Long‐Lived Charge‐Transfer State in Spiro Compact Electron Donor–Acceptor Dyads Based on Pyromellitimide‐Derived Rhodamine: Charge Transfer Dynamics and Electron Spin Polarization

Abstract We observed a long‐lived charge transfer (CT) state in a novel orthogonal compact electron donor–acceptor dyads, with closed form of rhodamine (Rho) as electron donor and pyromellitimide (PI),or thionated PI, as electron acceptor. The two parts in the dyads are connected via a spiro quaternary carbon atom, thus the torsion between the donor and acceptor is completely inhibited, which is beneficial to reduce the reorganization energy and to exploit the Marcus inverted region effect to prolong the CT state lifetime. Femtosecond transient absorption spectra show that the charge separation is rather fast, while nanosecond transient absorption spectra confirmed the formation of long‐lived CT state (2.6 μs). Time‐resolved electron paramagnetic resonance (TREPR) spectra determined the spin multiplicity of the long living state and assigned it to a 3CT state. Replacement of an oxygen atom in the PI part with a sulfur atom favoring classical intersystem crossing processes, causes a consistently shortening of the lifetime of the 3CT state (0.29 μs).

Synthesis of compound PI-1 and PI. Pyromellitic Dianhydride (9.2 mmol, 2.00 g) was added into a 100 mL twoneck bottle and dry DMF (35 mL) was added under N2 atmosphere. The mixture was heated at 80C, n-octylamine (1.2 mL, 9.2 mmol) with dry DMF (15 mL) were added into the mixture within an hour, then the temperature increased 110C for 11 h. After the reaction solution was cooled to room temperature, put the reaction solution in the refrigerator and stand overnight. The solid was collected by filtration, which is a disubstituted product (PI-1). DMF in the filtrate was distilled off under reduced pressure. Then dissolve the viscous mixture in dichloromethane (15 mL), the un-reacted anhydride was removed by filtration. The solvent was removed by rotary evaporator. Then disubstituted product (PI-1) was purified by column chromatography (DCM/PE = 1/1) to give a white solid, the compound (PI) was purified by column chromatography (DCM to DCM/MEOH 20:1 gradient elution) to give a white solid. PI was obtained, yield: 1.10 g (38%). 1

Synthesis of compound PI-O-Rho.
m-Cresole (0.65 g, 6 mmol), PI-1 (0.92, 2.79 mmol) and concentrated sulfuric acid (3 mL) were heated at 80 °C for mixture to room temperature, the reaction mixture was poured into ice-water mixture (22 mL), stirred vigorously, and put it in the refrigerator to stand overnight. The viscous solid was collected by filtration, the solid was added to ammonia water (10 mL) (NH3/H2O = 1/4), concentrated hydrochloric acid was added to bring pH < 2, under ice bath condition. The solid was collected by filtration. Then the solid was dissolved in DCM, the organic solution was dried over anhydrous Na2SO4. The crude product was purified by column chromatography (silica gel, DCM/ MeOH = 10/1, v/v). PI-O-Rho was obtained as purple solid. Yield: 610 mg (35%). 1

Synthesis of compound PI-Rho.
Under N2 atmosphere, PI-O-Rho (300 mg, 0.5 mmol) were dissolved in dry 1,2-dichloroethane (5 mL), then POCl3 (0.2 ml) was added dropwise into the mixture, the mixture was stirred and refluxed for 6 h. After cooling to room temperature, the solvent was removed under reduced pressure, the remaining mixture was dissolved in dry acetonitrile (10 mL). n-Butylamine (

Synthesis of compound PI-Rho-S.
Under N2 atmosphere, PI-Rho (100 mg, 0.15 mmol) and Lawesson's reagent (198 mg, 0.5 mmol) were dissolved in dry p-xylene (18 mL), then the mixture was stirred at 150C for 3 h. After reaction, saturated NaHCO3 aqueous solution (30 mL) was added, the mixture was extracted with ethyl acetate (3×10 mL). The brown organic layers were combined and MeOH (100 mL) was added. The solvent was evaporated under reduced pressure. The crude product was purified using column chromatography (silica gel, DCM: PE = 1:2, v/v) to give an orange solid. Yield: 20 mg (50%

Synthesis of compound RB-C.
Under N2 atmosphere, RB (100 mg, 0.23 mmol) were dissolved in dry 1,2-dichloroethane (5 mL), then POCl3 (0.1 mL) was added dropwise to the mixture. Then the reaction mixture was stirred and refluxed for 6 h. After cooling to room temperature, the solvent was removed by rotary evaporation, the remaining mixture was dissolved in dry acetonitrile (10 mL). Then under N2 atmosphere, n-butylamine (1 mL) and Et3N (0.3 mL) were added dropwise to the mixture at room temperature, the mixture was refluxed for 25 h. The solvent was evaporated under reduced pressure. The product was purified by column chromatography (silica gel, DCM/PE = 1/1) to give a pink solid. Yield: 101 mg (90%). 1

Synthesis of compound RB-S.
Under N2 atmosphere, RB-C (100 mg, 0.2 mmol) and Lawesson's Reagent (198 mg, 0.5 mmol) were dissolved in dry p-xylene (18 mL), then the mixture was stirred at 150C for 3 h. After reaction, saturated NaHCO3 aqueous solution (30 mL) was added, the mixture was extracted with ethyl acetate (3×10 mL), the brown organic layers were combined and MeOH (100 mL) was added. The solvent was evaporated under reduced pressure. The crude product was further purified using column chromatography (silica gel, DCM/ PE = 1/2) to give a white solid. Yield: 88 mg (95%). 1 Figure S20. Dihedral angle between xanthene and the PI planes of PI-Rho, the two color sheets show the planes of the PI and the RB moieties. Table S1. Single crystal X-ray diffraction data of PI-Rho (CCDC：2075565). Thermal ellipsoids are set at 50% probability. We used Hush eq S1 to calculate the electronic coupling matrix element (HAB) in neutral donor/acceptor systems. (1) where R is the separation between the center of electron donor and acceptor, in Å;

UVVis Absorption and Fluorescence Emission Spectra.
is the molar absorption coefficient at the maximum of the CT absorption band, in [M 1 cm 1 ];  is the absorption maximum of the CT absorption band in wavenumber scale, in cm 1 ; Δ 1/2 is the full width of the band. The HAB were calculated according to eq S1 and the data are presented in Table S2.                    For the femtosecond transient absorption, excitation at the CT absorption band at 370 nm were repeated in Figure S43 and S44. We don't expect drastic change of the photophysics of the D-A compounds upon photoexcitation at different wavelength, and the conclusion of the manuscript will not be changed by excitation at different wavelength. When using 370 nm excitation of the compound, the first species at 600-800 nm was attributed to 1 CT state with incomplete charge separation that appears around 12 ps. Another long-lived species (>3.3 ns) centred 545 nm was attributed to 3 CT absorption.