Composition of molecular aggregates during film formation revealed using simulated absorption spectra

The features in linear absorption spectra can be exquisitely sensitive to the electronic coupling between organic molecules in a molecular aggregate. The spectral signatures of molecular aggregation are the result of electronic coupling, which is determined by the physical arrangement of the molecules. In this work, the absorbance of pseudoisocyanine (PIC) is measured in situ after solution drop casting to reveal a distinct intermediate stage during the aggregation process. A possible composition and structure for the molecular aggregates during this stage is inferred by using a Holstein-like Hamiltonian to calculate an absorption spectrum with spectral features that match those of the measured spectrum. More than one type of aggregate is required to compute a spectrum that agrees with the measured spectrum within this model. In this case, the spectrum can be fit with a trimer and an aggregate with 9 molecules with electronic coupling values of +600 cm^-1 and -600 cm^-1, respectively. We report a procedure to compute spectra that agree with measured spectra and limits the number of iteratively fit parameters. This strategy will enable the interpretation of in situ absorption data for other conjugated molecules during molecular aggregation and provide insight into the evolving composition of aggregates during the process of film formation.