銥磷光配合物的合成、顏色調(diào)控方法及應(yīng)用研究現(xiàn)狀

常橋穩(wěn)，王姿奧，晏彩先，張選東，姜 婧，劉偉平，崔 浩 *

銥磷光配合物的合成、顏色調(diào)控方法及應(yīng)用研究現(xiàn)狀

常橋穩(wěn)1, 2，王姿奧1，晏彩先1，張選東1，姜 婧1，劉偉平1，崔 浩1 *

(1. 昆明貴金屬研究所，貴研鉑業(yè)股份有限公司 稀貴金屬綜合利用新技術(shù)國家重點(diǎn)實(shí)驗(yàn)室，昆明 650106；2. 昆明理工大學(xué) 材料科學(xué)與工程學(xué)院，昆明 650093)

銥磷光配合物具有良好的熱穩(wěn)定性，相對短的激發(fā)態(tài)壽命，發(fā)光效率高以及發(fā)光顏色容易調(diào)節(jié)等優(yōu)勢，成為迄今性能最為優(yōu)異的有機(jī)發(fā)光材料，在OLED中的應(yīng)用備受關(guān)注。圍繞銥磷光配合物的結(jié)構(gòu)、合成方法、顏色調(diào)控方法以及應(yīng)用等方面展開綜述，獲得了一些銥磷光配合物的構(gòu)效關(guān)系，可用于指導(dǎo)銥磷光配合物的高效研發(fā)。闡述了銥磷光配合物發(fā)展過程中遇到的問題，并指明了銥磷光配合物的發(fā)展趨勢和方向。

銥磷光配合物；合成；顏色調(diào)控；應(yīng)用；現(xiàn)狀

隨著可開采化石能源的日益枯竭和人類生存環(huán)境的惡化，能源的有效利用已成為我國乃至全世界面臨的一項(xiàng)極為緊迫的任務(wù)。有機(jī)發(fā)光二極管(Organic Light-emitting Diode，OLED)作為一種高效的電光轉(zhuǎn)換技術(shù)，可以將電能高效率地轉(zhuǎn)化為光能，應(yīng)用于顯示和照明領(lǐng)域[1-4]，從而實(shí)現(xiàn)能源的節(jié)流，是解決能源和環(huán)境問題的有效方案。

OLED技術(shù)和產(chǎn)業(yè)涉及到一系列新材料的應(yīng)用，新材料的研究對OLED的發(fā)展有著重要的影響。在OLED中使用了諸多有機(jī)新材料，有機(jī)發(fā)光材料是OLED最為核心和重要的關(guān)鍵原材料，它的發(fā)光性能直接影響著OLED器件性能。有機(jī)發(fā)光分子材料主要有熒光材料和磷光材料兩大類，其中熒光材料研究較早，應(yīng)用較多。但是，熒光材料發(fā)光效率低，而磷光材料具有高的發(fā)光效率，替代熒光材料可以顯著提升OLED的效率[5-7]。磷光材料主要是一些過渡金屬(Pt、Os、Ir、Au、Ru、Re和Cu等)的配合物分子。在這些配合物中，銥磷光配合物具有良好的熱穩(wěn)定性，相對短的激發(fā)態(tài)壽命，發(fā)光效率高以及發(fā)光顏色容易調(diào)節(jié)等方面的優(yōu)勢，成為迄今性能最為優(yōu)異的有機(jī)發(fā)光材料[8-12]。小分子綠光和紅光銥磷光配合物已經(jīng)成功應(yīng)用在OLED顯示產(chǎn)品中，并將有望應(yīng)用于OLED照明中。而藍(lán)光銥磷光配合物存在光譜不夠藍(lán)、效率不夠高、穩(wěn)定性差等問題，仍處于基礎(chǔ)研究階段。盡管紅光和綠光銥磷光配合物已成功應(yīng)用到OLED顯示產(chǎn)業(yè)中，但它們的廣泛應(yīng)用仍受到效率和發(fā)光純度不足的影響。隨著OLED產(chǎn)業(yè)的快速發(fā)展和人們對高清晰顯示的追求，科技和產(chǎn)業(yè)界還在繼續(xù)尋找效率更高和顏色更純的新型銥磷光配合物。

本文作者根據(jù)文獻(xiàn)報道，并結(jié)合所在實(shí)驗(yàn)室對銥磷光配合物的研究工作，對銥磷光配合物的結(jié)構(gòu)、合成方法、顏色調(diào)控方法及應(yīng)用等研究現(xiàn)狀進(jìn)行了綜述，并提出了銥磷光配合物所面臨的問題和今后的發(fā)展趨勢。

1 銥磷光配合物的結(jié)構(gòu)及合成方法

銥(III)的價電子構(gòu)型為d6，采用d2sp3雜化與環(huán)金屬配體和輔助配體的六個配位原子形成稍微扭曲的八面體配合物。根據(jù)分子結(jié)構(gòu)的不同和分子量的大小，銥磷光配合物可以分為小分子銥磷光配合物、樹枝狀銥磷光配合物和高分子銥磷光配合物。

1.1 小分子銥磷光配合物

小分子銥磷光配合物主要有4類(圖1)，分別是Ir(C^N)2(LX)型、Ir(C^N)3型、Ir(C^N)2(C`^N`)型和Ir(C^N)2(N^N)離子型銥磷光配合物。

圖1 小分子銥磷光配合物的化學(xué)結(jié)構(gòu)

