過渡金屬錯合物與雙光子吸收分子的光物理性質

林昭辰

doi:10.6342/NTU.2010.00034

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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 , Masters　　Advisor：周必泰　　

英文 DOI： 10.6342/NTU.2010.00034

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

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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.

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Chapter 1
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(6) Grätzel, M. Inorg. Chem. 2005, 44, 6841.
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