Spectral analysis using chemometrics is extensively used for quantitative chemical analysis in a mixture, but it works powerfully only when the peak intensity is solely proportional to the quantity of chemical components. In this sense, thin films on a solid substrate are not suitable for chemometric analysis, because the molecular orientation also influences the peak intensity via the surface selection rules. In the present study, this long-term analytical issue has readily been overcome by using p-polarized multiple-angle incidence resolution spectrometry (pMAIRS), which has a characteristic that the in-plane (IP) and out-of-plane (OP) vibrational spectra of a thin-film sample are obtained simultaneously in a common ordinate scale. Thanks to this unique power of pMAIRS, the average of the IP and OP spectra annihilates optical anisotropy, yielding an orientation-free spectrum, which enables us to perform the simultaneous quantitative analysis of both quantity change and molecular orientation of the constituents in a thin film. Now, we are ready to examine chemical reactions quantitatively in a thin film.

Solution-processable organic semiconductors having bulky substituent groups on the π-conjugated skeleton are rapidly gaining attention for their potential applications to large-area electronics. While the substituent groups contribute to the good solubility in organic solvents, they give rise to hopping sites in a thin film, affecting adversely the charge-carrier transport. As an alternative material, a solvent-soluble precursor compound with thermally cleavable functional groups is promising, which can be converted by heat treatment into a thin film to generate the desired material consisting solely of conjugated systems. This precursor approach is practically applied to various thin-film-based devices. The overall process of the thin film growth, however, remains unrevealed. In the present study, solution-processed pentacene thin films are prepared from a thermally convertible precursor, and the structural evolution during the chemical conversion reaction has been revealed by a combination of cutting-edge analytical tools of two-dimensional X-ray diffraction (2D-GIXD) and p-polarized multiple-angle incidence resolution spectrometry (pMAIRS). The highlight is that pentacene is crystallized in a stepwise manner in the thermally converted films, which is substantially different from a typical growth process. In addition, influences of the oxidation reaction of pentacene on the molecular arrangement are also discussed quantitatively. This study provides a fundamental schematic of thin films grown by the precursor method.