant reduction in the excited state lifetime of the HBC moiety, and this finding was discussed in terms of possible electron and energy transfer pathways. By comparing the experimental quenching rate constant (1.0 × 109 s−1) with the rate constant estimates for Marcus electron transfer (5.7 × 108 s−1) and Förster/Dexter energy transfers (8.1 × 106 s−1 and 1.0 × 1010 s−1), we conclude that both Dexter energy and Marcus electron transfer process are possible deactivation pathways in CoQD-A. No charge transfer or energy transfer intermediate was detected in transient absorption spectroscopy, indicating fast, subpicosecond return to the ground state. These results provide important insights into the factors that control the photophysical properties of photocatalytic chromophore-catalyst assemblies.

Long-Lived Excited State in a Solubilized Graphene Nanoribbon

“…These photocatalytic reactions have also been demonstrated for GQDs used in 0D/2D van der Waals heterojunctions with graphene . GQDs can also self-assemble into π-stacked structures that efficiently harvest light along the stack. − Furthermore, the photocatalytic properties of GQDs are enhanced upon coordination of a metal catalyst, such as cobaloximes for hydrogen evolution chemistry . Unlike GQDs, application of GNRs remains relatively unexplored, yet these compounds show great potential in photocatalysis since their absorption profiles cover much of the visible light region, making them efficient solar light absorbers.…”

“…8−10 Furthermore, the photocatalytic properties of GQDs are enhanced upon coordination of a metal catalyst, such as cobaloximes for hydrogen evolution chemistry. 11 Unlike GQDs, application of GNRs remains relatively unexplored, yet these compounds show great potential in photocatalysis since their absorption profiles cover much of the visible light region, making them efficient solar light absorbers. In addition to their excellent light harvesting, GNRs exhibit high charge carrier mobilities 12 that are expected to enable efficient delivery of charge carriers to the oxidation and reduction catalytic sites for photocatalysis.…”

“…34 However, the exciton lifetimes were found to be relatively short-lived due to excimer formation and the presence of dark states. 32,33 As part of our ongoing interest in the light-harvesting properties of nanographenes, 8,11,37 we investigate here the photophysical properties of hydrogenated and chlorinated GQDs and GNRs that contain bipyrimidine moieties in their edges (Scheme 1). Here, H-GQD and Cl-GQD can be thought of as the basic building blocks of H-GNR and Cl-GNR, respectively.…”

Long-lived Excited State in a Solubilized Graphene Nanoribbon

“…Furthermore, the photocatalytic properties of GQDs are enhanced upon coordination of a metal catalyst, such as cobaloximes for hydrogen evolution chemistry. 11 Unlike GQDs, application of GNRs remains relatively unexplored, yet these compounds show great potential in photocatalysis since their absorption profiles cover much of the visible light region, making them efficient solar light absorbers. In addition to their excellent light harvesting, GNRs exhibit high charge carrier mobilities 12 which are expected to enable efficient delivery of charge carriers to the oxidation and reduction catalytic sites for photocatalysis.…”

“…34 However, the exciton lifetimes were found to be relatively short-lived due to the excimer formation and the presence of dark states. 32,33 As part of our ongoing interest in light-harvesting properties of nanographenes 8,11,35 , we investigate here the photophysical properties of hydrogenated and chlorinated GQDs and GNRs that contain bipyrimidine moieties in their edges (Scheme 1). Here, H-GQD and Cl-GQD can be thought of as the basic building blocks of H-GNR and Cl-GNR, respectively.…”