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An insight into non-emissive excited states in conjugated polymers.

Hu Z, Willard AP, Ono RJ, Bielawski CW, Rossky PJ, Vanden Bout DA - Nat Commun (2015)

Bottom Line: Conjugated polymers in the solid state usually exhibit low fluorescence quantum yields, which limit their applications in many areas such as light-emitting diodes.In relatively polar solvents (ɛ>∼ 3), the aggregates exhibit a low fluorescence quantum yield (QY) of 2-5%, similar to bulk films, however, in relatively nonpolar solvents (ɛ<∼ 3) they demonstrate much higher fluorescence QY up to 20-30%.Fluorescence lifetime measurement reveals that the CT-type states exist as a competitive channel of the formation of emissive exciton-type states.

View Article: PubMed Central - PubMed

Affiliation: Center for Nano- and Molecular Science and Technology, Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, USA.

ABSTRACT
Conjugated polymers in the solid state usually exhibit low fluorescence quantum yields, which limit their applications in many areas such as light-emitting diodes. Despite considerable research efforts, the underlying mechanism still remains controversial and elusive. Here, the nature and properties of excited states in the archetypal polythiophene are investigated via aggregates suspended in solvents with different dielectric constants (ɛ). In relatively polar solvents (ɛ>∼ 3), the aggregates exhibit a low fluorescence quantum yield (QY) of 2-5%, similar to bulk films, however, in relatively nonpolar solvents (ɛ<∼ 3) they demonstrate much higher fluorescence QY up to 20-30%. A series of mixed quantum-classical atomistic simulations illustrate that dielectric induced stabilization of nonradiative charge-transfer (CT) type states can lead to similar drastic reduction in fluorescence QY as seen experimentally. Fluorescence lifetime measurement reveals that the CT-type states exist as a competitive channel of the formation of emissive exciton-type states.

No MeSH data available.


Absorption spectra.(a) Normalized absorption spectra of triblock in different mixtures of toluene and poor solvents (normalized to the maximum absorbance). The volume percent of poor solvent in each mixture are shown in parentheses. (b) Extracted absorption spectra of pure aggregates for each solvent mixture (normalized to the 0-1 transition peak). The absorption spectra of triblock molecular solution in toluene are included in (a) and (b) for comparison.
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f4: Absorption spectra.(a) Normalized absorption spectra of triblock in different mixtures of toluene and poor solvents (normalized to the maximum absorbance). The volume percent of poor solvent in each mixture are shown in parentheses. (b) Extracted absorption spectra of pure aggregates for each solvent mixture (normalized to the 0-1 transition peak). The absorption spectra of triblock molecular solution in toluene are included in (a) and (b) for comparison.

Mentions: Similar to what we observed previously, with increasing amount of poor solvent in toluene/poor solvent mixtures, red shifted spectra with an emergence and gradual intensity increase of vibronic structures at 560 and 610 nm due to interchain interaction were observed24. Eventually, the spectral change saturates at a certain volume ratio of toluene/poor solvent (Table 1) at which point there is a maximum fraction of aggregates contributing to the absorption spectrum. The final absorption spectra of triblock in different solvent mixtures are shown in Fig. 4a with volume percentage of poor solvents in the parentheses. The triblock absorption spectra in the solvent mixtures, however, contain contributions from both aggregated and molecular triblock chains1314. To extract pure spectra of aggregates, the molecular solution spectrum of triblock in toluene was scaled to the low wavelength shoulder of the spectra in solvent mixtures first and then was subtracted14. The extracted spectra of aggregates in different solvent media, as shown in Fig. 4b, are similar with slight differences in the relative intensity of vibronic transitions. A close examination reveals that the 0-0 electronic transition gradually increases with decreasing dielectric constant from toluene/acetonitrile to toluene/t-butanol, while it is similar for all of the low dielectric solvent mixtures except for toluene/dioxane. Within the framework of weak interchain coupling in the H-aggregate model, the interchain excitonic coupling (J0) can be estimated from the ratio of peak absorbance of A0-0 and A0-1, for which increased ratio implies reduced interchain coupling (Supplementary Fig. 5)25. For excitonic coupling, the screening factor by solvent medium is a function of optical dielectric constant (ɛopt), which is equivalent to the square of refraction index (n)2627. Since the n values of the solvent mixtures studied herein are very close, i.e., in the range of 1.37–1.45, the change in interchain coupling caused by the variation of solvents is anticipated to be small. We believe that the slight difference in interchain excitonic coupling for the aggregates studied in different solvent mixtures is mostly due to a subtle change in packing morphology of polymer chains. In addition, the well dissolved side-chains in nonpolar solvent environments would also be expected to reduce torsional disorder of P3HT backbone, therefore benefitting long conjugation length and high ordering along P3HT chain. The increased intrachain order, in turn, can also lead to a decrease in interchain excitonic coupling28293031.


