Dispersive Relaxation Dynamics of Photoexcitations in a Polyfluorene Film Involving Energy Transfer:  Experiment and Monte Carlo Simulations

Abstract

Time-resolved fluorescence spectroscopy is used to investigate relaxation of electronic excitations in films of π-conjugated polymer 1 in the ps time domain. The position of the fluorescence band and its width are measured as a function of time and excitation energy. Both low (15 K) and room-temperature behavior are investigated. For high energy excitation, the fluorescence band shows a continuous red shift with time. The energy associated with the maximum of the fluorescence band E is proportional to log(t), with t being the time after excitation. For excitation in the tail of the lowest absorption band, the fluorescence remains stationary and selective excitation of a subset of chromophoric chain segments is possible. At intermediate excitation energy the time required for the excitations to make their first jump depends on the excitation energy and is longer at lower energy. At low temperature and high energy excitation the fluorescence bands are found to narrow with time, while for low energy excitation a broadening with time is observed. The experimental data are consistent with dispersive relaxation dynamics for the photoexcitations by incoherent hopping between localized states. Monte Carlo simulations are performed to obtain the average energy and the width of the energy distribution for an ensemble of photoexcitations in an energetically disordered molecular solid assuming Förster type energy transfer. A Förster radius R 0 ∼ 30 Å is found to give good agreement between experiment and simulations. In addition, the measurements indicate that for excitation energies >2.94 eV additional relaxation processes, ascribed to ultrafast intrachain vibrational relaxation, are operative.

Charge recombination in a poly(para-phenylene vinylene)-fullerene derivative composite film studied by transient, nonresonant, hole-burning spectroscopy

“…Curiously, at longer time delays the standard deviation reduces again. Such a time-dependent narrowing has also been observed for the relaxation of neutral photoexcitations in conjugated polymers 25 and can be understood in terms of nonequilibrium dynamics. If quasiparticles with an energy above the average of the ensemble have a faster rate of relaxation or disappearance than the particles with lower energy, such a time-dependent narrowing has to occur.…”

Making Graphene Luminescent by Oxygen Plasma Treatment

“…Then, the optical properties of the PLG would resemble those of -conjugated polymer films, where a distribution of conjugation lengths translates into a strong inhomogeneously broadened density of states. 35 At room temperature, laser irradiation in the red would lead to selective excitation of a subset of quantum confined states. Then, spectral hole burning (i.e., the selective photobleaching of this subset of homogeneously broadened lines) should be possible.…”

“…In this case, spectral diffusion due to energy migration would lead to faster decay in the blue spectral range and a delayed signal rise on the same time scale in the red. 35 The decay traces can also be modeled using a stretched-exponential function. …”

Dispositivos poliméricos eletroluminescentes

“…Essa classe de polímeros, alguns dos quais estão mostrados na Figura 2, tem despertado o interesse dos estudiosos por suas propriedades elétricas e ópticas com potencial aplicação para fabricação de dispositivos emissores de luz (LEDs) [22][23][24][25][26][27][28][29][30][31][32][33] . Estes polímeros apresentam algumas propriedades interessantes para essa finalidade destacando-se, entre elas, grande estabilidade química, térmica e oxidativa 18,[34][35][36][37][38] ; alto rendimento quântico de fluorescência tanto em solução quanto no estado sólido 37,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] ; possibilidade de formarem materiais líquido-cristalinos 46,[58][59][60][61][62][63][64]…”