過渡金屬錯合物與雙光子吸收分子的光物理性質

Photophysics of Transition Metal Complexes and Two-Photon Absorbing Chromophores

林昭辰(Chao-Chen Lin)

指導教授 ： 周必泰

國立臺灣大學/理學院/化學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.00034

摘要

有鑑於能源危機日趨嚴重，節能有效的照明方式和太陽能電池的研究越來越受重視，其中包括可用於有機發光二極體(OLED)或光敏太陽能電池(DSSC)的過渡金屬錯合物。很有趣的是，論文第一章中所討論的一系列鋨二價錯合物同時具有螢光與磷光，且激發波長越短，磷光的相對比例越高。此一現象肇因於第一單重激發態中幾乎沒有金屬軌域的貢獻，故不易轉換至三重態；然而在較高的激發態時金屬軌域的參與增加，使得三重態的生成加快，因而磷光的比例提高。這個機制最直接的證據來自超快光譜的解析，配合上理論計算便可完整的解釋一切所觀察到的光物理性質。目前我們也在某些銀一價的錯合物得到同樣的結果。
論文的第二、三章則轉為討論具有雙光子吸收的分子，因其在生物影像及醫學上有很高的應用價值。以近紅外光做雙光子激發可有更好的穿透率、較小的激發體積與解析度，且能減少對樣品的傷害。第二章中發表首見將具有分子內質子轉移的發光團用於設計雙光子吸收分子，得到一、二、三支分叉的不同結構。此系列分子同時具有電荷轉移的性質，故質子轉移的速率顯著的受到溶劑極性的影響。從吸收光譜與雙光子吸收截面積可知每一分支間彼此互有關聯，然而從放光的模式與動力學的研究上發現三種結構具有類似的性質。最後從計算發現當最低激發態進行振動緩解時，分子構型的改變會導致軌域由均勻的分佈在各分支間轉變為侷限在特定的部分。最後一章則研究一系列放光在近紅外光的雙光子吸收分子，探討不同取代基和取代位置對雙光子吸收截面積大小的影響，並發現可簡單的由吸收光譜和計算結果得知參與雙光子吸收的能態。

關鍵字

過度金屬錯合物 ； 雙光子吸收 ； 動力學量測 ； 質子轉移 ； 電荷轉移

並列摘要

Aimed at energy conservation and low-cost clean energy, over the past two decades, research on second- and third-row transition metal complexes has aroused considerable attention for their applications in phosphorescent emitters and solar energy devices. In Chapter 1, using a series of Os(II) transition metal complexes we demonstrate for the first time remarkable ratiometric changes in phosphorescence (P) versus fluorescence (F) that are excitation wavelength dependent. The P/F intensity ratio can be increased by as large as eight folds upon tuning the electronic excitation from lowest to higher lying transitions in solution and in solid state. The result is in stark contrast to the long-standing and well established photophysical phenomena in condensed phase. For the titled complexes, while the S1 → T1 intersystem crossing (ISC) is measured to be several hundred picoseconds, which is anomalously slow, femto-picosecond dynamics reveals an accelerated ISC rate (1011–1012 s-1) in the highly electronically excited state (Sn → Tm, n, m > 1), either competitive to or faster than Sn → S1 internal conversion (IC) and/or vibrational relaxation (VR). The results are rationalized by the negligible contribution of metal-to-ligand charge transfer or ligand-to-metal charge transfer (MLCT or LMCT) character in lower lying electronically excited states (e.g. S1). Conversely, contribution of MLCT/LMCT is significant in the highly electronically excited states, which greatly enhances the spin-orbit coupling and hence the rate of intersystem crossing. This mechanism turns out to be universal for all titled Os(II) complexes bearing different ligands, such as isoquinoline-triazolate (1 – 4), isoquinoline-pyrazolate (5 – 6) and -diketonate (7). In theory, any late transition metal complexes fulfilling the state-dependent increase of MLCT% are expected to exhibit this unique property, and it has been proven that in Ag (I) complex incorporating quinoline-pyrrolate and bis[2-(diphenylphosphino)-phenyl]ether moieties (8), the same photophysical phenomenon is demonstrated, i.e. “harvesting highly electronically excited energy to triplet states”. To summarize, the study integrates nicely the ultrafast kinetic studies via femto-picosecond transient absorption pump-probe spectroscopy and the theoretical calculation in explaining the exceptional steady-state observations.
In Chapter 2, the topic is switched to the dynamics of excited-state intramolecular proton transfer (ESIPT) and two-photon absorption (TPA). Dipolar 4’-N,N- diphenylamino-3- hydroxyflavone (1) has been synthesized for investigation of its excited-state charge-transfer (ESCT) coupled ESIPT dynamics via femtosecond fluorescence upconversion. With increased donor strength compared to the dialkylamino analogue, ESIPT appears to cease in the more polar solvent of acetonitrile. Considering that the NPh3 moiety is a typical donor core in two-photon absorbing chromophores, the corresponding quadrupolar and octupolar two- and three-branched derivatives (2 and 3) were synthesized, and the TPA cross-sections (σ2) at 800 nm of 1, 2 and 3 were measured to be 150, 270 and 465 GM, respectively, with the Z-scan technique. Coupling between branches is manifested in the red-shifted and asymmetric absorption band of 2, while the absorption of 3 is governed by the decreased donor strength. Being a first case of multi-branched TPA chromophores incorporating ESIPT, cooperative enhancement in 2 is verified after normalization with the molecular weight (2/MW). In addition, the ESIPT dynamics of 2 and 3 are also strongly dependent on the solvent polarity, signifying the charge-transfer character of the normal excited states (N*) despite their symmetric structures. As evidenced by the theoretical approach, the frontier orbitals of vibrationally relaxed (geometry optimized) N*, from which ESIPT should take place, are localized on one specific branch, leading to similar emission patterns and dynamics, whereas the orbitals contributing to Franck-Condon excitation (absorption) spread over the entire molecule. The localization is found to be facilitated by rotation of a specific branch pivoting on the central nitrogen atom, while planarity is maintained within each 3-hydroxyflavone chromophore.
Continuing with the investigation of TPA chromophores, in Chapter 3, a new series of virtually centrosymmetic near-IR emitting 2,7-bis-(2-ethyl-hexyl)-benzo[lmn][3,8]- phenanthroline-1,3,6,8-tetraone (NDI) based quadrupolar molecules (1 – 6) were synthesized and their TPA cross-sections at 800 nm were measured with the Z-scan method. The spectral properties of these compounds can be fine-tuned by modification of the donor segments. The corresponding σ2 values range from 229 ± 15 to 1092 ± 59 GM owing to differences in conjugation length and/or position of substitution. Furthermore, by analyzing the one-photon absorption spectra in combination with the calculated energy levels, the intermediate and final states in the TPA process can be pinpointed.

並列關鍵字

transition metal complex ； two photon absorption ； dynamic measurement ； proton transfer ； charge transfer

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