Signatures of Melting and Recrystallization of a Bulky Substituted Poly(thiophene) Identified by Optical Spectroscopy

Abstract

As-cast and slowly crystallized films of conjugated polymers can contain (partially) ordered and less ordered (amorphous) regions with structural defects. Crystallization allows to generate chains with highly planarized backbones, embedded in structures exhibiting long-range order. In the present study, we used spatially resolved optical spectroscopy to quantify differences in the degree of order of a bulky substituted poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT). In particular, we compared absorption and photoluminescence (PL) measurements from large spherulitic crystals, and the same region rapidly recrystallized after melting, which allowed to identify characteristic features of ordered and less ordered regions. In addition, on the basis of temperature-dependent absorbance and PL measurements, we followed in situ melting and recrystallization processes, i.e., transitions between ordered and disordered phases. A multipeak analysis of absorption and PL spectra based on a modified Franck−Condon progression showed changes in for example the relative intensities of each peak, the excitonic bandwidth, and the vibronic energy as a function of temperature. Most importantly, at the phase transition temperature, a clear change in the positions of the peaks (i.e., their wavelengths, corresponding to the energy of the emitted photons) was detected. In particular, the relative absorption and PL intensities depended sensitively on the extent of order within PDOPT samples. Furthermore, on the basis of a comparison with calorimetric measurements, we have confirmed correlations between changes in the relative absorbance and PL intensities with variations in order/disorder occurring during melting and recrystallization.

Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes

“…The importance of the side groups for structural properties in PDOPT is also manifest in its crystallization behavior. Despite the absence of π−π stacking, spherulitic structures of several hundred micrometers diameter and a high degree of crystallinity can be grown under appropriate conditions (61). The main driving forces for spherulite formation are noncovalent interactions between side groups of neighboring chains (20).…”

Following isothermal and non-isothermal crystallization of poly(3-hexylthiophene) thin films by UV–vis spectroscopy

“…The low-energy absorbance vibronic peaks are attributed to chains in crystalline states, that is, π-stacked chains forming weakly interacting H-aggregates. The remaining high-energy contribution to the spectrum arises from intra-chain states of coiled (amorphous) chains [20][21][22]26,34,35]. The fraction of the spectrum attributed to absorption from aggregates is determined using a modified Franck-Condon fit (equation ( 1)) developed by Spano and co-workers [22]:…”

“…The parameters remaining free for fitting are m, the vibrational level, W, the free exciton bandwidth, E 0 , the transition energy of the lowest energy peak A 0-0 , and σ, the width of the Gaussian line shape functions. The summation ∑ n∕ =m … is over the vibrational quantum number n [21]. For simplicity, a single Gaussian line shape is used with the same width (σ) for each vibronic transition, introduced in the exponential term of the equation [22].…”

“…Another approach to extract the crystalline contribution is based on a subtraction of the amorphous contribution from the experimental absorption spectrum [20,21,26] by using the absorption spectrum of a The solid curve shows the modified Franck-Condon fit to the spectrum consisting of three peaks (m = 2), represented each by dotted curves, A 0-0 , A 0-1 and A 0-2 at 2.07, 2.24 and 2.42 eV, respectively (the peaks are spaced by 0.175 eV). See Appendix A (Table 1) for additional quantitative results of the fitting.…”

“…Charge transport is a complex multiscale process, extremely sensitive to the intra-and interchain order in these semiconducting polymers [2]. Hence, there is a continuous effort to improve the degree of order in the conjugated polymers, which is examined through different techniques such as small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) [11][12][13][14], nuclear magnetic resonance (NMR) [15,16], atomic force microscopy (AFM) [17], transmission electron microscopy (TEM) [18], differential scanning calorimetry (DSC) [19], infrared (IR) spectroscopy [16], and ultraviolet-visible (UV-vis) spectroscopy [20,21].…”