1.1.1Ir(C^N)2(LX)型銥磷光配合物

Ir(C^N)2(LX)型銥磷光配合物是由2個環(huán)金屬配體(C^N)和一個輔助配體(LX)配位構(gòu)成的中性發(fā)光配合物[13-19]。通過改變環(huán)金屬配體和輔助配體的結(jié)構(gòu)均能調(diào)節(jié)發(fā)光顏色和發(fā)射波長，可以獲得種類豐富的銥磷光配合物。對于Ir(C^N)2(LX)型銥磷光配合物，其合成路線見圖2，合成工藝相對簡單，主要有兩種方法：第一種是以水合三氯化銥為原料，在乙二醇單乙醚和水(體積比為3:1)的混合溶劑中，與相應(yīng)的環(huán)金屬配體反應(yīng)，得到銥的氯橋二聚體，然后在堿性條件下與輔助配體發(fā)生配位反應(yīng)，合成得到目標(biāo)銥磷光配合物。該方法是合成這類銥磷光配合物最常用的方法[20-21]；第二種方法也是以水合三氯化銥為原料，先合成得到環(huán)己二烯氯化銥二聚體，然后和輔助配體的鹽反應(yīng)形成環(huán)己二烯和輔助配體的配合物，最后和環(huán)金屬配體反應(yīng)，合成得到目標(biāo)銥磷光配合物[22]，該方法目前僅用于Ir(ppy)2(acac)的合成，但有望推廣到其它Ir(C^N)2(LX)型銥磷光配合物的合成中。

圖2 Ir(C^N)2(LX)型銥磷光配合物的合成路線

1.1.2Ir(C^N)3型銥磷光配合物

Ir(C^N)3型銥磷光配合物是由3個相同的環(huán)金屬配體(C^N)與銥(III)配位形成的均配型的中性分子[23-27]，存在面式()和經(jīng)式()兩種異構(gòu)體(如圖3)。在面式異構(gòu)體中，所有的Ir-C鍵處于吡啶的反位，Ir-N鍵處于苯環(huán)的反位，所有Ir-C鍵長和Ir-N鍵長各自相等。而在經(jīng)式異構(gòu)體中，Ir-C鍵和Ir-N鍵的鍵長各不相同。面式和經(jīng)式異構(gòu)體的形成與溫度有很大的關(guān)系。-Ir(C^N)3是熱力學(xué)穩(wěn)定產(chǎn)物，一般在高溫(≈200℃)條件下形成，-Ir(C^N)3是動力學(xué)穩(wěn)定產(chǎn)物，一般在較低溫度(≈150℃)下形成，二者在一定條件下可以相互轉(zhuǎn)化，如-Ir(C^N)3在較高溫度下可以轉(zhuǎn)化為-Ir(C^N)3[28]。如果溫度控制不好，在合成中往往會反應(yīng)生成兩種異構(gòu)體，二者之間分離難度大，給后續(xù)的提純工作帶來很大的困難。經(jīng)式和面式在構(gòu)型上的差異會導(dǎo)致材料光物理性質(zhì)的不同。通常情況下，與面式構(gòu)型相比較，經(jīng)式構(gòu)型發(fā)光光譜紅移，發(fā)光效率明顯降低[29]，這也是經(jīng)式構(gòu)型的研究較少，面式構(gòu)型則得到了廣泛的研究和應(yīng)用的主要原因。

圖3 [Ir(C^N)3]的面式和經(jīng)式立體結(jié)構(gòu)

Ir(C^N)3型銥磷光配合物的合成比Ir(C^N)2(LX)型銥磷光配合物的合成困難，隨之也發(fā)展了多種多樣的合成方法，文獻(xiàn)報道的合成方法主要有6種[27-28, 30-35]，合成路線見圖4。所有合成方法均是以水合三氯化銥為原料，首先合成出相應(yīng)的銥中間體，再進(jìn)一步合成得到目標(biāo)銥磷光配合物。第一種合成方法是銥氯橋二聚體和環(huán)金屬配體在堿性條件下，以甘油為溶劑，加熱回流反應(yīng)得到目標(biāo)分子材料；第二種合成方法是通過三氟甲基磺酸銀脫除銥氯橋二聚體中的氯離子后，再和環(huán)金屬配體反應(yīng)，溶劑為乙二醇單乙醚，反應(yīng)溫度為90~100℃，最終獲得目標(biāo)配合物；第三種合成方法以乙酰丙酮銥和環(huán)金屬配體在甘油溶劑中加熱回流反應(yīng)一步得到目標(biāo)配合物；第四種合成方法以Ir(C^N)2(O^O)型銥磷光配合物為原料，在甘油溶劑中和環(huán)金屬配體反應(yīng)得到目標(biāo)配合物；第五種合成方法是通過Na[Ir(acac)2Cl2]和環(huán)金屬配體在甘油溶劑中加熱回流反應(yīng)得到目標(biāo)配合物；第六種合成方法是通過Ir(acac-O,O)2(acac- C3)(H2O)和環(huán)金屬配體在甘油溶劑中加熱回流反應(yīng)得到目標(biāo)配合物。在這些合成方法中，除第二種方法外，其它方法均在高溫下反應(yīng)。第二種方法中，通過三氟甲基磺酸銀脫除氯離子，可以在較低的溫度(90~100℃)下實(shí)現(xiàn)面式結(jié)構(gòu)的合成，是目前合成Ir(C^N)3型銥磷光配合物最常用的方法。第六種合成方法使用的前驅(qū)體Ir(acac-O,O)2(acac-C3)(H2O)是乙酰丙酮銥合成過程中產(chǎn)生的副產(chǎn)物，使用該方法可以實(shí)現(xiàn)銥的高效利用，有利于降低Ir(C^N)3型銥磷光配合物的合成成本，也是本文作者的研究成果。