An insight into non-emissive excited states in conjugated polymers.

Hu Z, Willard AP, Ono RJ, Bielawski CW, Rossky PJ, Vanden Bout DA - Nat Commun (2015)

Absorption spectra.(a) Normalized absorption spectra of triblock in different mixtures of toluene and poor solvents (normalized to the maximum absorbance). The volume percent of poor solvent in each mixture are shown in parentheses. (b) Extracted absorption spectra of pure aggregates for each solvent mixture (normalized to the 0-1 transition peak). The absorption spectra of triblock molecular solution in toluene are included in (a) and (b) for comparison.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4595598&req=5

f4: Absorption spectra.(a) Normalized absorption spectra of triblock in different mixtures of toluene and poor solvents (normalized to the maximum absorbance). The volume percent of poor solvent in each mixture are shown in parentheses. (b) Extracted absorption spectra of pure aggregates for each solvent mixture (normalized to the 0-1 transition peak). The absorption spectra of triblock molecular solution in toluene are included in (a) and (b) for comparison.
Mentions: Similar to what we observed previously, with increasing amount of poor solvent in toluene/poor solvent mixtures, red shifted spectra with an emergence and gradual intensity increase of vibronic structures at 560 and 610 nm due to interchain interaction were observed24. Eventually, the spectral change saturates at a certain volume ratio of toluene/poor solvent (Table 1) at which point there is a maximum fraction of aggregates contributing to the absorption spectrum. The final absorption spectra of triblock in different solvent mixtures are shown in Fig. 4a with volume percentage of poor solvents in the parentheses. The triblock absorption spectra in the solvent mixtures, however, contain contributions from both aggregated and molecular triblock chains1314. To extract pure spectra of aggregates, the molecular solution spectrum of triblock in toluene was scaled to the low wavelength shoulder of the spectra in solvent mixtures first and then was subtracted14. The extracted spectra of aggregates in different solvent media, as shown in Fig. 4b, are similar with slight differences in the relative intensity of vibronic transitions. A close examination reveals that the 0-0 electronic transition gradually increases with decreasing dielectric constant from toluene/acetonitrile to toluene/t-butanol, while it is similar for all of the low dielectric solvent mixtures except for toluene/dioxane. Within the framework of weak interchain coupling in the H-aggregate model, the interchain excitonic coupling (J0) can be estimated from the ratio of peak absorbance of A0-0 and A0-1, for which increased ratio implies reduced interchain coupling (Supplementary Fig. 5)25. For excitonic coupling, the screening factor by solvent medium is a function of optical dielectric constant (ɛopt), which is equivalent to the square of refraction index (n)2627. Since the n values of the solvent mixtures studied herein are very close, i.e., in the range of 1.37–1.45, the change in interchain coupling caused by the variation of solvents is anticipated to be small. We believe that the slight difference in interchain excitonic coupling for the aggregates studied in different solvent mixtures is mostly due to a subtle change in packing morphology of polymer chains. In addition, the well dissolved side-chains in nonpolar solvent environments would also be expected to reduce torsional disorder of P3HT backbone, therefore benefitting long conjugation length and high ordering along P3HT chain. The increased intrachain order, in turn, can also lead to a decrease in interchain excitonic coupling28293031.

Bottom Line: Conjugated polymers in the solid state usually exhibit low fluorescence quantum yields, which limit their applications in many areas such as light-emitting diodes.In relatively polar solvents (ɛ>∼ 3), the aggregates exhibit a low fluorescence quantum yield (QY) of 2-5%, similar to bulk films, however, in relatively nonpolar solvents (ɛ<∼ 3) they demonstrate much higher fluorescence QY up to 20-30%.Fluorescence lifetime measurement reveals that the CT-type states exist as a competitive channel of the formation of emissive exciton-type states.

View Article: PubMed Central - PubMed

Affiliation: Center for Nano- and Molecular Science and Technology, Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, USA.

ABSTRACT
Conjugated polymers in the solid state usually exhibit low fluorescence quantum yields, which limit their applications in many areas such as light-emitting diodes. Despite considerable research efforts, the underlying mechanism still remains controversial and elusive. Here, the nature and properties of excited states in the archetypal polythiophene are investigated via aggregates suspended in solvents with different dielectric constants (ɛ). In relatively polar solvents (ɛ>∼ 3), the aggregates exhibit a low fluorescence quantum yield (QY) of 2-5%, similar to bulk films, however, in relatively nonpolar solvents (ɛ<∼ 3) they demonstrate much higher fluorescence QY up to 20-30%. A series of mixed quantum-classical atomistic simulations illustrate that dielectric induced stabilization of nonradiative charge-transfer (CT) type states can lead to similar drastic reduction in fluorescence QY as seen experimentally. Fluorescence lifetime measurement reveals that the CT-type states exist as a competitive channel of the formation of emissive exciton-type states.

No MeSH data available.