Vibrational exciton nanoimaging of phases and domains in porphyrin nanocrystals

Abstract

Much of the electronic transport, photophysical, or biological functions of molecular materials emerge from intermolecular interactions and associated nanoscale structure and morphology. However, competing phases, defects, and disorder give rise to confinement and many-body localization of the associated wavefunction, disturbing the performance of the material. Here, we employ vibrational excitons as a sensitive local probe of intermolecular coupling in hyperspectral infrared scattering scanning near-field optical microscopy (IR s-SNOM) with complementary small-angle X-ray scattering to map multiscale structure from molecular coupling to long-range order. In the model organic electronic material octaethyl porphyrin ruthenium(II) carbonyl (RuOEP), we observe the evolution of competing ordered and disordered phases, in nucleation, growth, and ripening of porphyrin nanocrystals. From measurement of vibrational exciton delocalization, we identify coexistence of ordered and disordered phases in RuOEP that extend down to the molecular scale. Even when reaching a high degree of macroscopic crystallinity, identify significant local disorder with correlation lengths of only a few nanometers. This minimally invasive approach of vibrational exciton nanospectroscopy and -imaging is generally applicable to provide the molecular-level insight into photoresponse and energy transport in organic photovoltaics, electronics, or proteins. infrared spectroscopy | molecular vibrations | scattering-scanning near-field optical microscopy (s-SNOM) | vibrational exciton | molecular energy transport

2D Vibrational Exciton Nanoimaging of Domain Formation in Self-Assembled Monolayers

“…The coupling manifests itself in electronic or vibrational wave function delocalization that gives rise to hybridization with vibrational mode splitting and resonance energy shifts as spectroscopic observables. In particular, vibrational excitons, delocalized across domains in dense, well-ordered molecular systems, are sensitive to disorder and domain size. , However, with dimensions typically on the few-nanometer scale for many alkanethiol and aromatic thiol SAMs, , probing disorder in SAMs with the desired simultaneous high spatial resolution, monolayer sensitivity, and spectroscopic specificity has long remained difficult.…”

Vibrational exciton delocalization precludes the use of infrared intensities as proxies for surfactant accumulation on aqueous surfaces

“…In solids, where these effects are far more pronounced, the influence of vibrational excitons on intensity is almost always reported as signal loss. 3,[9][10][11] The correlation analysis between frequencies and intensities in the presence or absence of the transition dipole coupling suggests that the reduction in oscillator strength in NaCl solution that is observed experimentally corresponds to precisely this type of signal reduction.…”

“…The long-range exponential function determines the delocalization length, and integration suggests that 90% of the wavefunction is contained in the relatively large delocalization length of 23 Å (which is still an order of magnitude smaller than delocalization length scales reported in solids). 3 Despite the sizable delocalization length, the differences in the splittings as a function of mean molecular area do not become particularly pronounced until after the monolayer is compressed into the region governed by the short-range exponential, ∼6 Å (36 Å 2 /molecule). Interestingly, this is precisely consistent with experiment, where the differences in integrated C-H peak intensities are not pronounced until a compression of 25 Å 2 /molecule.…”

“…1 Moreover, surfactant molecules spontaneously aggregate into micelles at high concentrations, 2 and strong vibrational couplings play a central role in the spectra of self-aggregating nanostructures. 3 In the present work, we apply vibrational exciton theory 4,5 to understand the role of vibrational coupling between surfactant molecules at the air/water interface. In this approach, which is the vibrational analogue of the Frenkel-Davydov exciton theory developed to understand collective electronic excitations in molecular crys-tals, 6,7 the vibrational wavefunction is expressed as a superposition of local-mode wavefunction on individual vibrational chromophores.…”

“…Collective vibrational "excitons" emerge from coupling between quasi-degenerate local modes, and these have the effect of depleting local signal intensity in one-dimensional (1D) spectroscopic measurements. 3,[8][9][10][11] Total oscillator strength (when integrated over all wavelengths) is conserved in the presence of excitonic coupling, and the apparent loss in signal at a particular wavelength is a result of intensity borrowing leading to a depletion of oscillator strength from the nominal bright state(s). In the presence of significant coupling, much of the oscillator strength may be "lost" in a diffuse background that may be difficult to resolve experimentally.…”