光致異構化、激發態質子轉移以及重金屬提升磷光放光之光物理性質與光譜動力學之探討

Abstract

這份論文主要分成四個部份，第一部份針對 Salen type ligand, 2,3-bis(4-(di-p-tolylamino)-2-hydroxybenzylideneamino) maleonitrile 照光受激發後，所進行的異構化機制加以探討。異構化發生在該分子的 2 號和 3 號碳碳之間的雙鍵，根據合成的反應機制，所得到的最終產物應該是順式的結構，但經過 X 光單晶分析卻得到反式的結構。然而在避光下所取得的晶體經相同的分析後，才得到預期的順式結構，因此整個研究的方向便轉朝光致異構化來加以解釋。在Salen type ligand中，此一現象乃首度發現，相關性質上的差異也在本論文中被加以研究。 第二部分關於具有雙路徑激發態質子轉移的 3-hydroxyflavone 衍生物， 3-hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one, 在其質子轉移機制上所做的研究。雙路徑質子轉移在 3-hydroxyflavone 衍生物中，此分子為首例，透過分子的設計，質子轉移的路徑可以發生在 hydroxyl group 和 pyridinic group 之間或是 hydroxyl group 和 carbonyl group 之間。固態時，由 X-ray 的分析結果可知，僅前者會發生，而在溶液態時，兩個路徑皆可進行，但前者因 pyridinic group 會旋轉而不放光。透過酸鹼控制或保護基的引入﹙於溶液態時﹚，可將 pyridinic group protonate 或 protect 起來，此時質子轉移僅只能發生在 hydroxyl group 和 carbonyl group 之間。雙路徑質子轉移，在經過適當的調控後，可切換回以往的單一路徑，進而在螢光光譜或吸收光譜上呈現出差異。 最後，第三和第四部份為鋨 (II) 和鉑 (II) 金屬錯合物於發光材料上，光物理性質與機制的探討。因鋨和鉑為重原子，透過重原子效應， intersystem crossing 得以更有效地發生，和放螢光的 ligand 形成金屬錯合物後，放光會改以磷光的機制為主，藉由對 ligand 做不同的取代基修飾進而達到調整金屬錯合物的放光波長波長的目的。

Part I. The strategic design of a Salen ligand possessing strong excited-state charge transfer properties leads to a previously unrecognized feature in the photo-induced isomerization. The synthesis of 2,3-bis(4-(di-p-tolylamino)-2-hydroxybenzylideneamino) maleonitrile (1) rendered Z and E conformers in the absence and presence of room light, respectively. We conclude that the 1Z → 1E isomerization in the excited state is triggered by the rotation of the maleonitrile C(2)-C(3) (1Z) bond, which becomes single-bond like due to the photoinduced charge transfer reaction. Part II. The dual function of the excited-state intramolecular proton transfer (ESIPT) in the newly synthesized 3-hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one has been recognized. One of the two ESIPT routes typically happens between the hydroxyl group and the carbonyl group in a five-membered ring configuration. The other occurs between the hydroxyl group and the pyridinic group in a six-membered ring configuration. The switchable ESIPT implies the potential applications of the pH fluorescent indicator and the anion chemosensor. The dual function in the 3-hydroxyflavone derivative is believed to further spark large interests for the utilization of the ESIPT in a more practical way. Part III. Cyclometalated osmium complexes with the formulas [Os(ppy)2(CO)2] (1a,b), [Os(dfppy)2(CO)2] (2a,b), and [Os(btfppy)2(CO)2] (3a,b) have been synthesized, for which the chelating chromophores ppyH, dfppyH, and btfppyH denote 2-phenylpyridine, 2-(2,4-difluorophenyl)pyridine, and 2-(2,4-bis(trifluoromethyl)phenyl)pyridine, respectively. UV–vis and emission spectra were measured, revealing the lowest excited state for all complexes as a nominally ligand-centered 3ππ* state mixed with certain MLCT character. Introduction of the electron-withdrawing substituents on the cyclometalated chelates or replacement of one CO ligand with phosphine at the metal center increased the MLCT contribution in the first excited state, giving a broad and featureless emission with greatly enhanced quantum yields. Part IV. A series of Pt(II) complexes Pt(fpbpy)Cl (1), Pt(fpbpy)(OAc) (2), Pt(fpbpy)(NHCOMe) (3), Pt(fpbpy)(NHCOEt) (4) and [Pt(fpbpy)(NCMe)](BF4) (5) with deprotonated 6-(5-trifluoromethyl-pyrazol-3-yl)-2,2’-bipyridine terdentate ligand were prepared, among which 1 was converted to complexes 2 ~ 5 via simple ligand substitution. In contrast to 3, which exhibits concentration-independent red phosphorescence at 604 nm, complex 1 shows strong concentration-dependent emission, giving normal (560 nm) and excimeric-like dual emission (~660 nm), manifesting its strong Pt(II)-Pt(II) interaction in the exited state. In sharp contrast, complex 3 exhibits ligand centered ππ* phosphorescence at 505 nm in solid state versus red shifted emission at 604 nm recorded in CH2Cl2 solution.