Theoretical predictions of red and near-infrared strongly emitting X-annulated rylenes

The optical properties of rylenes are extremely interesting because their emission colors can be tuned from blue to near-infrared by simply elongating the chain length. However, for conjugated chains, the dipole-allowed odd-parity 1B_u excited state often lies above the dipole-forbidden even-parity 2A_g state as the chain length increases, thus preventing any significant luminescence according to Kashas rule. We systemically investigated the 1B_u2A_g crossover behaviors with respect to the elongating rylene chain length with various quantum chemistry approaches, such as time-depended density functional theory (TDDFT), complete active space self-consistent field theory (CASSCFCASPT2), multireference configuration interaction (MRCI)Zerners intermediate neglect of diatomic overlap (ZINDO), and MRCImodified neglect of differential overlap. The calculated results by CASSCFCASPT2 and MRCIZINDO are completely coherent: the optical active 1B _u state lies below the dark B_3g or 2A_g state for perylene and terrylene, which results in strong fluorescence; while a crossover to S₁ = 2A_g occurs and leads to much weaker fluorescence for quaterrylene. Then we put forward a molecular design rule on how to recover fluorescence for the longer rylenes by introducing heteroatom bridges. Several heteroatom-annulated rylenes are designed theoretically, which are predicted to be strongly emissive in the red and near-infrared ranges. These are further confirmed by theoretical emission spectra as well as radiative and nonradiative decay rate calculations by using the vibration correlation function formalisms we developed earlier coupled with TDDFT.