ABSTRACT
The controlled delocalization of molecular excitons remains an important goal towards the application of organic chromophores in processes ranging from light-initiated chemical transformations to classical and quantum information processing. In this study, we present a methodology to couple optical and magnetic spectroscopic techniques and assess the delocalization of singlet and triplet excitons in model molecular chromophores. By comparing the steady-state and time-resolved optical spectra of Zn-porphyrin monomers and weakly coupled dimers, we show that we can use the identity of substituents bound at specific positions of the macromolecules' rings to control the inter-ring delocalization of singlet excitons stemming from their B states through acetylene bridges. While broadened steady-state absorption spectra suggest the presence of delocalized B state excitons in mesityl-substituted Zn-tetraphenyl porphyrin dimers (Zn2U-D), we confirm this conclusion by measuring an enhanced ultrafast non-radiative relaxation from these inter-ring excitonic states to lower lying electronic states relative to their monomer. In contrast to the delocalized nature of singlet excitons, we use time-resolved EPR and ENDOR spectroscopies to show that the triplet states of the Zn-porphyrin dimers remain localized on one of the two macrocyclic sub-units. We use the analysis of EPR and ENDOR measurements on unmetallated model porphyrin monomers and dimers to support this conclusion. The results of DFT calculations also support the interpretation of localized triplet states. These results demonstrate researchers cannot conclude triplet excitons delocalize in macromolecular based on the presence of spatially extended singlet excitons, which can help in the design of chromophores for application in spin conversion and information processing technologies.
ABSTRACT
Conjugated polymers are anisotropic in shape and with regard to electronic properties. Little is known as to how electronic anisotropy impacts the underlying characteristics of the electron spin, such as the coupling to orbital magnetic moments. Using multifrequency electrically detected magnetic resonance spectroscopy extending over 12 octaves in frequency, we explore the effect of spin-orbit coupling by examining the pronounced broadening of resonance spectra with increasing magnetic field. Whereas in three commonly used materials, the high-field spectra show asymmetric broadening, as would be expected from anisotropic g-strain effects associated with the molecular structure, in the conducting polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) the spectra broaden isotropically, providing a direct measure of the microscopic distribution in g-factors. This observation implies that effective charge-carrier g-tensors are isotropic, which likely originates from motional narrowing in this high-mobility material.
