New bioinspired luminescent compounds based on lumazine: A photophysical study in solution and solid state

Two polycyclic lumazine derivatives with different side chains (alkyl-acid and alkyl-ester chains) have been synthesized. Both novel bioinspired compounds exhibit interesting luminescent properties. Herein, the structural and optical properties of these compounds have been analyzed by single-crystal X-ray analysis, absorption, fluorescence and vibrational Raman spectroscopy together with DFT calculations. Their spectroscopic behavior depends on the five-fused core, which remains almost planar in the excited electronic state, and also on the nature of the side chain. In the solid phase, bathochromic shifts were observed with respect to solution for both molecules. These spectroscopic shifts were particularly notorious for the acid derivative and they might be related to the differences found in the molecular packing depending on the nature of the side chain. Two different theoretical strategies were employed to study the molecular packing in the crystalline state. Firstly, the energy of the S0→S1 transition was calculated for a molecule surrounded by a cluster of neighboring molecules extracted from the crystal structure. Secondly, the lowest-energy electronic transitions were computed for different-size molecular clusters extracted from the crystal structure. Thus, the J-aggregate photophysical behavior experimentally observed in the solid phase, and the spectroscopic differences between both compounds, were explained on the basis of short-range excitonic coupling. These results provide valuable insights for the rational design of new bioinspired compounds with luminescent properties.