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Efficient and Long-lived Room Temperature Organic Phosphorescence: Theoretical Descriptors for Molecular Designs Huili Ma, Qian Peng, Zhongfu An, Wei Huang, and Zhigang Shuai J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11224 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018
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Journal of the American Chemical Society
Efficient and Long-lived Room Temperature Organic Phosphorescence: Theoretical Descriptors for Molecular Designs Huili Ma†, ‡, Qian Peng*, §, Zhongfu An‡, Wei Huang‡, Zhigang Shuai*, † Department of Chemistry and MOE Key Laboratory of Organic Optoelectronics and Molecular Engineering, Tsinghua University, 100084 Beijing, China. § Key Laboratory of Organic Solids and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, 100190 Beijing, China. ‡ Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211800, China. †
ABSTRACT: Room-temperature phosphorescence (RTP) with long afterglow from pure organic materials has attracted great attentions for their potential applications in biological imaging, digital encryption and optoelectronic device and so on. Organic materials have been long considered to be non-phosphorescence owing to the weak molecular spin-orbit coupling and highly sensitive to temperature. However, recently, some purely organic compounds can demonstrate highly efficient RTP with long afterglow upon aggregation while others fail. Namely, it remains a challenge to expound the underline mechanisms. In this study, we present the molecular descriptors to character the phosphorescence efficiency and lifetime. For prototypical RTP system consists of carbonyl group and π-conjugated segments, the excited states can be regarded as admixture of nπ* (with portion ) and ππ* (portion ). Starting from phosphorescent process and El-Sayed rule, we deduced that (i) the intersystem crossing (ISC) rate of S1Tn is mostly governed by the modification of the product of and , and (ii) the ISC rate of T1S0 is determined by the value of T1. Thus, the descriptors (γ=×, ) can be employed to describe the RTP character of organic molecules. From hybrid quantum mechanics and molecular mechanics (QM/MM) calculations, we illustrated the relationships amongst the descriptors (γ, ), phosphorescent efficiency and lifetime, as well as spin-orbit coupling constants. We stressed that the large γ and values are favorable to the strong and long-lived RTP in organic materials. Experiments have reported confirmations of these molecular design rules.
1. INTRODUCTION More recently, room-temperature phosphorescence (RTP) from pure organic materials has attracted great attentions owing to not only fundamental interests, but also to application potentials in optoelectronics and biotechnologies such as organic lightemitting diodes, security systems, digital encryption, optical recording devices, sensing, and imaging, etc.1-9 Phosphors are conventionally limited to inorganic materials (such as rare earth ions Eu, Ce and Pr etc.)10 and organometallic complex materials (Ir, Pt etc.).11 Pure organic phosphors with long persistent RTP are extremely rare but strongly desired for their advantages of low cost, higher processability, biocompatibility, synthetic flexibility and appreciable stability etc. Having a look at the current pure organic materials, we can find that some compounds exhibit long RTP lifetime of hundreds of millisecond even second order but low quantum efficiency of less than 5% in crystal, 12-14 named as type I in Figure 1a; and some compounds have strong RTP but very short lifetime of ca. 1.0 ms or microsecond order 15, called as type II in Figure 1b. Obviously, the long lifetime and high quantum efficiency are conflict. More recently, several groups have been devoted to overcome this challenge by designing aromatic carbonyl compounds containing n/π-groups.16-21 However, this strategy seems invalid in some compounds because of the low Φp (
