Abstract
Red emission in crystals has been observed with an ultra-small-single-benzenic meta-fluorophore (MF) with a molecular weight (MW) of only 197 Da, bettering the literature report of fluorophores with the lowest MW = 252 Da. Supramolecular extensive hydrogen-bonding and J-aggregate type centrosymmetric discrete-dimers or a 1D chain of MFs led to red emission (λ (max) (em) = 610-636 nm) in MF crystals. Unlike in the solution phase showing one absorption band, in thin films and in crystals the transition from the S(0) state to both the S(1) (-) state and S(1) (+) state becomes feasible. The angle between the transition dipole moments has been obtained to be 66.99° and the exciton splitting energy has been obtained to be (-) 55.7 meV. Significant overlap have been observed and the extent of overlaps integrals between the HOMOs and the LUMOs were assessed to be 0.0068 and (-) 0.00024, respectively. Planar molecules are shown to be involved in anti-parallel stacking with a slip-angle of 44.05° and an inter-planar longitudinal distance of 3.40 Å. A large magnitude of ΔE (ES) (energy difference between the S(1) (-) state and S(1) (+) state) (0.83 eV) has been obtained. A much higher magnitude of the CT coupling constant (-0.708 for MF2) has been noted in comparison to the coulombic coupling constant (0.016 for MF2). The excited-state-lifetime has been shown to increase from 5.98 ns (in hexane) to 30.90 ns in the crystal. All these extra-ordinary optical properties point to the existence of a charge-transfer mediated J-aggregation phenomenon in these MF crystals. Based on these fascinating observations, highly stable, bright and colour pure white LEDs could be generated.