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PNAS Plus Significance Statements

Higher acenes are promising organic semiconductors with versatile electronic properties. A better understanding of their groundand electronic excited states will benefit further molecular design and future applications. However, their instability and multireference character have impeded experimental and theoretical studies. Here, we use the recently developed particle–particle random-phase approximation in combination with a diradical analysis to unveil the nature of their groundand electronic excited states. The excitation energies are presented, along with a detailed description of the bonding nature, which switches from regular molecules to full diradicals, and then even to polyradicals. The likelihood of singlet fission is briefly discussed from an energetic perspective. (See pp. E5098–E5107.)