熱活化延遲螢光(thermally activated delayed fluorescence，TADF)材料，憑藉其第一單重激發態(S1)與第一三重激發態(T1)之間很小的能量差，約小於等於100 meV，使其三重態激子能發生逆向系統間跨越(reverse intersystem crossing，RISC)進而達到更高的放光量子效率。故科學家以光譜技術研究TADF材料的光激放光動力學過程，但所提及之動力學模型多為經驗式且缺乏正確定義之動力學參數。本實驗吾人將使用對稱雙硼基底結構之TADF材料tBuCzDBA，並以螢光強度時間側寫偵測系統測量其固體薄膜及其摻雜50 %w/w PMMA之固體薄膜受波長355 nm、440 nm及500 nm的脈衝雷射激發後之光激放光動力學過程。搭配密度泛函理論計算之能量輔助建立動力學模型解釋其螢光放光過程，並推測其輻射與非輻射緩解途徑的動力學和熱力學特性。
根據tBuCzDBA理論計算結果及其穩態紫外/可見光吸收光譜，吾人推測355 nm之脈衝雷射得將其激發至S_4，而S4→S1及S4→S1'之內轉換過程需要部份時間延遲進而使其螢光上升時間較慢。而440 nm及500 nm激發則主要將其激發至S1及部分S1'，伴隨後續的螢光緩解。吾人以一個四能態熱平衡之動力學模型解釋tBuCzDBA熱活化延遲螢光的動力學過程並用以擬合螢光強度時間側寫之緩解部分可獲得相關速率常數、tBuCzDBA之第一單重激發態與第一三重激發態的能量差(∆E_ST)及由S1'→S0輻射緩解放光之貢獻程度(I_S1')。擬合結果顯示，兩種tBuCzDBA的樣品受355 nm之脈衝雷射激發後的k(F)、k(ISC) 、k(RISC)及k(S1')皆較440 nm及500 nm激發為大，吾人推論係由於S4→S1及少部分S4→S1'之內轉換過程將提供多餘的能量或是發生其他非輻射緩解途徑而高估緩解速率。此外，依據分析S1'→S0輻射緩解放光之貢獻程度(I_S1')後，放光波形由S1及S1'的貢獻比例未有重大差異，故未造成激發波長相依之穩態螢光光譜的差異。此外，由分析k(ISC)及k(RISC)後所得之∆E_ST(18.6±3.1 meV)與密度泛函理論計算之結果(26 meV)相近。吾人在本研究中成功建立一個適當的動力學模型描述對稱雙硼基底螢光材tBuCzDBA之波長相依熱活化延遲螢光動力學過程與熱力學參數。

The thermally activated delayed fluorescence material is capable of achieving high efficiency in organic light-emitting diode by converting triplet excitons to singlet state via reversed intersystem crossing (RISC) which requires a small energy gap between the lowest singlet (S1) and triplet (T1) excited states(∆E_ST), ca. ≤100 meV. As a result, the photophysical dynamics of TADF materials have been extensively investigated with spectroscopic techniques. However, the present kinetics models, in term of prompt and delayed fluorescence rates, are not appropriate for analyzing the evolution of fluorescence. In this work, we investigated the fluorescence evolutions of intrinsic tBuCzDBA, which has been reported as a high-yield TADF material, and tBuCzDBA immobilized in PMMA(50 %w/w) upon excitation at 355, 440, and 500 nm. The complete kinetics modelling and time-dependent density functional theory calculations are employed to unravel the origins of these constituent fluorescence characteristics, in terms of k(F) 、k(ISC)、k(RISC) and ∆Est. Moreover, the contribution of S1' in the steady-state fluorescence contour(I_S1') can be estimated with the above-mentioned rate coefficients.
The rise of fluorescence upon excitation at 440 and 500 nm is faster than that at 355 nm, denoting that the excitation at 355 nm leads to the high electronic state (S4) and the late fluorescence is attributed to the internal conversion of S4→S1 and S4→S1'. Moreover, there is no significant difference among the contribution portions of S1' upon excitation at 355, 440, and 500 nm. Therefore, the steady-state fluorescence contours do not significantly depend on the excitation wavelengths and are nearly identical. Besides, the kinetics components including k(F)、k(ISC)、k(RISC) are accelerated owing to the excess energy provided by the internal conversion from upper state to the fluorescent states upon the excitation at 355 nm. In addition, the averaged ∆E_ST is 18.6±3.1 meV, consisting with the predicted values of 26 meV by time-dependent density functional theory calculations. Therefore, we demonstrated an appropriate kinetics model to unravel the mechanisms of the TADF molecules.

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