圖4 Ir(C^N)3型銥磷光配合物的合成路線

1.1.3Ir(C^N)2(C`^N`)型銥磷光配合物

與Ir(C^N)2(LX)型銥磷光配合物相比較，Ir(C^N)3型銥磷光配合物的三個配體相同，發(fā)射波長和發(fā)光顏色的調(diào)控相對單調(diào)，于是發(fā)展了Ir(C^N)2(C`^N`)型銥磷光配合物[36-40]，使得其中的兩個環(huán)金屬配體相同，改變第3個環(huán)金屬配體的化學(xué)結(jié)構(gòu)，形成了Ir(C^N)2(C`^N`)型銥磷光配合物。第三個環(huán)金屬配體的引入，使發(fā)光顏色調(diào)節(jié)更容易，同時也可以獲得更多結(jié)構(gòu)不同的銥磷光配合物，其中Ir(C^N)2(C`^N`)型銥磷光配合物的綠光材料已在OLED器件中得到廣泛應(yīng)用。與Ir(C^N)3型銥磷光配合物相似，Ir(C^N)2(C`^N`)型銥磷光配合物的合成方法主要有3種[41-43]，合成路線見圖5。第一種方法是以銥氯橋二聚體和第三環(huán)金屬配體為原料，在堿性條件下和甘油溶劑中加熱回流反應(yīng)得到目標(biāo)分子材料；第二種方法是通過三氟甲基磺酸銀脫除銥氯橋二聚體中的氯離子，然后和第三環(huán)金屬配體在90~100℃反應(yīng)得到目標(biāo)銥磷光配合物；第三種方法是以甲醇、乙腈、乙醇等為溶劑，通過三氟甲基磺酸銀脫除銥氯橋二聚體中的氯離子，形成Ir(C^N)2(solvent)中間體，然后再和第三環(huán)金屬配體反應(yīng)，即得到目標(biāo)分子材料。對比三種合成方法，第一種合成方法反應(yīng)溫度高、容易產(chǎn)生副產(chǎn)物，第二種和第三種合成方法均能在較溫和的條件下實(shí)現(xiàn)，并且能夠獲得更高純度的銥磷光配合物，已在Ir(C^N)2(C`^N`)型銥磷光配合物的合成中得到廣泛應(yīng)用。

圖5 Ir(C^N)3(C`^N`)型銥磷光配合物的合成路線

1.1.4Ir(C^N)2(N^N)離子型銥磷光配合物

離子型Ir(C^N)2(N^N)銥磷光配合物通常是陽離子型銥磷光配合物，中心結(jié)構(gòu)由2個環(huán)金屬配體和一個中性的N^N輔助配體構(gòu)成配陽離子，外界為六氟磷酸根陰離子，組成離子型的小分子銥磷光配合物[10, 44-50]。這類銥磷光材料可以通過增加或減小配體的共軛程度、或引入極性基團(tuán)等結(jié)構(gòu)修飾實(shí)現(xiàn)光物理特性的調(diào)控，也可以通過改變陰離子的種類改善發(fā)光分子的光物理性能。離子型銥磷光材料具有較好的親水性和生物相容性，在光催化、發(fā)光電化學(xué)池、化學(xué)傳感等領(lǐng)域都具有潛在的應(yīng)用價值。這類材料的合成路線與Ir(C^N)2(LX)型銥磷光配合物的合成路線相似，但不需要像合成Ir(C^N)2(LX)型銥磷光配合物那樣在高溫條件下進(jìn)行，銥氯橋二聚體與相應(yīng)的N^N輔助配體在二氯甲烷和甲醇混合溶劑中，加熱回流反應(yīng)，然后冷至室溫，加入含對應(yīng)的陰離子鹽即可制得目標(biāo)銥磷光配合物。由此可見，離子型銥磷光配合物的合成過程和方法較中性銥磷光分子簡單、容易。

1.2 樹枝狀銥磷光配合物和高分子銥磷光聚合物材料

小分子銥磷光配合物在OLED上已經(jīng)取得了相當(dāng)高的器件效率，并實(shí)現(xiàn)了商業(yè)化應(yīng)用，但由于小分子銥磷光配合物在發(fā)光層容易產(chǎn)生團(tuán)聚和濃度淬滅現(xiàn)象，人們開始轉(zhuǎn)向大分子銥磷光配合物的研究，于是發(fā)展了樹枝狀銥磷光配合物和高分子銥磷光聚合物材料[51-61]。這兩類材料既有小分子銥磷光配合物優(yōu)異的發(fā)光性能，又具備溶解性好、優(yōu)良的機(jī)械性能(如繞曲和加工成型)和良好的成膜性能，容易實(shí)現(xiàn)大尺寸和柔性屏體的制備。樹枝狀銥磷光配合物和高分子銥磷光聚合物材料的合成方法與小分子銥磷光配合物的合成方法類似，最為關(guān)鍵的是樹枝狀配體和高分子配體的合成。

