J-Aggregates

The following sections are included:

• Preface

• CONTENTS

The following sections are included:

• Absorption of Isolated Molecules

• Interaction of two dye molecules in proximity
  • Sandwich dimers
  • Arbitrary arrangement of two equal dye molecules
  • Extended dipole model

• Absorption of J-aggregates
  • Absorption of aggregates of N molecules
  • The Scheibe coin-pile model
  • The brickstone model of J-aggregates
  • Purposely constructed brickstone arrangement
  • J-band shift in electric field to probe π-electron distribution

• Emission of J-aggregates
  • Life Time of Emission. Superradiance
  • Estimating N and its temperature dependence. Size and shape of coherent exciton
  • Soliton Motion through aggregate: test by incorporating acceptor as exciton trap
  • Modelling exciton motion
  • Long range energy transfer from J-aggregate to acceptor
  • Aggregates of finite size
  • Oscillating charge distribution in excition
  • J-band shape for N = 9

• Brickstone model extension
  • Spectral features
  • Exciton motion
  • Fluorescence lifetime and delocalization

• Conclusions

• References

A new model of hierarchical structure in J-aggregates, mesoaggregates and macroaggregates, is proposed on the basis of the linear and nonlinear optical properties of the oriented J-aggregates prepared by a novel vertical spin-coating method. The concentration dependence of dichroic spectra, wavelength dependence of hole-burning efficiency, and polarization dependence of electroabsorption are studied. The mesoaggregate is characterized by coherent exciton delocalized over the mesoscopic size, while the macroaggregate can be oriented by macroscopic centrifugal force of solvent flow. The macroaggregate is constituted by an incoherent ensemble of the mesoaggregates alternately stacked.

A synopsis is provided of the studies dealing with Raman scattering by aggregates that have been reported from the author's laboratory; the vast majority of which have dealt with the structure of 2,2'-cyanine. A discussion of exciton emission dynamics is also provided, followed by a report on ongoing studies regarding the structure and fluorescence decay dynamics of a meso-tetrakis(p-sulfonatophenyl)porphyrin, referred to herein as either H₂TSPP⁻⁴ or TSPP, which aggregates in highly acidic homogeneous solution. The fluorescence lifetime of the aggregate is found to depend on whether electronic excitation is to the B (Soret) state or the Q-state. This finding is interpreted as indicating that the aggregate's effective size differs in the two cases. Specific structural changes which occur within the porphyrin upon N-protonation and incorporation within the aggregate are also discussed. Additionally, vibrational frequencies are assigned to specific bonds of the porphinato macrocycle, and comparisons are made between bond lengths in the aggregate environment to those of the monomeric dianion.

The following sections are included:

• Introduction

• The aggregate Hamiltonian

• One-exciton states in a perfect aggregate

• Exciton-phonon coupling

• Solvent fluctuations

• Electronic excitations beyond the simple exciton picture

• References

The following sections are included:

• Introduction

• General theory of the pump-probe spectrum
  • Relation of the pump-probe spectrum to optical response functions
  • Sum-over-states expressions for the differential absorption spectrum

• Low-intensity pump-probe spectra of molecular aggregates
  • Chains of two-level molecules with excitation transfer
    • Model
    • Eigenstate analysis
    • Homogeneous chains, segment model, and motional narrowing theory
    • Two-color differential absorption spectrum for general disorder
    • Short-pulse pump-probe spectrum in the segment model
    • Long-pulse pump-probe spectrum in the segment model
    • Exciton coherence length versus saturation length
  • Two-level molecules with exciton-exciton interactions
    • Model Hamiltonian and eigenstates
    • Short-pulse pump-probe spectrum in the presence of exciton-exciton interactions
    • Comparison to experiment
    • Long-pulse pump-probe spectrum in the presence of exciton-exciton interactions
  • Chains of three-level molecules
    • Hamiltonian and level structure
    • Short-pulse pump-probe spectrum for aggregates of three-level molecules
    • Two-color pump-probe spectrum for aggregates of three-level molecules
    • Discussion of the pump-probe spectra for aggregates of three-level molecules

• Strong-field pump-probe spectrum of J-aggregates
  • Strong-field pump-probe spectrum for a model aggregate
  • Comparison to experiment

• Concluding remarks

• Acknowledgments

• References

The femtosecond dynamics of one-dimensional excitons in J-aggregates is studied. Time-resolved difference absorption spectrum of J-aggregates was measured by femtosecond pump-probe spectroscopy. The induced absorption near J-band at 20 K was assigned to the transitions from n-exciton states to (n+1)-exciton states (n ≥ 1) with most contribution from n=2 exciton state. The decay time of n(≥ 2)-exciton states and the one-exciton state are determined to be about 200 fs and 1.5 ps, respectively. The dynamics of the one-exciton state is determined by the exciton-exciton annihilation among mesoaggregates, which incoherently constitute a whole macroaggregate. The average size of the exciton delocalization in each mesoaggregate is measured to be about 23 molecules. Inverse Raman process under the sub-resonant excitation is also observed in Langmuir-Blodgett (LB) films of J-aggregates. The time-resolved inverse Raman spectrum is calculated analytically using a simplified phenomenological three-level model of a Frenkel exciton in J-aggregates accompanied by a vibrational mode.

The following sections are included:

• Introduction

• Nonlinear optical dynamics in J-aggregates driven by strong light pulses
  • Materials and methods
  • Resonance DFWM
    • Mixing efficiency and intensity dependence of the signal
    • Intensity dependence of temporal response
    • Pump-and- probe data
    • Discussion

• Nonlinear susceptibility

• Conclusion

• Acknowledgments

• References

The following sections are included:

• Introduction

• The physical size of J-aggregates from exciton annihilation

• Phthalocyanines

• Photosynthetic systems

• Discussion

• Acknowledgements

• References

The following sections are included:

• Introduction

• Sensitizing Dyes and Their Aggregates

• Adsorption of Sensitizing Dyes to Silver Halide Grains

• Electronic Energy Levels of Sensitizing Dyes Adsorbed on Silver Halides

• Quantum Yield of Light–Induced Electron Transfer

• Kinetics of Light–Induced Electron Transfer

• Back Reaction of Light–Induced Electron Transfer

• References