Electron transfer dynamics and excited state branching in a charge-transfer platinum(<scp>ii</scp>) donor–bridge-acceptor assembly
作者:Paul A. Scattergood、Milan Delor、Igor V. Sazanovich、Oleg V. Bouganov、Sergei A. Tikhomirov、Alexander S. Stasheuski、Anthony W. Parker、Gregory M. Greetham、Michael Towrie、E. Stephen Davies、Anthony J. H. M. Meijer、Julia A. Weinstein
DOI:10.1039/c4dt01682c
日期:——
A linear asymmetric Pt(II) trans-acetylide donorâbridge-acceptor triad designed for efficient charge separation, NAPPt(PBu3)2PhâCH2âPTZ (1), containing strong electron acceptor and donor groups, 4-ethynyl-N-octyl-1,8-naphthalimide (NAP) and phenothiazine (PTZ) respectively, has been synthesised and its photoinduced charge transfer processes characterised in detail. Excitation with 400 nm, â¼50 fs laser pulse initially populates a charge transfer manifold stemming from electron transfer from the Pt-acetylide centre to the NAP acceptor and triggers a cascade of charge and energy transfer events. A combination of ultrafast time-resolved infrared (TRIR) and transient absorption (TA) spectroscopies, supported by UV-Vis/IR spectroelectrochemistry, emission spectroscopy and DFT calculations reveals a self-consistent photophysical picture of the excited state evolution from femto- to milliseconds. The characteristic features of the NAP-anion and PTZ-cation are clearly observed in both the TRIR and TA spectra, confirming the occurrence of electron transfer and allowing the rate constants of individual ET-steps to be obtained. Intriguingly, 1 has three separate ultrafast electron transfer pathways from a non-thermalised charge transfer manifold directly observed by TRIR on timescales ranging from 0.2 to 14 ps: charge recombination to form either the intraligand triplet 3NAP with 57% yield, or the ground state, and forward electron transfer to form the full charge-separated state 3CSS (3[PTZ+âNAPâ]) with 10% yield as determined by target analysis. The 3CSS decays by charge-recombination to the ground state with â¼1 ns lifetime. The lowest excited state is 3NAP, which possesses a long lifetime of 190 μs and efficiently sensitises singlet oxygen. Overall, molecular donorâbridge-acceptor triad 1 demonstrates excited state branching over 3 different pathways, including formation of a long-distant (18 Ã
) full charge-separated excited state from a directly observed vibrationally hot precursor state.
设计了一种用于高效电荷分离的线性非对称Pt(II)反式乙炔供体-桥-受体三联体,含强电子受体和供体基团的NAPPt(PBu3)2Ph-CH2-PTZ(1)(4-乙炔基-N-辛基-1,8-萘酰亚胺(NAP)和吩噻嗪(PTZ)),已合成并详细表征了其光诱导电荷转移过程。用400nm、~50fs激光脉冲激发,首先在Pt-乙炔中心和NAP受体间发生电子转移,形成电荷转移系,并触发一系列级联的电荷和能量转移事件。在UV-Vis/IR光谱电化学、发射光谱和DFT计算的支持下,结合超快时间分辨红外(TRIR)光谱和瞬态吸收(TA)光谱,揭示了从飞秒到毫秒量级自洽的光物理图像,即激发态的演变过程。在TRIR和TA光谱中清楚地观察到NAP阴离子和PTZ阳离子的特征,证实了电子转移的发生,并得到了单个电子转移步骤的速率常数。有趣的是,1在0.2~14ps的时间尺度上,通过TRIR直接观察到非热化电荷转移系的三个独立的超快电子转移通道:电荷复合,分别形成萘酰亚胺三重激发态3NAP,得率57%,或形成基态;以及正向电荷转移,形成完全电荷分离态3CSS,得率10%,根据目标分析得出。3CSS通过电荷复合回到基态,寿命约为1ns。最低激发态为3NAP,具有190μs的长寿命,能高效地敏化单线态氧。总体而言,分子供体-桥-受体三联体1展示了在三种不同的途径上的激发态分支,包括从直接观察到的振动性热的先驱态形成长距离(18Å)的完全电荷分离激发态。