2 銥磷光配合物的波長和顏色調(diào)控方法

銥磷光配合物的發(fā)射波長和發(fā)光顏色，可以通過改變環(huán)金屬配體和輔助配體的化學(xué)組成和結(jié)構(gòu)在整個可見光區(qū)域內(nèi)調(diào)控[62-69]。大量研究表明，對于絕大多數(shù)銥磷光配合物，HOMO 能級主要是分布在環(huán)金屬配體的苯環(huán)和中心金屬銥上，LUMO能級主要局域在環(huán)金屬配體的含氮雜環(huán)上。通過改變環(huán)金屬配體的共軛程度、引入不同種類和數(shù)量的取代基、引入不同的雜原子或原子團(tuán)和改變輔助配體等方法都可以調(diào)節(jié)銥磷光配合物的HOMO-LUMO能級，從而達(dá)到調(diào)控銥磷光配合物發(fā)光波長和發(fā)光顏色的目的。

2.1 環(huán)金屬配體對發(fā)光顏色的調(diào)控

環(huán)金屬配體對銥磷光配合物的能級和發(fā)光顏色有著最重要的影響[70]，通過改變環(huán)金屬配體的共軛程度、取代基的種類和位置、引入雜原子或原子團(tuán)等方式，可以實(shí)現(xiàn)銥磷光配合物發(fā)光波長的調(diào)控，從而達(dá)到調(diào)節(jié)發(fā)光顏色的目的。

2.1.1共軛效應(yīng)

對于Ir(C^N)2(acac)型銥磷光配合物，發(fā)射波長主要由環(huán)金屬配體(C^N)決定，隨著C^N配體共軛程度的增加和配體剛性增強(qiáng)，降低了配體的能隙，最大發(fā)射波長發(fā)生紅移。如圖6中，Ir(ppy)2(acac)、Ir(piq)2(acac)、Ir(niq)2(acac)和Ir(azp)2(acac)的環(huán)金屬配體的共軛程度越來越大，發(fā)射波長分別為516、620、664和932 nm，向長波方向發(fā)生了移動[71]；對比另外3個銥磷光分子Ir(pbi)2(acac)、Ir(nbi)2(acac)和Ir(fbi)2(acac)，隨著C^N的剛性和共軛程度越大，對應(yīng)的發(fā)射波長也發(fā)生明顯紅移(530、608和651 nm)[20]。

圖6 基于不同共軛程度環(huán)金屬配體的Ir(C^N)2(acac)型銥磷光配合物

對于Ir(C^N)3型銥磷光配合物，其發(fā)射波長完全由環(huán)金屬配體(C^N)決定，隨著C^N配體π共軛程度和配體剛性增強(qiáng)，發(fā)射波長發(fā)生紅移。如圖7中[30]，改變C^N配體的剛性和共軛程度，實(shí)現(xiàn)了最大發(fā)射波長從545到652 nm范圍內(nèi)的調(diào)控。例如，與Ir(thpy)3相比，Ir(tiq)3、Ir(t-5t-pyhpy)3和Ir(btpy)3的C^N配體的剛性和共軛程度更大，發(fā)射波長發(fā)生了明顯紅移，最多的紅移了96 nm。Ir(fliq)3的C^N配體的剛性和共軛程度大于Ir(piq)3的C^N配體，發(fā)射波長向長波方向移動了32 nm。Ir(fliq)3的C^N配體的剛性和共軛程度明顯大于Ir(flpy)3的配體，發(fā)射波長紅移了107 nm。

圖7 基于不同共軛程度環(huán)金屬配體的Ir(C^N)3型銥磷光配合物

2.1.2取代基效應(yīng)

通過改變?nèi)〈姆N類和位置可以實(shí)現(xiàn)銥磷光配合物發(fā)射波長的調(diào)控。在銥磷光配合物中環(huán)金屬配體1,2-二苯基苯并咪唑的苯基上引入-OMe或-CN，或者同時引入-OMe和-CN，或?qū)?OMe和-CN引入在不同的位點(diǎn)(圖8)[72]，都可以改變配合物的發(fā)射波長。在沒有引入取代基團(tuán)之前，Ir(pbm)2(acac)的最大發(fā)射波長為520 nm。在對位引入-OMe后形成的Ir(p-MeO-pbm)2(acac)，最大發(fā)射波長紅移至555 nm，在間位引入-OMe后得到的Ir(m-MeO- pbm)2(acac)，最大發(fā)射波長藍(lán)移至507 nm，兩者的最大發(fā)射波長相差48 nm。與之相反，在對位引入-CN和在間位引入-CN時，得到的兩個配合物的最大發(fā)射波長相差45 nm；當(dāng)同時引入-OMe和CN時，且OMe處于對位而-CN處于間位，產(chǎn)生的配合物最大發(fā)射波長紅移至605 nm；當(dāng)-OMe和-CN交換取代位點(diǎn)，配合物的最大發(fā)射波長則藍(lán)移24 nm至496 nm；OMe處于對位、-CN處于間位的配合物和-OMe處于間位、-CN處于對位的配合物，兩者的最大發(fā)射波長相差109 nm。由此可見，對于Ir(pbm)2(acac)母體結(jié)構(gòu)，在間位引入-OMe時，最大發(fā)射波長紅移，在間位引入-CN時，最大發(fā)射波長藍(lán)移，在對位引入-OMe時，發(fā)射波長藍(lán)移，在間位引入-CN時，最大發(fā)射波長紅移。

圖8 MeO和CN取代的銥磷光配合物

Tsuzuki等[73]在Ir(ppy)2(acac)銥磷光配合物的苯基吡啶環(huán)金屬配體的不同點(diǎn)位引入具有強(qiáng)拉電子性質(zhì)的五氟苯基合成了4種銥磷光分子(圖9)，實(shí)現(xiàn)了發(fā)射波長在513 nm到578 nm范圍內(nèi)調(diào)控，最大外量子效率可達(dá)17%。

