Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, USA.
Inorg Chem. 2011 Aug 15;50(16):7820-30. doi: 10.1021/ic200974h. Epub 2011 Jul 15.
The synthesis and photophysics of a new Re(I)-carbonyl diimine complex, Re(PNI-phen)(CO)(3)Cl, where the PNI-phen is N-(1,10-phenanthroline)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide is reported. The metal-to-ligand charge transfer (MLCT) emission lifetime was increased approximately 3000-fold at room temperature with respect to that of the model complex [Re(phen)(CO)(3)Cl] as a result of thermal equilibrium between the emissive (3)MLCT state and a long-lived triplet ligand-centered ((3)LC) state on the PNI chromophore. This represents the longest excited state lifetime (τ = 651 μs) that has ever been observed for a Re(I)-based CT photoluminescence at room temperature. The energy transfer processes and the associated rate constants leading to the establishment of the excited state equilibrium were elucidated by a powerful combination of three techniques (transient visible and infrared (IR) absorption and photoluminescence), each applied from ultrafast to the micro/milliseconds time scale. The MLCT excited state was monitored by transient IR using CO vibrations through time intervals where the corresponding signals obtained in conventional visible transient absorption were completely obscured by overlap with strong transients originating from the pendant PNI chromophore. Following initial excitation of the (1)LC state on the PNI chromophore, energy is transferred to form the MLCT state with a time constant of 45 ps, a value confirmed in all three measurement domains within experimental error. Although transient spectroscopy confirms the production of the (3)MLCT state on ultrafast time scales, Förster resonance energy transfer calculations using the spectral properties of the two chromophores support initial singlet transfer from (1)PNI* to produce the (1)MLCT state by the agreement with the experimentally observed energy transfer time constant and efficiency. Intersystem crossing from the (1)MLCT to the (3)MLCT excited state is believed to be extremely fast and was not resolved with the current experiments. Finally, triplet energy was transferred from the (3)MLCT to the PNI-centered (3)LC state in less than 15 ns, ultimately achieving equilibrium between the two excited states. Subsequent relaxation to the ground state occurred via emission resulting from thermal population of the (3)MLCT state with a resultant lifetime of 651 μs. The title chromophore represents an interesting example of "ping-pong" energy transfer wherein photon excitation first migrates away from the initially prepared (1)PNI* excited state and then ultimately returns to this moiety as a long-lived excited triplet which disposes of its energy by equilibrating with the photoluminescent Re(I) MLCT excited state.
报告了一种新的 Re(I)-羰基二亚胺配合物 Re(PNI-phen)(CO)(3)Cl 的合成和光物理性质，其中 PNI-phen 是 N-(1,10-菲咯啉)-4-(1-哌啶基)萘-1,8-二羧酸二酰亚胺。由于 PNI 发色团上的发射（3）MLCT 态和长寿命三重态配体中心（3）LC 态之间的热平衡，使得金属-配体电荷转移（MLCT）发射寿命在室温下相对于模型配合物 [Re(phen)(CO)(3)Cl] 增加了约 3000 倍。这代表了迄今为止在室温下观察到的基于 Re(I)的 CT 光致发光的最长激发态寿命（τ=651μs）。通过三种技术（瞬态可见和红外（IR）吸收和光致发光）的强大组合，阐明了导致激发态平衡建立的能量转移过程和相关速率常数，每种技术都从超快时间尺度应用到微/毫秒时间尺度。通过 CO 振动在瞬态红外中监测 MLCT 激发态，通过时间间隔进行监测，在该时间间隔中，在常规可见瞬态吸收中获得的相应信号完全被来自悬垂 PNI 发色团的强瞬态掩盖。在 PNI 发色团上的（1）LC 态的初始激发后，能量转移以 45 ps 的时间常数形成 MLCT 态，该值在所有三个测量域内都在实验误差范围内得到确认。尽管瞬态光谱证实了在超快时间尺度上产生（3）MLCT 态，但使用两个发色团的光谱性质进行Förster 共振能量转移计算支持从（1）PNI* 到（1）MLCT 态的初始单重态转移，这与实验观察到的能量转移时间常数和效率一致。从（1）MLCT 到（3）MLCT 激发态的系间窜越被认为是非常快的，并且当前实验无法分辨。最后，三重态能量在不到 15 ns 内从（3）MLCT 转移到 PNI 中心（3）LC 态，最终在两个激发态之间达到平衡。随后通过（3）MLCT 态的热填充而发生到基态的弛豫，导致 651μs 的寿命产生发射。标题发色团代表了“乒乓”能量转移的有趣示例，其中光子激发首先从最初制备的（1）PNI* 激发态迁移，然后最终返回到该部分作为长寿命激发三重态，通过与光致发光 Re(I) MLCT 激发态平衡来释放其能量。
