3號位取代硫二苯胺的初步光物理特性

Abstract

無論在藥物、染料敏化太陽能電池，或是有機發光二極體上，硫二苯胺（phenothiazine，PTZ）都有許多的應用。但由於它螢光量子產率低（在環己烷中小於1%），在應用上受到部分限制。有趣的是，當這個低螢光量子產率與硝基（-NO2），一個普遍公認螢光淬滅體，結合形成3-硝基硫二苯胺（3-nitrophenothiazine，PTZ-NO2）時，竟有極高的螢光量子產率（在環己烷中100%）。為了更進一步調查此現象及其背後的物理化學機制，我們設計並合成了一系列的C3取代硫二苯胺來做比較，其中取代基包含拉電子的腈基（-CN）、甲醯基（-CHO），及硝基（-NO2），以及推電子的甲氧基（-OMe）。與無取代的硫二苯胺及加了推電子基的硫二苯胺相比，拉電子基造成螢光量子產率明顯提高。在化學計算的幫助之下，我們發現π到π*的電子躍遷在PTZ及PTZ-OMe中是部分禁制躍遷，因其π*與硫上的未鍵結軌域混合；相反地，由於拉電子基降低了LUMO的能量，使它不再與未鍵結軌域混合，而造成π到π*變為容許躍遷。此研究展示了如何利用分子設計一窺PTZ中HOMO跟LUMO的能量，進而達到高螢光量子產率。

Phenothiazine (PTZ) is a versatile compound that possesses many applications in pharmaceutical chemistry, dye-sensitized solar cell, etc. However, its weak-fluorescent character (quantum yields less than 1% in toluene) impedes its further applications. Besides, the nitro group (-NO2) is widely considered as a fluorescence quencher. Interestingly, we can obtain a highly fluorescent chromophore by combining these two moieties, forming 3-nitrophenothiazine (PTZ-NO2). To make a fair comparison, a series of PTZ substituted with various electron-withdrawing groups (cyano -CN and formyl -CHO) and electron-donating group (methoxy -OMe) group at C3-position was designed and synthesized. As we observed, the three molecules with different electron-withdrawing groups exhibit brilliant quantum yields compared with the non-substituted PTZ and electron-donating group substituted derivative. With the aid of computational approaches, the results reveal that the electronic transitions in PTZ and PTZ-OMe are partially forbidden π to π* transition, in which the π* orbital is mixed with the nonbonding orbital character of the sulfur atom. On the contrary, the transitions in the electron-withdrawing substituted analogies show allowed π to π* transition, which is due to the addition of electron-withdrawing group lowering the energy level of LUMO to diminish their mixing with non-bonding orbitals. This work demonstrates a judicious chemical design to exploit the energy level of HOMO and LUMO in PTZ analogies to achieve the high quantum yields.