Spectroscopic Signature of Two Distinct H-Aggregate Species in Poly(3-hexylthiophene)

Abstract

In an endeavor to correlate the optoelectronic properties of π-conjugated polymers with their structural properties, we investigated the aggregation of P3HT in THF solution within a temperature range from 300 to 5 K. By detailed steady-state, site-selective, and time-resolved fluorescence spectroscopy combined with Franck–Condon analyses, we show that below a certain transition temperature (265 K) aggregates are formed that prevail in different polymorphs. At 5 K, we can spectroscopically identify two H-type aggregates with planar polymer backbones yet different degree of order regarding their side chains. Upon heating, the H-character of the aggregates becomes gradually eroded, until just below the transition temperature the prevailing “aggregate” structure is that of still phase-separated, yet disordered main and side chains. These conclusions are derived by analyzing the vibrational structure of the spectra and from comparing the solution spectra with those obtained from thin films that were cooled slowly from the melting temperature to room temperature and that had been analyzed previously by various X-ray techniques. In addition, site selectively recorded fluorescence spectra show that there isdependent on temperatureenergy transfer from higher energy to lower energy aggregates. This suggests that they must form clusters with dimensions of the exciton diffusion length, i.e., several nanometers in diameter.

The effect of intermolecular interaction on excited states in p − DTS(FBTTH2)2

“…In the same ways as in polymers, we observe that aggregation proceeds in a three-step process, where the actual process of dimer/aggregate formation is preceded by planarization of the molecular backbone, and followed by further optimization of the aggregate geometry. 13,24 It appears that the process of planarization is required a priori to enable the intermolecular interaction.…”

“…[13][14][24][25][26] For these polymers, the process of aggregation upon cooling was found to proceed in three distinct phases, i.e. (i) a swelling up and planarization upon cooling,…”

Salt-induced thermochromism of a conjugated polyelectrolyte

“…It was shown that the absorption and emission spectra, as well as the excited-state properties, are strongly determined by the intrachain polymer conformation and by interchain aggregation. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Conjugated polymers can be synthesised with ionic side-chains, yielding conjugated polyelectrolytes (CPEs), which combine the optoelectronic properties of p-conjugation in the backbone with solubility in highly polar solvents and the electrostatic behaviour of polyelectrolytes. 23 CPEs provide the unique opportunity to manipulate the conformation of conjugated polymers in water and to correlate this to their photophysical behaviour.…”

π‐Conjugated Donor Polymers: Structure Formation and Morphology in Solution, Bulk and Photovoltaic Blends

“…At 5 K in a frozen THF glass, the emission from P3HT was observed to be the superposition of two nearly identical, well-resolved spectra that differ by about 70 meV in the position of their origin. [49] Comparison with X-ray data showed that these two emissions can be assigned to two different structures. [53] In both arrangements, the chain backbones are planar and π-stacked in the same way, so that there is no difference between the two structures within one stack, and this is consistent with both emissions showing the same vibronic progression.…”

“…[47] Alternatively, one may keep the solvent yet reduce the temperature. [48,49] Since the "quality" of a solvent depends on temperature, cooling has the same effect than using a poorer solvent. [50] When a good solution of P3HT is cooled from room temperature to, e.g., 160 K, as illustrated in Figure 10a, the absorption spectra show distinct changes.…”