以2-苯基苯并噻唑?yàn)榄h(huán)金屬配體，在苯并噻唑環(huán)的6位上引入氟原子和在苯基的間位上分別引入三氟甲基、氟、甲基和甲氧基，得到4種銥磷光配合物(圖10)：Ir(4-CF3-Btp)2(acac)、Ir(4-F-Btp)2(acac)、Ir(4-Me-Btp)2(acac)和Ir(4-MeO-Btp)2(acac)，最大發(fā)射波長分別為567、544、559和544 nm，與母體分子Ir(Btp)2(acac)的最大發(fā)射波長560 nm相比，差異不大，但是量子效率要高出1~2倍[74]。

圖9 取代基位置不同的銥磷光配合物

圖10 具有不同取代基的銥磷光配位

在2-苯基吡啶的吡啶環(huán)的不同位置引入苯環(huán)形成喹啉或異喹啉環(huán)(圖11)，得到銥磷光配合物的最大發(fā)射波長也不同，Ir(npy)2(acac)[75]、Ir(pq)2(acac)[20]和Ir(piq)2(acac)[76]的最大發(fā)射波長分別為551、597和620 nm，對應(yīng)的發(fā)光顏色的變化從黃綠色-橙紅色-深紅色。

圖11 取代基位置不同的銥磷光配合物

在(C^N)2Ir(acac)型銥磷光配合物中，當(dāng)環(huán)金屬配體為對稱性的2,5-二芳基吡啶，芳基按從苯基、4-氟苯基、4-甲氧基苯基和9,9-二甲基芴的順序改變時(圖12)，發(fā)射波長紅移，從525紅移至595 nm，提示當(dāng)銥磷光配合物的π共軛程度增大時，發(fā)射波長會發(fā)生紅移，實(shí)現(xiàn)發(fā)射波長和發(fā)光顏色調(diào)控[77]。

圖12 對稱性芳烴取代的銥磷光配合物

對于Ir(piq)2(acac)，在環(huán)金屬配體上苯基的同一位置引入不同的取代基或在苯基的不同位置引入相同的取代基(圖13)，均可以實(shí)現(xiàn)發(fā)光波長和發(fā)光顏色的調(diào)控。在苯基異喹啉的苯環(huán)的間位分別引入甲基、三氟甲基和甲氧基時，對應(yīng)的配合物的發(fā)射波長分別為628、631和623 nm；在苯基異喹啉的苯環(huán)的對位分別引入甲基、三氟甲基和甲氧基時，對應(yīng)配合物的發(fā)射波長分別為645、644和632 nm。由此可以看出，在Ir(piq)2(acac)系列銥磷光配合物中，在同一位置引入不同的取代基，最大發(fā)射波長稍有變化，但在不同位置引入同一取代基，最大發(fā)射波長的變化更明顯[78]。

圖13 取代基位置不同的異喹啉銥磷光配合物

與(mpq)2Ir(acac)銥磷光配合物相比，在2-苯基-4-甲基喹啉環(huán)金屬配體苯基的間位和對位上引入甲氧基(圖14)，并且當(dāng)甲氧基的位置處于間位時，所對應(yīng)的配合物發(fā)射波長藍(lán)移17 nm，而當(dāng)甲氧基的位置處于對位時，最大發(fā)射波長則紅移45 nm。甲氧基位于間位和對位的兩種不同的配合物，最大發(fā)射波長相差62 nm[79]。

圖14 甲氧基處于不同位置的銥磷光配合物

同時，改變?nèi)〈鶖?shù)量也可以實(shí)現(xiàn)銥磷光配合物發(fā)射波長調(diào)節(jié)。含氟官能團(tuán)對銥磷光配合物的發(fā)射有著顯著的影響(圖15)，由于氟的吸電子誘導(dǎo)效應(yīng)，與(mpq)2Ir(acac)相比，氟取代后的分子發(fā)射光譜發(fā)射顯著藍(lán)移，隨氟原子個數(shù)變化，顏色從橙紅光至紅光[80]，(mpq-4F)2Ir(acac)、(mpq-2,4F)2Ir(acac)、(mpq-2,3,4F)2Ir(acac)和(mpq-2,3,4,5F)2Ir(acac)的發(fā)射波長分別為574、577、584和564 nm。

圖15 氟原子數(shù)量不同的的銥磷光配合物

由以上可見，取代基種類、位置和數(shù)量等均對銥磷光配合物的光物理性質(zhì)調(diào)控起著重要的作用。

2.1.3雜原子或原子團(tuán)效應(yīng)

雜原子和原子團(tuán)的引入也可以實(shí)現(xiàn)銥磷光配合物最大發(fā)射波長的調(diào)節(jié)。對比圖16中(bpo)2Ir(acac)和(bpt)2Ir(acac)的最大發(fā)射波長，由硫取代氧后，由于硫原子的可極化性大于氧原子，導(dǎo)致分子最大發(fā)射波長紅移30 nm；用萘基取代苯基，共軛程度增大，分子的最大發(fā)射波長則紅移60 nm；如果氧被硫取代的同時苯基也被萘基取代，分子的熒光光譜也相應(yīng)紅移80 nm[81]。Zhou等[82]合成了圖17中的7個銥磷光配合物，在Ir(ppy)3的苯基間位引入含主族元素的基團(tuán)，材料的最大發(fā)射波長在497~530 nm范圍內(nèi)變化，發(fā)光顏色從藍(lán)綠色到黃綠色的轉(zhuǎn)變。

