Long-Lived Excited State in a Solubilized Graphene Nanoribbon

Abstract

Graphene nanoribbons have excellent light-absorbing properties but often exhibit short excited-state lifetimes that prevent their applications in photocatalysis. Here, we report a long-lived charge-transfer triplet excited state in a well-solubilized, chlorinated graphene nanoribbon (Cl-GNR) with edges modified by bipyrimidine (bpm) moieties. The photophysical behavior of Cl-GNR was observed and characterized by steady-state UV–vis absorption and emission spectroscopy, transient absorption spectroscopy on the ps-ms timescale, and density functional theory (DFT) calculations. Both the Cl-GNR and its monomeric subunit, chlorinated graphene quantum dot (Cl-GQD), were synthesized using bottom-up techniques to produce the H-analogs of the compounds followed by edge-chlorination to achieve soluble products. The absorption spectra of Cl-GQD and Cl-GNR appear in the UV–vis range with lowest-energy peaks at 375 and 600 nm, respectively. The excitons in Cl-GNR were found to exhibit charge-transfer character with the bpm edges serving as electron acceptors. DFT calculations indicate that the excitons are relatively localized, spreading over at most two monomeric units of the GNR. Transient absorption spectroscopy shows that singlet excited states of Cl-GQD and Cl-GNR undergo intersystem crossing with ∼300 ps lifetime to form triplet states that last for 15.7 μs (Cl-GQD) and 106 μs (Cl-GNR). These properties, combined with the ability of the bpm sites to coordinate transition metals, make Cl-GNRs promising light-harvesting motifs for photocatalytic applications.

Interfacial Electrochemistry of Catalyst-Coordinated Graphene Nanoribbons

“…The PDS of GNR-2DNS showed a broad absorption in the 400–700 nm range (Figure S6a), which is consistent with the UV/vis absorption spectrum reported for the analogous GNR in solution . Likewise, the calculated spectrum of a model compound with reduced size ( Flake , structure shown in Figure S8) exhibits the lowest energy transitions at 462 and 478 nm.…”

“…This effect is expected to be even more pronounced as one goes from Rh-Flake to RhGNR-2DNS since the ligandcentered transitions decrease in energy with the introduction of additional monomer units. 68 Given this reasoning, we conclude that the electronic spectroscopy of RhGNR-2DNS is dominated by the π−π* transitions located on the GNR, making the spectra less sensitive to Rh-coordination. Ligand-Centered Reduction.…”

“…67 The PDS of GNR-2DNS showed a broad absorption in the 400−700 nm range (Figure S6a), which is consistent with the UV/vis absorption spectrum reported for the analogous GNR in solution. 68 Likewise, the calculated spectrum of a model compound with reduced size (Flake, structure shown in Figure S8) exhibits the lowest energy transitions at 462 and 478 nm. These low energy transitions in Flake were assigned to the π−π* transitions with significant charge transfer character to the bipyrimidine unit, as illustrated by the molecular orbitals involved in the transition (Figure 1c).…”

Ultrafast photophysics of an orange–red thermally activated delayed fluorescence emitter: the role of external structural restraint

“…Upon UV excitation, high-lying S n states can be populated, followed by rapid internal conversion (IC, S n / S 1 ) to the longlived S 1 state, which explains the dramatically reshaped ESA in the 1 ps delay time. 64,65 For the subsequent delay times of 1 ps to 1 ns, fs-TA evolution visualizes wavepacket motion that highly depends on the topology of S 1 PES. 41,[66][67][68] For instance, the role of the excited-state structural relaxation (ES-SR) in the S 1 state decay of DA-and DAD/ADA-type TADF emitters has been intensively reported.…”

Electronic Structure of Isolated Graphene Nanoribbons in Solution Revealed by Two-Dimensional Electronic Spectroscopy

“…Solution-phase synthesis of GNRs allows easy process scale-up, of great importance for applications, but results in strong aggregation effects due to π–π stacking interactions. Aggregation also hinders characterization of the linear and nonlinear optical properties of the GNRs, − since it quenches the photoexcited states and prevents the study of the properties of the isolated nanostructures. Recently, we have demonstrated the synthesis of a new kind of GNR, decorated with pending Diels–Alder cycloadducts of anthracenyl units and N - n -hexadecyl maleimide (AHM) .…”