Long‐Lived Charge‐Transfer State Induced by Spin‐Orbit Charge Transfer Intersystem Crossing (SOCT‐ISC) in a Compact Spiro Electron Donor/Acceptor Dyad

Dongyi Liu,Zhijia Wang,Mariangela Di Donato,Jianzhang Zhao,Laura Bussotti,Ahmed M. El‐Zohry,Maria Taddei,Omar F. Mohammed,Stefan Weber,Clemens Matt

doi:10.1002/ange.202003560

Abstract

AbstractWe prepared conceptually novel, fully rigid, spiro compact electron donor (Rhodamine B, lactam form, RB)/acceptor (naphthalimide; NI) orthogonal dyad to attain the long‐lived triplet charge‐transfer (3CT) state, based on the electron spin control using spin‐orbit charge transfer intersystem crossing (SOCT‐ISC). Transient absorption (TA) spectra indicate the first charge separation (CS) takes place within 2.5 ps, subsequent SOCT‐ISC takes 8 ns to produce the 3NI* state. Then the slow secondary CS (125 ns) gives the long‐lived 3CT state (0.94 μs in deaerated n‐hexane) with high energy level (ca. 2.12 eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as 1NI*→1CT→3NI*→3CT. With time‐resolved electron paramagnetic resonance (TREPR) spectra, an EEEAAA electron‐spin polarization pattern was observed for the naphthalimide‐localized triplet state. Our spiro compact dyad structure and the electron spin‐control approach is different to previous methods for which invoking transition‐metal coordination or chromophores with intrinsic ISC ability is mandatory.

Highlights

Mimicking the natural photosynthetic reaction centers with the aim to attain a CT state with high energy level and long lifetime has attracted much attention.[1 6]
-conjugation between the N atom and the NI’s framework is significant for RB-NI-N, which will decrease the energy level of 3LE below that of the CT state
The long-lived CT state is based on electron spin control, i.e. to initiate the final charge separation (CS) with a triplet precursor, which is achieved with the spin orbit charge transfer intersystem crossing (SOCT-ISC), without invoking of any heavy atom effect or a chromophore with intrinsic ISC ability

Summary

Introduction

[e] ICCOM-CNR, via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy. Supporting information for this article is given via a link at the end of the document. Radical pair intersystem crossing (RP ISC) mechanism can be used as an electron spin control method, in which the probability of 1CT 3CT transition increases, prolonging the lifetime of the CT states. One possibility is to use a transition-metal coordination framework to attach the electron D and A because the coordination center has usually a ultrafast ISC, through a 1MLCT 3MLCT 3CT process.[22,24] For instance, a long-lived 3CT state We report a new electron spin control approach, based on the spin orbit charge transfer ISC (SOCT ISC) mechanism, to obtain a long-lived 3CT state (0.94 s, at room temperature),[22,26 28] using a novel, simple, fully rigid, spiro electron D (lactam form Rhodamine B, RB)/A (naphthalimide, NI) compact dyad to induce orthogonal orientation and a fully rigid connection between RB and NI units. SOCT ISC is supposed to render 1CT 3NI* process fast, and the close proximity of NI and the RB units to increase the J value, suppressing the undesired backward 3CT 3NI* 1CT processes

Results and Discussion

NI-NH RB-NI d

Conclusion

Conflict of interest