圖16 含雜原子的[Ir(ppy-L)2(acac)]型銥磷光配合物

圖17 主族元素基團(tuán)修飾的銥磷光配合物

2.2 輔助配體對能級及發(fā)光顏色的調(diào)控

環(huán)金屬配體在銥磷光配合物顏色調(diào)控起重要作用，輔助配體對顏色調(diào)節(jié)也有重要影響[83-84]。在銥磷光配合物中，常見的輔助配體有O^O型[20, 85-87]、N^O型[88-91]、S^S型[92-93]、N^N型[94-95]和P^P型[96]等(圖18)，其中常用輔助配體為O^O型。β-二酮作為O^O雙齒配體，能與銥(III)形成穩(wěn)定的配合物(圖19)，是最常見的一類輔助配體。其中乙酰丙酮為最簡單的β-二酮，廣泛用作銥磷光配合物輔助配體。對Ir(C^N)2(O^O)型銥磷光配合物，β-二酮的結(jié)構(gòu)和三重態(tài)能級對銥磷光配合物的發(fā)光性能影響較大。β-二酮的三重態(tài)能級較高時，在溶液中有較強(qiáng)的磷光發(fā)射；當(dāng)β-二酮配體三重態(tài)能級較低時，在溶液中不發(fā)光[85, 97]。在摻雜型器件中，要求β-二酮配體具有較高的三重態(tài)能級。最近研究表明，對于Ir(dmippiq)2(deacac)[98]，3,7-二乙基-4,6-壬二酮的引入可以改善材料的光譜特性，使發(fā)射光譜窄化，提高色純度，還可降低銥磷光配合物的升華溫度，改善升華效果，增加OLED器件的最大外量子效率。

圖18 不同類型的輔助配體的結(jié)構(gòu)

圖19 含不同β-二酮配體的銥磷光配合物

以2-苯基苯并吡喃為環(huán)金屬配體，改變不同結(jié)構(gòu)的N^O輔助配體可以實(shí)現(xiàn)配合物的發(fā)光顏色從綠光到橙光的調(diào)控，例如Ir(bo)2(pic)、Ir(bo)2(prz)、Ir(bo)2(bop)和Ir(bo)2(btp) (圖20)的最大發(fā)射波長分別為531、591、539和598 nm[99]。以苯基吡啶為環(huán)金屬配體，改變不同N^O輔助配體可以實(shí)現(xiàn)銥磷光配合物發(fā)光顏色從藍(lán)色到橙紅色的調(diào)控，例如，Ir(ppy)2(ImPyC)、Ir(ppy)2(ImPmC)、Ir(ppy)2(ImPzC)和Ir(ppy)2(ImTzC)(圖20)的發(fā)射波長分別為501、414、592和504 nm[100]。

以苯基吡啶為環(huán)金屬配體，不同結(jié)構(gòu)脒基N^N作輔助配體，實(shí)現(xiàn)了配合物發(fā)光波長在528~548 nm范圍內(nèi)調(diào)控[93, 100-102](圖21)。

圖21 基于N^N輔助配體的銥磷光分子

3 銥磷光配合物的應(yīng)用研究現(xiàn)狀

發(fā)光效率高、性能穩(wěn)定的紅、綠、藍(lán)三種顏色的磷光材料是實(shí)現(xiàn)全色顯示和固態(tài)照明必不可少的三基色有機(jī)發(fā)光分子材料，黃/黃綠光磷光材料可以和藍(lán)光材料組合實(shí)現(xiàn)結(jié)構(gòu)簡單的白光器件。銥磷光配合物通過改變配體結(jié)構(gòu)可以實(shí)現(xiàn)在整個可見光范圍內(nèi)調(diào)控，發(fā)光顏色主要包括紅光、綠光、藍(lán)光和黃光等。

3.1 綠光銥磷光配合物

綠光銥磷光配合物研制的時間最早，現(xiàn)已發(fā)展成為最成熟的一類磷光材料，基于這類材料構(gòu)建的OLED器件具有最好的效率和最長的壽命。最經(jīng)典的綠光銥磷光配合物是Ir(ppy)3、Ir(ppy)2(acac)及其衍生物(圖22)，最大發(fā)射波長介于510~520 nm，色坐標(biāo)在(0.32，0.64)左右。1985年[103]Ir(ppy)3被首次合成出來，最大發(fā)射波長為510 nm，在甲苯中的量子產(chǎn)率為0.97。1999年[7]Thompson 等將其首次應(yīng)用到OLED中，經(jīng)器件結(jié)構(gòu)優(yōu)化后，獲得最大外量子效率21.3%，發(fā)光強(qiáng)度為68.1 cd/A，CIE坐標(biāo)(0.27, 0.63)。2001年[28]發(fā)現(xiàn)Ir(ppy)2(acac)與Ir(ppy)3有相似的性質(zhì)，最大發(fā)射波長為516 nm，并有長激發(fā)態(tài)壽命和高的量子產(chǎn)率，以Ir(ppy)2(acac)為客體材料的OLED器件，獲得的最大外量子效率(EQE)為23.7%，發(fā)光效率為91.6 cd/A，CIE坐標(biāo)(0.31，0.64)[104]。在Ir(ppy)3和Ir(ppy)2(acac)的基礎(chǔ)上，為了獲得更高穩(wěn)定性、更高效率和更高色純度等高性能的發(fā)綠光材料，通過改變輔助配體和環(huán)金屬配體研制了許多其他的綠光銥磷光配合物[105-117]，其中一些小分子綠光銥磷光配合物已在OLED產(chǎn)業(yè)中得到了應(yīng)用。

