Generating Long-Lived Triplet Excited States in Narrow Bandgap Conjugated Polymers

Abstract

Narrow bandgap conjugated polymers are a heavily studied class of organic semiconductors, but their excited states usually have a very short lifetime, limiting their scope for applications. One approach to overcome the short lifetime is to populate long-lived triplet states for which relaxation to the ground state is forbidden. However, the triplet lifetime of narrow bandgap polymer films is typically limited to a few microseconds. Here, we investigated the effect of film morphology on triplet dynamics in redemitting conjugated polymers based on the classic benzodithiophene monomer unit with the solubilizing alkyl side chains C 16 and C 2 C 6 and then used Pd porphyrin sensitization as a further strategy to change the triplet dynamics. Using transient absorption spectroscopy, we demonstrated a 0.45 ms triplet lifetime for the more crystalline nonsensitized polymer C 2 C 6 , 2−3 orders of magnitude longer than typically reported, while the amorphous C 16 had only a 5 μs lifetime. The increase is partly due to delaying bimolecular electron−hole recombination in the more crystalline C 2 C 6, where a higher energy barrier for charge recombination is expected. A triplet lifetime of 0.4 ms was also achieved by covalently incorporating 5% of Pd porphyrin into the C 16 polymer, which introduced extra energy transfer steps between the polymer and porphyrin that delayed triplet dynamics and increased the polymer triplet yield by 7.9 times. This work demonstrates two synthetic approaches to generate the longest-lived triplet excited states in narrow bandgap conjugated polymers, which is of necessity in a wide range of fields that range from organic electronics to sensors and bioapplications.

Space-Arranged Intramolecular Modular Assembled Dyad as Superior-Performance “Superoxide Anion Engine” for Boosting Photodynamic Therapy

“…Due to the deshielding effect of the An approximately arranged by the π-stacking in PS-An , the chemical shift of protons in the pyran nitrile part of PS-An has significantly shifted toward the low field compared with that of PS-Be and p-PS-An . The π-stacked conformation could largely inhibit the molecular rotation and vibration and reduce the nonradiative inactivation of the excited states. ,,, Indeed, ascribed to the rigid plane of the unit Be , PS-Be behaved enhanced fluorescence quantum yield compared with PS (Table S1). Intuitively, PS-An with a bigger π-stacked rigid plane of An should have exhibited a higher fluorescence quantum yield than that of PS-Be .…”

“…The π-stacked conformation could largely inhibit the molecular rotation and vibration and reduce the nonradiative inactivation of the excited states. 36,37,35,38 Indeed, ascribed to the rigid plane of the unit Be, PS-Be behaved enhanced fluorescence quantum yield compared with PS 38 (Table S1).…”

A narrow-bandgap photothermal material based on a donor–acceptor structure for the solar–thermal conversion application

“…The small band gap value indicated that 2TP-BBT had a relatively broad solar absorption range. Generally, the molecules are in the ground state S 0 , and when 2TP-BBT absorbs the energy of excitation light, the electrons transition from the ground state to the lowest excited state (S 1 ) or the triple excitation state (T 1 ) 56,57 (Fig. S7 and S8, ESI†).…”

Appending Coronene Diimide with Carbon Nanohoops Allows for Rapid Intersystem Crossing in Neat Film

“…Consequently, the low‐energy triplets generated through SF are challenging to harness effectively [7] . An alternative and more commonly employed method for generating triplet excitons involves ISC, wherein the triplet state is directly produced from the singlet state [8,9] . However, ISC is generally slow, occurring on ns to microsecond (μs) timescales.…”

“…[7] An alternative and more commonly employed method for generating triplet excitons involves ISC, wherein the triplet state is directly produced from the singlet state. [8,9] However, ISC is generally slow, occurring on ns to microsecond (μs) timescales. As a result, ISC-based triplet materials often resort to incorporation of heavy atoms into conjugated compounds.…”