Interconversion processes between singlet and triplet excited states of organic semiconductors play vital roles in determining the performance of organic electronics;1-4 for example, in organic light emitting diodes (OLEDs), triplet exciton is the major product of charge recombination and hence highly efficient device cannot be realized without exploiting it.2 On the one hand, phosphorescent OLEDs devices take advantage of efficient intersystem crossing from singlet to triplet excited states. Recently, on the other hand, a new route to make use of triplet excitons via effective intersystem crossing from triplet to singlet state is proposed. So-called thermally activated delayed fluorescence (TADF) phenomenon has attracted enormous attention because it can pave a new road of light harvest from triplet excitons without heavy metal atoms such as Ir or Pt.3,4 One of the key parameters for this process is the energy difference between singlet and triplet excited state. Therefore, from a theoretical perspective, it is very important to calculate singlet-triplet energy difference exactly and establish a structure-property relationship for a given molecular system. In this talk, recent theoretical results of the natures of singlet and triplet states as well as the origin of singlet-triplet energy difference of pi-conjugated molecules will be presented.5-7 In the case of carbazole and dicyanobenzene-based TADF molecules,4 the effects of topology on singlet-triplet energy difference and hence the structure-photophysical property relathionship will be addressed.5 Furthermore, the effects of intermolecular interactions on singlet-triplet energy difference will also be discussed in detail.6-7
References
1. Smith, M. B.; Michl, J. Chem. Rev. 2010, 110, 6891-6936.
2. Köhler, A.; Bässler, H. Mater. Sci. Eng. R Rep. 2009, 66, 71-109.
3. Lee, S. Y.; Yasuda, T.; Nomura, H.; Adachi, C. Appl. Phys. Lett. 2012, 101, 093306.
4. Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Nature 2012, 492, 234-238.
5. Lee, K; Kim, D. unpublished
6. Kim, D. Bull. Kor. Chem. Soc. 2014, 35, 2738-2742.
7. Kim, D. Submitted