圖22 經(jīng)典的綠光銥磷光配合物

3.2 紅光銥磷光配合物

與綠光銥磷光配合物相比，受制于能帶定律和窄的能隙，紅光銥磷光配合物的發(fā)展滯后，在色純度、效率和壽命方面還需進(jìn)一步提升。這類銥磷光配合物在2-苯基吡啶上增加環(huán)金屬配體的共軛程度來實(shí)現(xiàn)紅光發(fā)射。但隨著環(huán)金屬配體共軛程度的增大，分子間的π-π相互作用增強(qiáng)，容易引起分子的聚集，產(chǎn)生團(tuán)聚和濃度淬滅現(xiàn)象，導(dǎo)致器件效率和壽命的雙重降低[118]。阻礙紅光銥磷光配合物商用的另一個問題是高效率和高色純度難于同時實(shí)現(xiàn)，同時具備高效率、穩(wěn)定性、亮度、CIE坐標(biāo)(0.67，0.33)的紅光銥磷光配合物非常少。盡管如此，在人們合成的許多紅光銥磷光配合物中[119-130]，還是篩選出滿足OLED產(chǎn)業(yè)基本需求的紅光銥磷光配合物。

紅光銥磷光配合物的環(huán)金屬配體主要有2-苯基苯并噻唑、1-芳基異喹啉、2-芳基喹啉及其衍生物(圖23)。2001年，Lamansky團(tuán)隊(duì)[131]報道了第一個紅光銥磷光配合物Ir(btp)2(acac)，外量子效率在4.6%到6.6%之間、最大發(fā)射波長為616 nm、CIEx,y色坐標(biāo)為(0.68, 0.32)。2003年，Tsuboyama團(tuán)隊(duì)[76]合成均配型的異喹啉銥磷光配合物Ir(piq)3，最大發(fā)射波長為620 nm、色坐標(biāo)為(0.68，0.32)、流明效率為8.0 lm/w、外量子效率達(dá)到10.3%。通過對Ir(piq)3進(jìn)行改性，得到Ir(piq)2(acac)[132]，色坐標(biāo)為(0.68，0.32)、最大外量子效率9.7%。Thompson等發(fā)展了Ir(pq)2(acac)，其最大發(fā)射波長為597 nm、在溶液中的量子效率為10%[23]。鑒于紅光銥磷光配合物共軛程度大、存在著較強(qiáng)的分子間相互作用力，會產(chǎn)生濃度淬滅，導(dǎo)致發(fā)光效率和壽命的雙重降低。但是，在環(huán)金屬配體上引入取代基或引入體積更大的輔助配體，可以改善銥磷光配合物的發(fā)光性能。例如，Ir(mpq)2(acac)和Ir(mpmq)2(tmd)[133]的發(fā)射波長分別為594和586 nm、CE分別為20.3 cd/A和30.1 cd/A、流明效率分別為23.9和32 lm/w、CIE分別為(0.65，0.34)和(0.65，0.35)，基本滿足了OLED產(chǎn)業(yè)的需求，曾經(jīng)在OLED產(chǎn)線上得到應(yīng)用，但是存在色純度不夠高的缺點(diǎn)。為了提高色純度，對Ir(piq)2(acac)進(jìn)行結(jié)構(gòu)改造，在異喹啉的6位引入異丙基，將輔助配體乙酰丙酮替換成3,7-二乙基-4,6-壬二酮，研制出一款性能優(yōu)良的深紅光銥磷光配合物，已廣泛應(yīng)用到OLED顯示產(chǎn)業(yè)中[98]。

圖23 經(jīng)典的紅光銥磷光配合物

3.3 藍(lán)光銥磷光配合物

銥磷光配合物發(fā)射藍(lán)光主要通過如下兩個手段來擴(kuò)大配合物分子的能隙，增加電子躍遷所需的能量：(1)在2-苯基吡啶的苯環(huán)上引入拉電子基團(tuán)降低HOMO能級，增大HOMO-LUMO能級差，發(fā)射波長藍(lán)移；(2)在吡啶環(huán)上引入給電子基團(tuán)提高LUMO能級，增大HOMO-LUMO能級差，發(fā)射波長藍(lán)移。藍(lán)光銥磷光配合物能隙大，與之匹配的寬能隙主體材料很少，導(dǎo)致在色純度、效率、壽命和穩(wěn)定性等方面的性能尚不能滿足OLED產(chǎn)業(yè)的需求，仍處于基礎(chǔ)研究階段，開發(fā)高性能藍(lán)光銥磷光配合物仍然面臨很多挑戰(zhàn)。

圖24 經(jīng)典的藍(lán)光銥磷光配合物

FIrpic是第一個天藍(lán)色銥磷光配合物[134](圖24)，最大發(fā)射波長為470 nm、CIE色坐標(biāo)(0.14, 0.30)，備受關(guān)注[135-136]。Firpic雖然發(fā)光性較好，但發(fā)射波長太長，不是真正意義上的藍(lán)光。因此，人們更希望得到深藍(lán)光銥磷光配合物[137-149]。對Firpic的化學(xué)結(jié)構(gòu)進(jìn)行改性，增大能隙，可以使其發(fā)射波長藍(lán)移。例如，將2,4-二氟苯基吡啶中與氟原子直接鍵合的兩個碳之間的碳原子替換成氮原子，得到深藍(lán)光銥磷光配合物N-Firpic[150]，最大發(fā)射波長藍(lán)移了25 nm至445 nm；改變Firpic的輔助配體得到Fir6[151]和FirN4[152]，發(fā)射波長均藍(lán)移。Thompson[153]開發(fā)出了第一個含有卡賓結(jié)構(gòu)的深藍(lán)光銥磷光配合物Ir(pmb)3，色坐標(biāo)(0.17,0.06)；我們和北京大學(xué)黃春輝院士課題組合作設(shè)計合成了一類深藍(lán)光銥磷光配合物Ir(PMN)3，色坐標(biāo)為(0.15, 0.11)、最大外量子效率達(dá)到22.5%[154]。

