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UV–Vis Spectroscopy of Conjugated Systems01:32

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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
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Exciton Absorption Spectra by Linear Response Methods: Application to Conjugated Polymers.

Martín A Mosquera1, Nicholas E Jackson1,2, Thomas J Fauvell1

  • 1Department of Chemistry and the Materials Research Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.

Journal of the American Chemical Society
|February 23, 2017
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Summary

This study presents an efficient two-step method for calculating exciton spectra in conjugated polymers. The new approach accurately predicts near-infrared absorption, aiding in understanding material properties and guiding future spectroscopy developments.

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Area of Science:

  • Computational chemistry
  • Spectroscopy
  • Materials science

Background:

  • Calculating exciton spectra is crucial for understanding excited-state dynamics in materials.
  • Conventional methods face challenges with computational cost and accuracy using common density functionals.
  • Previous work introduced a two-step calculation to address these limitations.

Purpose of the Study:

  • To apply a novel two-step theoretical method for calculating near-infrared absorption spectra of excitons.
  • To investigate exciton behavior in oligomers of poly(3-hexylthiophene) (P3HT), poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), and poly(benzodithiophene-thieno[3,4-b]thiophene) (PTB7).
  • To identify dominant orbital excitations and establish a rule for predicting longest-wavelength absorption peaks.

Main Methods:

  • A two-step calculation involving two linear-response time-dependent density functional theory (TDDFT) steps.
  • The first TDDFT step generates orbitals perturbed by the excitonic state.
  • The second TDDFT step calculates the excitation spectrum relative to the excitonic state.

Main Results:

  • Calculated spectra for P3HT and MEH-PPV oligomers show convergence for 10 monomer units, aligning with experimental data.
  • Exciton spectral features in MEH-PPV were found to overlap with bipolaron formation.
  • Exciton absorption bands in PTB7 oligomers were identified in transient absorption spectra.
  • Dominant orbital excitations contributing to optically active transitions were reported for all studied polymers.

Conclusions:

  • The developed methodology accurately predicts near-infrared absorption spectra of excitons in conjugated polymers.
  • The findings provide insights into exciton-bipolaron interactions and validate experimental transient absorption spectra.
  • The method offers a foundation for advanced theoretical transient spectroscopy, including nonadiabatic effects and charge-transfer states.