Role of positional disorder in the spectra of conjugated polymer aggregates: a conical intersection of potential energy surfaces

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Here we have studied the effect of disorder on the luminescence spectra due to the orientational disposition of monomeric conjugated polymer chain units forming a dimer aggregate. The dipolar interchain interactions are described considering static as well as dynamic disorder created by the torsion or shearing degree of freedom. For static disorder, the shearing angle is treated parametrically for different frozen configurations of the positional defect. In the case of dynamical disorder the torsional motion is treated on an equal footing with the vibrational degree of freedom by including the shearing coordinate explicitly in the calculation. The inclusion of torsion angle dependence in the interchain interaction not only saves the 0–0 transition from total quenching but also generates a conical intersection in the adiabatic potential energy surfaces of the dimer aggregate. Due to the vanishing of the interchain coupling at the conical intersection point, the two chains become independent and the dimeric spectra pass through a monomeric pattern which can serve as an example of the presence of conical intersection in the aggregates of conjugated polymer chains. Also the presence of the low-frequency torsion mode explicitly considered in the case of dynamic disorder can be detected from a significant Stokes shift in the steady state spectra.