Fluorescence from graphene nanoribbons of well-defined structure

Abstract

Graphene nanoribbons synthesized by the bottom-up approach with optical energy gaps in the visible are investigated by means of optical spectroscopy. The optical absorption and fluorescence spectra of two graphene nanoribbons with different structures are reported as well as the life-time of the excited states. The possibility of the formation of excimer states in stacks of individual graphene nanoribbons is discussed in order to interpret the broad and highly Stokes-shifted luminescence lines observed on both structures. Finally, combined atomic force microscopy and confocal fluorescence measurements have been performed on small aggregates, showing the ability of graphene nanoribbons to emit light in the solid state. These observations open interesting perspectives for the use of graphene nanoribbons as near-infrared emitters.

Diels–Alder polymerization: a versatile synthetic method toward functional polyphenylenes, ladder polymers and graphene nanoribbons

“…represented a broad emission peak at 695 nm with a large Stokes shift of ~135 nm, which might originate from excimer-like states in stacks of individual GNRs or other aggregation-induced effects and is currently under further investigations (Figure 3b). 96 The corresponding photoluminescence excitation was consistent with the absorption band, proving that the observed Stokes shifted luminescence arose from the GNRs. 96 The scanning tunneling microscopy of GNR 65a and atomic force microscopy (AFM)…”

Toward Thiophene‐Annulated Graphene Nanoribbons

“…[15] The slightly lower band gap of oT-GNR can be attributed to the presence of electron-rich thiophene units on the edge of the [n]phenacene backbone.R emarkably,t he solid-state absorption spectrum is only slightly broader than that in solution, meaning that very weak intermolecular p interactions take place in the solid state.U V/Vis analysis in solution showed that the spectrum of oT-GNR is concentration-independent, ruling out the possibility of aggregation in solution to explain such asimilarity between the two spectra. [17] TheS tokes shift in solution for both oT-GNR and pT-GNR are relatively large (206 and 170 nm, respectively) and in the same range as that observed for ladder conjugated polymers with ap oly(paraphenylene) backbone. [16] Interestingly, pT-GNR possesses amuch broader absorption band with a l max value of 390 nm and ab and gap of 2.61 eV.T he lower band gap of pT-GNR compared to oT-GNR can be attributed to abetter electronic communication between the thiophene units (para versus ortho)a nd the presence of an acene moiety (anthracene) within the backbone.S imilar to oT-GNR, the solid-state spectrum is very comparable to that in solution, proving the rigid nature of pT-GNR.…”

“…Both oT-GNR and pT-GNR are photoluminescent in solution and exhibit featureless emission band peaking at 500 and 560 nm, respectively (Figure 3, inset) with quantum yield (f F )of3and 6%,respectively.T ime-resolved photoluminescence spectroscopy have been performed and fluorescence lifetime values of 1.1 and 1.7 ns have been measured for oT-GNR and pT-GNR, respectively,which is in the same order of magnitude as that measured for all-phenyl GNRs (Supporting Information, Figures S32 and S33). [17] TheS tokes shift in solution for both oT-GNR and pT-GNR are relatively large (206 and 170 nm, respectively) and in the same range as that observed for ladder conjugated polymers with ap oly(paraphenylene) backbone. [12b] Notably,the fluorescence excitation spectrum of both pT-GNR and oT-GNR matches well with the absorption spectra, meaning that the emission originated from the GNRs (Supporting Information, Figures S28 and S30).…”

“…[16] Interestingly, pT-GNR possesses amuch broader absorption band with a l max value of 390 nm and ab and gap of 2.61 eV.T he lower band gap of pT-GNR compared to oT-GNR can be attributed to abetter electronic communication between the thiophene units (para versus ortho)a nd the presence of an acene moiety (anthracene) within the backbone.S imilar to oT-GNR, the solid-state spectrum is very comparable to that in solution, proving the rigid nature of pT-GNR. [17] Both oT-GNR and pT-GNR have ac ontorted structure and can adopt different conformations.T ostudy the different conformers and their relative energy,D FT calculations have been performed. [17] TheS tokes shift in solution for both oT-GNR and pT-GNR are relatively large (206 and 170 nm, respectively) and in the same range as that observed for ladder conjugated polymers with ap oly(paraphenylene) backbone.…”

“…[12b] Notably,the fluorescence excitation spectrum of both pT-GNR and oT-GNR matches well with the absorption spectra, meaning that the emission originated from the GNRs (Supporting Information, Figures S28 and S30). [17] Both oT-GNR and pT-GNR have ac ontorted structure and can adopt different conformations.T ostudy the different conformers and their relative energy,D FT calculations have been performed. Calculations suggest that pT-GNR adopts av ery rigid geometry due to the steric congestion at the thiophene H-contorted region (Supporting Information , Figure S36) for which the inversion into the other isomer is not possible.O nt he contrary, oT-GNR may adopt several geometries.A ss hown on Figure 4, steric congestion in the S-contorted region of the polymer forces whether ah elical (C1) or an alternated (C5) conformation occurs.According to…”