Photophysical and Spectroscopic Investigations on (Oligo)Thiophene-Arylene Step-ladder Copolymers. The Interplay of Conformational Relaxation and On-Chain Energy Transfer

Abstract

An optical spectroscopy and photophysics study on four (oligo)thiophene-phenylene and (oligo)thiophene-naphthylene step-ladder type copolymers in solution (room and low temperature) and in the solid state (thin film) is presented. The study involves absorption, emission, and triplet-singlet difference spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation), excited-state lifetimes, and singlet and triplet energies. The overall data allow for a determination of the rate constants for all decay processes and from these several conclusions could be drawn: (1) in solution the main deactivation channels are radiationless processes (S(1) approximately -->S(0) internal conversion and S(1) approximately -->T(1) intersystem crossing); (2) from time-resolved fluorescence decays in the picosecond time domain three decay components are seen: a fast decay (10-20 ps) at short wavelengths, which becomes a rising component at longer wavelengths, an intermediate decay component (120-190 ps) most probably associated to isolated conjugated segments, and a third exponential related to the emission of the fully relaxed polymer. The assignment of the fast decay component to an on-chain energy transfer/migration is based of the dependence of the decay time on the solvent viscosity in combination with the investigation of an oligomeric model compound. Here, the absence of any significant changes of the decay parameters (decay times and pre-exponential factors) upon going from a less (toluene) to a more viscous (decalin) solvent together with the monoexponential fluorescence decay of the oligomeric model compound allow us to differentiate between deactivation of the singlet excited state by conformational relaxation and on-chain energy/transfer migration.

Excited-State Dynamics of Water-Soluble Polythiophene Derivatives: Temperature and Side-Chain Length Effects

“…Because of the stronger hydrogen-bonding interactions, the longer side chains will be capable of accommodating a greater number of backbone conformations. According to previous experimental ,,− and theoretical studies, − the spectroscopic and photophysical properties of conjugated polymers depend strongly on their backbone conformations. This would indicate that presence of different number of backbone conformations should give rise to distinct excited-state decay behavior.…”

Ultrafast Relaxation of the Poly(3-hexylthiophene) Emission Spectrum

“…After they have formed, the singlet excitons can hop to lower-energy localized states, allowing a change of their spatial position and energy . The process has often been described as excitation energy transfer, mainly by the Förster mechanism, between polymer segments of different conjugation lengths that exist due to disorder (kinks and defects in the polymer chain). ,,,,,− Cascading down-energy exciton migration typically leads to a progressive red shift of the emission spectrum, which slows down in time as the number of nearby states with lower energy decreases. Multiphasic time scales ranging from about half a picosecond (for one-step hops) to hundreds of picoseconds (for multistep hops) were reported for various polymers, so that we conclude that exciton hopping contributes to the three time constants we found for the spectral relaxation of P3HT in solution and film.…”

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

“…[64,65] This is clearly much less than the typical length scale of about 10-20 nm in optimized bulk heterojunctions, estimated by structural characterization in optimized morphologies. [27][28][29][30][31][32] Furthermore, point-like exciton diffusion by the hopping mechanism has been measured on numerous occasions to take 0.5-1 ps for a single hopping step, [39][40][41][42][44][45][46][49][50][51]88,89] which is again too slow to explain how excitons reach a fullerene interface in <100 fs. In agreement with this, an ultrafast transient absorption study of poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(4,7′-di-2-thienyl-2′,1′,3-benzothiadiazole)] (APFO3) films with various concentrations studies on pristine polymers in our group, we have pointed out that the ultrafast charge separation can compete with relaxation processes such as excited-state localization and exciton diffusion.…”