郭續(xù)更,朱秋玲
(河南大學(xué) 化學(xué)化工學(xué)院,河南 開封 475004)
3CzIPN分子熱活化延遲熒光機制的理論研究
郭續(xù)更*,朱秋玲
(河南大學(xué) 化學(xué)化工學(xué)院,河南 開封 475004)
運用含時密度泛函理論(TD-DFT)方法,在以苯為溶劑的可極化連續(xù)模型(PCM)下,研究了2,4,6-三(9-咔唑基)-間苯二腈(3CzIPN)分子發(fā)生熱活化延遲熒光(TADF)的反應(yīng)機制. 計算結(jié)果表明,3CzIPN分子的單-三態(tài)能量差非常小,僅為0.124 eV,這對反系間竄越(RISC)非常有利. 此外,3CzIPN分子的RISC速率達(dá)到了104數(shù)量級,表明3CzIPN分子可能是一個潛在的TADF發(fā)射體.
含時密度泛函理論;熱活化延遲熒光;系間竄越;反系間竄躍
有機發(fā)光二極管(OLEDs)在平板顯示器和一般照明設(shè)備中起著重要的作用[1],在OLEDs中,形成單線態(tài)和三線態(tài)激子的概率分別為25%和75%;與單線態(tài)激子相比,三線態(tài)激子的能量更低且壽命更長. 由于三線態(tài)激子只能發(fā)生無輻射弛豫,造成大部分激發(fā)態(tài)能量損失,導(dǎo)致傳統(tǒng)的熒光OLED分子的內(nèi)量子效率很低,僅為25%[2]. 自從BERBERAN-SANTOS和GARCIA在C70富勒烯中發(fā)現(xiàn)了不尋常的強延遲熒光以來[3],具有熱活化延遲熒光(TADF)特性的OLEDs因其接近100%的內(nèi)量子效率引起了人們的極大關(guān)注[4-8]. TADF分子能夠產(chǎn)生單線態(tài)激子,并立即通過快速的系間竄越 (ISC) 生成三線態(tài)激子. 由于TADF分子的單三態(tài)能量差非常小,三線態(tài)激子可以通過反系間竄越 (RISC)重新形成單線態(tài)激子. 最終,所有的單線態(tài)激子能夠通過發(fā)出即時熒光或延遲熒光的形式回到該分子的電子基態(tài).
近年來,很多課題組都在致力于獲得高效的TADF OLEDs材料[9-12]. 例如,ADACHI課題組設(shè)計了一系列基于咔唑基二氰基苯基的高效TADF發(fā)射體,發(fā)現(xiàn)它們的電致發(fā)光效率與磷光OLEDs的電致發(fā)光效率相當(dāng)[13]. 目前,許多具有延遲熒光特性的分子被合成,其外量子效率明顯高于傳統(tǒng)的熒光OLEDs. 最近,實驗和理論研究都發(fā)現(xiàn),2,4,5,6-四(9-咔唑基)-間苯二腈 (4CzIPN)分子是一個效率很高的TADF發(fā)射體[14-16]. 基于4CzIPN分子的結(jié)構(gòu),我們設(shè)計了一個與之類似的新分子,即2,4,6-三(9-咔唑基)-間苯二腈(3CzIPN)(如圖1所示),打算通過理論計算的方式來預(yù)測其發(fā)生延遲熒光的可能性. 為此,我們采用含時密度泛函理論(TD-DFT)方法計算了3CzIPN的基態(tài)和激發(fā)態(tài)的幾何結(jié)構(gòu)以及光譜性質(zhì),獲得了單三態(tài)能量差、熒光和磷光速率、自旋軌道耦合常數(shù)、系間竄越速率和反系間竄越速率等重要信息. 希望目前的理論計算結(jié)果有助于加深人們對TADF發(fā)射體的內(nèi)在電致發(fā)光性質(zhì)的理解.
圖1 3CzIPN的結(jié)構(gòu)Fig.1 Structure of 3CzIPN
運用密度泛函理論(DFT)方法優(yōu)化得到了3CzIPN分子的基態(tài)(S0)和最低三線態(tài)(T1)的平衡構(gòu)型. 采用含時密度泛函理論(TD-DFT)方法優(yōu)化得到了3CzIPN的最低單線態(tài)(S1)的穩(wěn)定結(jié)構(gòu)。在S0和S1結(jié)構(gòu)下,借助TD-DFT方法分別計算了3CzIPN的吸收和熒光光譜. 為了考察溶劑對3CzIPN光譜性質(zhì)的影響,以上計算都采用了以苯為溶劑的極化連續(xù)模型(PCM)[17]. 選取的密度泛函是B3LYP[18],基組是6-31G*[19],所有的DFT和TD-DFT計算都是在Gaussian 09[20]程序下完成的.
