Correlating Nanoscale Optical Coherence Length and Microscale Topography in Organic Materials by Coherent Two-Dimensional Microspectroscopy

Abstract

Many nanotechnology materials rely on a hierarchical structure ranging from the nanometer scale to the micrometer scale. Their interplay determines the nanoscale optical coherence length, which plays a key role in energy transport and radiative decay and, thus, the optoelectronic applications. However, it is challenging to detect optical coherence length in multiscale structures with existing methods. Techniques such as atomic force microscopy and transmission electron microscopy are not sensitive to optical coherence length. Linear absorption and fluorescence spectroscopy methods, on the other hand, were generally limited by inhomogeneous broadening, which often obstructs the determination of nanoscale coherence length. Here, we carry out coherent two-dimensional microspectroscopy to obtain a map of the local optical coherence length within a hierarchically structured molecular film. Interestingly, the nanoscale coherence length is found to correlate with microscale topography, suggesting a perspective for controlling structural coherence on molecular length scales by appropriate microscopic growth conditions.

On the Low-Lying Electronically Excited States of Azobenzene Dimers: Transition Density Matrix Analysis

“…On the other hand, an approximate Formula ( 19 ) provides results very similar to those obtained with more complex Formulae ( 21 )–( 23 ) if a small basis set (6-31G* in our case) is employed (see Figure 7 ). Therefore, the use of this formula (Equation ( 19 )) can be justified for analysis of local and charge transfer contributions to the excited states of noncovalent dimers and larger aggregates in the case of quantum chemical calculations employing rather small basis sets [ 25 , 78 ]. We also note that FTDM matrices including the AO overlap (Equations ( 21 )–( 23 )) are basis set dependent, similarly to Mulliken and Löwdin population analyses.…”

“…In some cases, e.g., at short distances between monomers (when an overlap of molecular orbitals belonging to different chromophores is large), these theories should be amended by accounting for charge transfer excitations between monomers [ 23 ]. For aggregates composed of a relatively small number of chromophores it is possible to perform a full quantum chemical calculation instead of using exciton theories, thus avoiding approximations inherent to the latter [ 24 , 25 ]. (Naturally, the smallness of the system decreases with the increasing computational cost associated with a particular quantum chemical method.)…”

Coherent Two-Dimensional and Broadband Electronic Spectroscopies

“…250,252 Brixner and co-workers applied this approach to study the optical coherence length in organic films, informing the design of microscopic growth conditions that optimally control the molecular-level structural coherence. 452 Overall, 2DES microscopy is shown to be an elegant approach to probe the electron dynamics of optoelectronic devices as well as the in vivo dynamics of fluorescent biological samples.…”

“…When a photoinitiated process causes a change in the spatial wave function beyond the optical diffraction limit, spatial resolution is necessary to monitor the phenomena. The combination of optical microscopy and femtosecond spectroscopy enables the observation of the time evolution of the electronic wave function over space. , Recently, it has been proposed that the combination between optical microscopy and 2DES reveals spatially resolved dynamics of heterogeneous samples with a submicron spatial resolution and femtosecond time resolution. , Brixner and co-workers applied this approach to study the optical coherence length in organic films, informing the design of microscopic growth conditions that optimally control the molecular-level structural coherence . Overall, 2DES microscopy is shown to be an elegant approach to probe the electron dynamics of optoelectronic devices as well as the in vivo dynamics of fluorescent biological samples.…”

Higher-Order Multidimensional and Pump–Probe Spectroscopies

“…For any device, the processing into thin films or bulk materials can result in different phases that contain varying local structures and thus modified interactions between the constituents (i.e., molecules). Such processing typically leads to a change of properties from solution to bulk material contained in an actual device. , Spatially resolved higher-order spectroscopy could characterize the local properties and determine the influence of local structure, such as in domains, at interfaces, or around local defects. For example, one might envision using fifth-order spectroscopy to resolve the local spatial variations in exciton diffusion within a thin film.…”