Conjugated organic materials have been studied widely as a means to create cheaper, more efficient optoelectronic devices. As conducting materials, understanding the spectral changes in these films as a result of molecular orientation and exciton coupling is critical. Here we investigate the nature of model systems, perylene diimide (PDI) derivatives, in the solid state. By varying alkyl side chains attached to the imide N- positions, we controlled electronic coupling and crystallization present in thin films and microstructures. Using polarization absorption microscopy, we could pinpoint the effects of the electric field vector on the absorption spectrum of individual PDI microstructures. These results were compared to atomic force microscopy, fluorescence spectroscopy, and x-ray diffraction measurements. One particular PDI derivative, N,N′-Bis(3-pentyl)perylene-3,4,9,10-bis(dicarboximide) (B2), forms microcrystals of varying size depending on preparation conditions. The resulting spectra show both a large bathochromic shift (2200 cm-1) and a smaller hypsochromic shift (600 cm-1) compared to the spectrum of B2 in chloroform, indicating the presence of two exciton states. The polarization dependent isosbestic point is at 520 nm (19200 cm-1), which indicates that these states are orthogonal to one another. The lower energy portion of the film spectra has an absorption polarization ratio of 1.0 while the higher energy spectrum has a polarization ratio of -0.2. The higher polarization ratio and larger spectral shift of the lower energy transition is attributed to enhanced pi-pi coupling, orbital overlap, and long range order in these aggregates. We then compared our B2 results to PDI compounds having a) longer branched chains and b) linear side chains. The crystals made from the linear side chains show a greater degree of electronic coupling, and the vibronic structure is not observed. The polarization ratio of the low energy spectrum is typically reduced to a maximum value of 0.4. These comparisons shed light as to the effects of electron coupling, film morphology, and crystal size on the overall spectrum of PDI samples, which is a direct indicator of electron transport.

Correlating spectral shifts, polarization, and molecular orientation in conjugated organic thin films and microstructures

251st National Meeting (2016)

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