此外,在ADF程序[21]下,分別計算得到了3CzIPN的熒光速率kf(S1→ S0)和磷光速率kp(T1→ S0).
(R)ISC是一種非輻射衰減過程. 為了簡便起見,(R)ISC衰減速率可以用半經(jīng)典的Marcus速率方程計算得到[22-24]:
kB是玻爾茲曼常數(shù),T是溫度(設(shè)為300 K),V是S1態(tài)和T1態(tài)之間的自旋軌道耦合,ΔEST是S1態(tài)和T1態(tài)之間的能量差,λ是重組能. 在計算ISC速率時,ΔEST=E(T1) -E(S1).
為了確定3CzIPN能否產(chǎn)生延遲熒光,ΔEST是一個非常重要的參數(shù). 為此,采用6種不同的泛函(B3LYP、PBE0、BMK、CAM-B3LYP、M06-2X和M06-HF)計算得到了3CzIPN的ΔEST值.
2.1 單三態(tài)能量差
一般來說,ΔEST是研究分子能否發(fā)生RISC的一個重要的參數(shù),而在TD-DFT計算中,不同的密度泛函對ΔEST的結(jié)果影響很大. 為了給出一個合理的預(yù)測,運用6種不同的泛函(B3LYP、PBE0、BMK、CAM-B3LYP、M06-2X和M06-HF)計算得到了3CzIPN的ΔEST值,計算的結(jié)果被列于表1中. 從表1中可以看出,使用PBE0、BMK、CAM-B3LYP、M06-2X和M06-HF計算得到的ΔEST都比B3LYP的計算結(jié)果稍大. 前人的研究已經(jīng)指出[25],對于擁有咔唑基團(tuán)的TADF分子來說,B3LYP方法能夠?qū)ΖST值給出更為合理的預(yù)測,因此,B3LYP方法計算的3CzIPN的ΔEST結(jié)果應(yīng)該也是合理可靠的. 值得注意的是,這一結(jié)果與UOYAMA等[14]報道的TADF分子4CzIPN的ΔEST值(0.083 eV)十分接近,表明3CzIPN分子應(yīng)該也是一個潛在的TADF發(fā)射體.
表1 運用不同密度泛函計算得到的3CzIPN的S1和T1態(tài)的垂直激發(fā)能以及單三態(tài)能量差(單位:eV)
2.2 吸收和發(fā)射光譜
為了更好地理解3CzIPN的激發(fā)態(tài)性質(zhì),在TD-B3LYP/6-31G*水平下,借助以苯為溶劑的PCM模型,計算得到了3CzIPN的吸收和熒光光譜,如圖2所示. 從圖2中可以看出,3CzIPN的S0→ S1吸收峰位于465 nm處,其振子強度為0.100 6. 它的S1→ S0的發(fā)射峰位于636 nm處,在紅光范圍內(nèi),因此,3CzIPN分子可能是潛在的紅光材料.
圖2 TD-B3LYP模擬得到的3CzIPN在苯溶劑中的吸收和發(fā)射光譜Fig.2 TD-B3LYP-simulated absorption and emission spectra for 3CzIPN using benzene as solvent
2.3 電荷轉(zhuǎn)移距離和其他的光物理參數(shù)
為了考察3CzIPN分子S1態(tài)的電荷轉(zhuǎn)移性質(zhì),分析了電子激發(fā)期間的電荷轉(zhuǎn)移長度指數(shù)Δr以及空穴和電子之間的重疊積分S,相應(yīng)數(shù)據(jù)列于表2中. 從表2中可以看出,3CzIPN的電荷轉(zhuǎn)移距離為0.347 5 nm,S值非常小,僅為0.027,這表明3CzIPN的S1態(tài)是電荷轉(zhuǎn)移態(tài). 3CzIPN的S1態(tài)的電荷轉(zhuǎn)移特征可以從最高占據(jù)分子軌道(HOMO)和最低未占據(jù)分子軌道(LUMO)的電子分布中獲得更直觀的證據(jù). 如圖3所示,3CzIPN的HOMO上的電子主要分布在供體上,而LUMO上的電子主要分布在中心受體上,這意味著當(dāng)該分子激發(fā)到S1態(tài)時,分子的電子從供體基團(tuán)很容易轉(zhuǎn)移到中心的受體基團(tuán).
