Thermally Activated Delayed Fluorescence in a Y₃N@C₈₀ Endohedral Fullerene: Time‐Resolved Luminescence and EPR Studies

Abstract

Abstract:The endohedral fullerene Y 3 N@C 80 exhibits luminescence with reasonable quantum yield and extraordinary long lifetime.B yv ariable-temperatures teady-state and timeresolved luminescence spectroscopy, it is demonstrated that above6 0Kthe Y 3 N@C 80 exhibits thermally activated delayed fluorescence with maximum emission at 120 Kand anegligible prompt fluorescence.B elow 60 K, ap hosphorescence with al ifetime of 192 AE 1msi so bserved. Spin distribution and dynamics in the triplet excited state is investigated with X-and W-band EPR and ENDOR spectroscopies and DFT computations.Finally,electroluminescence of the Y 3 N@C 80 /PFO film is demonstrated opening the possibility for red-emitting fullerene-based organic light-emitting diodes (OLEDs).Fullerenes usually exhibit low quantum yields (QY) of fluorescence (in the range of 10 À4 )owing to the slow radiative decay and af ast and efficient inter-system crossing (ISC), leading to an almost quantitative formation of triplet states with slow non-radiative decay.E xohedral chemical derivatization can substantially modify the p-system of fullerenes and produce highly luminescent multi-adducts.[1] Another way to enhance the quantum yield of luminescence of fullerenes is based on the thermal repopulation of the S 1 state,v ia the T 1 state,i ft he S 1 -T 1 gap is sufficiently small. This process is known as thermally activated delayed fluorescence (TADF) and has been found to increase the fluorescence quantum yield of C 70 up to 0.08 at high temperatures.[2] TADF is currently an extensively examined phenomenon in the field of organic light-emitting diodes (OLEDs) as it allows to harvest up to 100 %ofelectrically formed excitons. [3] With the exception of several Er-and Tm-based endohedral metallofullerenes (EMFs), [4] with metal-based emission, EMFs are also non-luminescent.[5] Thei nternal heavy-atom effect leads to even faster ISC in EMFs than in empty fullerenes.T he lifetime of the S 1 state in Sc 3 N@C 80 is only 48 ps.[6] Surprisingly,Y 3 N@C 80 exhibits luminescence with reasonable quantum yield of ca 1% or even 8% in ac ycloadduct, [7] and an unusually long luminescence lifetime of 200-800 ns at room temperature.H erein, we explore the photoemission mechanism of Y 3 N@C 80 with variable-temperature time-resolved luminescence spectroscopy and EPR spectroscopy.The former reveals the evolution of the emission spectra and lifetimes with temperature,w hereas the latter provides information on relaxation dynamics and spin density distribution in the triplet state of Y 3 N@C 80 .W eshow that efficient photoemission of Y 3 N@C 80 is caused by the small S 1 -T 1 gap leading to TADF above 60 K. In ab lend with polyfluorene (PFO), Y 3 N@C 80 exhibits electroluminescence,w hich suggests that after suitable optimization and boost of the QY, Y 3 N@C 80 may be used for red or NIR-emitting OLEDs.In deoxygenated toluene solution, Y 3 N@C 80 exhibits luminescence with aQ Yo f1 .7 %a nd lifetime of 0.95 msa t 296 K. Saturation with air leads to athreefold reduction of...

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

“…Thermally-activated delayed uorescence (TADF) described for Y 3 N@C 80 (ref. 51) and empty fullerenes 56 probably take place in all Y x Sc 3Àx N@C 80 compounds. This hypothesis is further corroborated via temperature-dependent PL study.…”

“…49,50 Recently we found that the strong photoemission of Y 3 N@C 80 is caused by the small singlet-triplet S 1 -T 1 gap of less than 0.1 eV, which enables thermally activated delayed uorescence (TADF) via thermal population of the emitting S 1 state from the T 1 "reservoir". 51 As the triplet state plays a crucial role in the process, in the same work we also characterized its dynamics and spin distribution by light-induced EPR and ENDOR spectroscopy and found that yttrium atoms have negligible spin populations. TADF is a crucial phenomenon for this work, and Fig.…”

“…11 . The spectra obtained with Y 3 N@C 80 were already analyzed in our previous work 51 and are reproduced here for completeness. The EPR spectrum of Y 3 N@C 80 shows a well resolved ZFS fine structure and the ENDOR spectra were recorded at field positions corresponding to the turning points of the ZFS pattern, providing a sufficient orientation selection.…”

“…The VT-PL studies of Y 3 N@C 80 reported by us earlier revealed three regimes outlined above. 51 When cooling from room temperature down to 120 K, the PL peak of Y 3 N@C 80 at 705 nm enhances its intensity approximately 20-fold, which is accompanied by a gradual shi to 701 nm and narrowing of the emission peaks. In this temperature range, TADF is the only emissive pathway.…”

“…The spatial separation of the HOMO and LUMO over the fullerene cage leads to the relatively small D ST gap in Y 3 N@C 80 and enables efficient TADF. 51 Enhancement of the spatial separation of the frontier orbitals due to the adherence of the LUMO to Sc atoms and the increase of the FMCT character of the rst excitation across the Y x Sc 3Àx N@C 80 series is the reason for an even further decrease of the D ST gap in the Y x Sc 3Àx N@C 80 molecules as x decreases from 3 to 0. And since the FMCT character is enhanced in excited-state conformers such as conf 3 (Fig.…”