3.4 黃/黃綠光銥磷光配合物

與三基色銥磷光配合物相比，黃/黃綠光銥磷光配合物的研究較晚。隨著OLED照明技術(shù)的發(fā)展，基于三基色的白光器件性能雖然優(yōu)異，但是器件結(jié)構(gòu)復(fù)雜、制作成本高，不利于產(chǎn)業(yè)化和大規(guī)模的應(yīng)用。黃/黃綠光銥磷光配合物可以和藍(lán)光材料互補(bǔ)，實(shí)現(xiàn)結(jié)構(gòu)更為簡單的白光器件。因此，有很多黃/黃綠光銥磷光配合物被合成出來[157-164]，開展了器件性能測試研究。其中，最具代表性的一些黃/黃綠光銥磷光配合物見圖25。然而，現(xiàn)有的黃/黃綠光銥磷光配合物的發(fā)光性能不及相應(yīng)的三基色銥磷光配合物，仍需進(jìn)一步改進(jìn)。

圖25 代表性的黃/黃綠光銥磷光配合物

4 結(jié)語與展望

銥磷光配合物是性能最優(yōu)異的有機(jī)發(fā)光材料，具有較高的發(fā)光效率、良好的熱穩(wěn)定性、發(fā)光顏色容易調(diào)節(jié)等特點(diǎn)，是過去20年發(fā)光材料領(lǐng)域的研究熱點(diǎn)。通過學(xué)術(shù)界和產(chǎn)業(yè)界的共同努力，小分子紅光和綠光銥磷光配合物已成功應(yīng)用到OLED顯示產(chǎn)業(yè)中，而藍(lán)光銥磷光配合物存在光譜不夠藍(lán)、效率不夠高、穩(wěn)定性差等問題，仍處于基礎(chǔ)研究階段。相比紅光、綠光和藍(lán)光三基色銥磷光配合物，黃/黃綠光銥磷光配合物的研究仍較少，有待進(jìn)一步的深入研究。今后應(yīng)從以下幾個方面開展銥磷光配合物的研究開發(fā)：

1) 開展原始創(chuàng)新，設(shè)計合成具有新穎結(jié)構(gòu)高性能銥磷光配合物，以突破專利封鎖，促進(jìn)我國材料及OLED產(chǎn)業(yè)的持續(xù)健康發(fā)展；

2) 為滿足OLED產(chǎn)業(yè)需求，仍需繼續(xù)尋找效率更高和顏色更純的新型銥磷光配合物；

3) 構(gòu)建銥磷光分子材料數(shù)據(jù)庫，獲得結(jié)構(gòu)與性能之間的構(gòu)效關(guān)系，獲得影響發(fā)光性能的主要因素，用于指導(dǎo)新型高性能銥磷光配合物的高效研發(fā)；

4) 發(fā)展低成本、高效率的批量合成新技術(shù)，降低銥磷光配合物的成本，有利于促進(jìn)我國OLED產(chǎn)業(yè)的發(fā)展。

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Synthesis, Color-Controlling Methods and Application Research of Iridium Phosphorescent Complexes

CHANG Qiao-wen1, 2, WANG Zi-ao1, YAN Cai-xian1, ZHANG Xuan-dong1, JIANG Jing1, LIU Wei-ping1, CUI Hao1 *

(1. Kunming Institute of Precious Metals, State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Group Metals, Sino-Platinum Metals Co. Ltd., Kunming 650106, China; 2. Faculty of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China)

Iridium phosphorescent complexes have good thermal stability, relatively short excited state lifetime, high luminous efficiency and easy adjustment of luminous color, and they has become the most excellent organic luminescent materials, and their applications in OLED have attracted much attention. In this paper, the structure, synthesis, color control and application of iridium phosphorescent complexes were reviewed. The structure-activity relationships of several iridium phosphorescent complexes have been obtained, which can be used to guide the efficient research and development of iridium phosphorescent complexes. The problems encountered in the development of iridium phosphorescent complexes were also described, and the development trend and direction of iridium phosphorescent complexes were pointed out.

iridium phosphorescent complexes; synthesis; color regulation; application; development.

O614.82+5

A

1004-0676(2020)03-0094-21

2019-12-30

國家自然科學(xué)基金(21861023)、云南省應(yīng)用基礎(chǔ)研究項(xiàng)目(2019FA047和2018FD141)、云南省轉(zhuǎn)制院所技術(shù)開發(fā)研究專項(xiàng)(202004AR040001) 、稀貴金屬綜合利用新技術(shù)國家重點(diǎn)實(shí)驗(yàn)室開放課題(SKL-SPM-201807)

常橋穩(wěn)，男，博士，高級工程師，研究方向：貴金屬功能配合物研究開發(fā)。E-mail：changqiaowen@126.com

崔 浩，男，高級工程師，研究方向：貴金屬新材料研究開發(fā)。E-mail：cuihao@ipm.com.cn