Self-assembly of quaternary benzo[c]phenanthridine plant alkaloids into dimer in aqueous solution

•Quaternary benzo[c]phenanthridine alkaloids self-assemble into dimers in water.•Nitidine dimer formation is the most exothermic and modifies the fluorescent behaviour.•Sanguinarine and chelerythrine dimers leads to rapidly dissociateion to singlet-excited and ground-state monomers upon light absorption.•The dimerization enthalpy is substantially compensated by the unfavourable entropic term.