Comparison of Charge-Transfer Dynamics of Naphthalenediimide Triads in Solution and π-Stack Architectures on Solid Surfaces

Abstract

The femto- to microsecond excited-state dynamics of an electron donor–acceptor–donor triad, consisting of two red core-substituted naphthalenediimides (rNDI) and one colorless naphthalenediimide (pNDI), in solution has been compared to that of a supramolecular surface architecture, constituted of coaxial stacks of rNDI and pNDI and prepared by self-organizing surface initiated polymerization (SOSIP). In the triad, charge separation between an excited rNDI and pNDI takes place in highly polar solvents only and for a subensemble of molecules, around 30%, with a folded conformation. Other processes, such as singlet and triplet excitation energy transfer from pNDI to rNDI and intersystem crossing, are also operative. Additionally, bimolecular symmetry-breaking charge separation upon triplet–triplet annihilation is observed on the microsecond time scale in polar solvent. In the surface architecture, excitation of an rNDI is followed by an ultrafast symmetry breaking-charge separation resulting in a charge-transfer exciton, which either recombines or dissociates into a charge-separated state with the electron and the hole in different stacks. The same charge-separated state can also be populated upon excitation of pNDI, either via a charge-transfer pNDI exciton or after excitation energy transfer to rNDI. Charge recombination in the SOSIP film takes place on a wide range of time scales, ranging from a few picoseconds to several hundreds of microseconds.