This is a theoretical chemistry study to design singlet fission chromophores. Quantum chemistry methods MRSF-TDDFT and NEVPT2 are used to design chromophores with the building blocks of cyclic (alkyl)(amino)carbenes (CAACs). CAAC dimers with C 2 , C 4 and p-phenylene spacers are considered. The substitutions with trifluoromethyls and fluorine atoms at the position are investigated. The electronegative substituents enhance the accepting capability of the C, while maintaining it as a quaternary C

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... atom. The phenylene-connected dimers with the two substitutions are identified as promising candidates for the singlet fission chromophores. The cylindrically symmetric C 2 and C 4 spacers allow for substantial structural reorganizations in the S 0 -to-S 1 and S 0 -to-T 1 excitations. Although the two substituted dimers with the C 4 spacer satisfy (or very close to satisfy) the primary thermodynamics criterion for the singlet fission, the significant structural reorganizations result in high barriers so that the fission is kinetically unfavorable. iii Acknowledgement First of all, I would like to express my sincere gratitude to my supervisor Dr. Tao Zeng for his patient and immense support during my research. I consider myself lucky to work with him and share his knowledge in academic and professional life. My project couldn't have a successful one without him.