表2 3CzIPN的躍遷偶極矩(μ,Debye),電荷轉(zhuǎn)移距離(Δr, nm),重疊積分(S),HOMO、LUMO能級及能隙(eV),電荷轉(zhuǎn)移量,以及S0 → S1躍遷可能性
Table 2 Transition dipole moment (μ, in Debye), charge transfer distance (Δr, in nm), overlapping integral (S), HOMO,LUMO energy and energy gap (, in eV), charge transfer amount (q), and S0→ S1 transition possibility of 3CzIPN
表2 3CzIPN的躍遷偶極矩(μ,Debye),電荷轉(zhuǎn)移距離(Δr, nm),重疊積分(S),HOMO、LUMO能級及能隙(eV),電荷轉(zhuǎn)移量,以及S0 → S1躍遷可能性
μΔrSHOMOLUMOeqmainconfiguration0.1790.34750.027-5.74-2.433.310.7456HOMO→LUMO(97.7%)
圖3 3CzIPN的前線分子軌道Fig.3 Frontier molecular orbital (FMO) for 3CzIPN
圖4 (a)S1態(tài)的電子-空穴分布,藍(lán)色和綠色分別表示空穴和電子; (b) 電子-空穴對于分子的S1態(tài)的重疊;(c)S1態(tài)的自然躍遷軌道 (NTO)Fig.4 (a) Electron-hole distribution in the S1 state, blue and green represent the hole and electron, respectively; (b) Overlap of the electron-hole for the S1 state; (c) natural transition orbital (NTO) of the S1 state for 3CzIPN
為了更深入地理解3CzIPN激發(fā)態(tài)的性質(zhì),該分子的電子-空穴分布被展示在圖4(a)中.從圖4(a)可以看出,3CzIPN的電子主要分布在受體上,而空穴主要分布在供體上,這進(jìn)一步支持了它的S1態(tài)具有電荷轉(zhuǎn)移的特征. 圖4(b)是電子-空穴對于S1態(tài)的重疊圖,重疊區(qū)域主要位于中心的苯環(huán)上,也就是說,S1態(tài)有一些貢獻(xiàn)是來自于局域激發(fā)三重態(tài). 此外,通過分析S1態(tài)的自然躍遷軌道(圖4(c)),也能說明3CzIPN具有明顯的電荷轉(zhuǎn)移特征;更重要的是,還說明了在3CzIPN中心區(qū)域的兩個過渡軌道之間存在一些重疊. 眾所周知,兩個軌道之間的重疊是發(fā)生熒光的必要條件,而兩個軌道之間的分離可以產(chǎn)生較小的S-T能量差,有利于3CzIPN發(fā)生延遲熒光.
表3 3CzIPN的S1和T1態(tài)之間的能隙差,重組能,自旋軌道耦合,S1和T1態(tài)的輻射躍遷速率,系間竄越速率和反系間竄越速率
憑借TD-DFT理論計算,研究了3CzIPN分子的單三態(tài)能量差、自旋軌道耦合常數(shù)、熒光和磷光速率、系間竄躍和發(fā)系間竄躍速率等性質(zhì). 計算結(jié)果表明,3CzIPN的單三態(tài)能量差僅為0.124 eV,是RISC順利進(jìn)行的關(guān)鍵所在. 進(jìn)一步計算表明,3CzIPN的系間竄躍速率非常大(9.94×106s-1),有利于形成的單線態(tài)激子通過系間竄躍快速轉(zhuǎn)換成三線態(tài)激子. 此外,3CzIPN的反系間竄越速率也達(dá)到了104的數(shù)量級,使得其三線態(tài)激子能夠很容易重新生成單線態(tài)激子. 由此可見,3CzIPN分子應(yīng)該是一個有前途的TADF發(fā)射體.
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[責(zé)任編輯:吳文鵬]
Theoretical study on thermally activated delayed fluorescence mechanism of 3CzIPN molecule
GUO Xugeng*, ZHU Qiuling
(CollegeofChemistryandChemicalEngineering,HenanUniversity,Kaifeng475004,Henan,China)
With time-dependent density functional theory (TDDFT) method and the polarizable continuum model (PCM) using benzene as solvent the thermally activated delayed fluorescence (TADF) mechanism of 2,4,6-tri(9H-carbazol-9-yl)isophthalonitrile (3CzIPN) was investigated. The present studies reveal that the singlet-triplet energy difference of the 3CzIPN molecule is very small (0.124 eV), which is favorable for the reverse intersystem crossing (RISC). In addition, the RISC rate of 3CzIPN is calculated to be in 104order of magnitude, which means that the 3CzIPN molecule could be a potential TADF emitter.
TD-DFT; TADF; intersystem crossing; reverse intersystem crossing
2017-05-25.
國家自然科學(xué)基金項目(21503069).
郭續(xù)更 (1982-),男,講師.研究方向:理論與計算化學(xué).*
,E-mail:xgguo@henu.edu.cn.
O64
A
1008-1011(2017)04-0416-05