Competitive Electron Transfer and Enhanced Intersystem Crossing in Photoexcited Covalent TEMPO-Perylene-3,4:9,10-bis(dicarboximide) Dyads: Unusual Spin Polarization Resulting from the Radical-Triplet Interaction
Competitive Electron Transfer and Enhanced Intersystem Crossing in Photoexcited Covalent TEMPO-Perylene-3,4:9,10-bis(dicarboximide) Dyads: Unusual Spin Polarization Resulting from the Radical-Triplet Interaction
复制标题
DOI：
10.1021/jp909212c
复制
发表时间：
2010-02-04
影响因子：
2.9
通讯作者：
Wasielewski, Michael R.
中科院分区：
文献类型：
--
作者：
Colvin, Michael T.;Giacobbe, Emilie M.;Wasielewski, Michael R.
将稳定的2，2，6，6-四甲基-1-哌啶氧基（克里思）自由基在其4-位共价连接到二萘嵌苯-3，4：9，10-双（二甲酰亚胺）（PDI）的酰亚胺氮原子上以产生TEMPO-PDI，1，其在克里思和PDI之间具有明确的距离和取向。在选择性光激发TEMPO-PDI中的PDI发色团之后在甲苯中的瞬态光吸收实验显示，在tau = 45 +/- 1 ps时发生增强的系间交叉，导致克里思-（3）*PDI的形成，而在THF中的相同实验表明，电子转移反应克里思-（1）*PDI ->克里思+*-PDI-* 在τ = 1.2 +/-时发生0.2 ps，从而有效地与增强的系统间交叉竞争。时间分辨EPR（TREPR）光谱的光生三自旋系统克里思-（3）*PDI在甲苯中在295 K最初显示了一个广泛的信号分配给自旋极化的3*PDI，在较长的时间热化，并伴随着形成的发射极化克里思自由基。在295 K时，在THF中没有观察到信号.克里思-（3）*PDI在甲苯中在85 K下的TREPR光谱示出了由于克里思引起的发射/吸收信号和由于3*PDI引起的宽三重态信号，其具有其T-0子能级的过布居的自旋极化图案特征。这种不寻常的自旋极化模式不是由自由基对系间交叉引起的，因为在85 K下不发生电子转移。观察到的（3）*PDI的自旋极化不能很容易地解释先前讨论的机制，导致我们提出了一个新的自旋极化机制。这要求自由基和连接的三重态处于弱交换状态。
A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with tau = 45 +/- 1 ps, resulting in formation of TEMPO-(3)*PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-(1)*PDI -> TEMPO+*-PDI-* occurs with tau = 1.2 +/- 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-(3)*PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized 3*PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-(3)*PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to 3*PDI having a spin polarization pattern characteristic of overpopulation of its T-0 sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of (3)*PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism. which requires that the radical and attached triplet are in the weak exchange regime.