skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Ultrafast Relaxation Dynamics of Photoexcited Zinc-Porphyrin: Electronic-Vibrational Coupling

Abstract

Cyclic tetrapyrroles are the active core of compounds with crucial roles in living systems, such as hemoglobin and chlorophyll, and in technology as photocatalysts and light absorbers for solar energy conversion. Zinc-tetraphenylporphyrin (Zn-TPP) is a prototypical cyclic tetrapyrrole that has been intensely studied in past decades. Because of its importance for photochemical processes the optical properties are of particular interest, and, accordingly, numerous studies have focused on light absorption and excited-state dynamics of Zn-TPP. Relaxation after photoexcitation in the Soret band involves internal conversion that is preceded by an ultrafast process. This relaxation process has been observed by several groups. Until now, it has not been established if it involves a higher lying ”dark” state or vibrational relaxation in the excited S 2 state. Here we combine high time resolution electronic and vibrational spectroscopy to show that this process constitutes vibrational relaxation in the anharmonic 2 potential.

Authors:
 [1];  [2];  [1]
  1. Univ. of Delaware, Newark, DE (United States). Dept. of Chemistry and Biochemistry
  2. Univ. of Delaware, Newark, DE (United States). Dept. of Physics and Astronomy
Publication Date:
Research Org.:
Univ. of Delaware, Newark, DE (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1345807
Grant/Contract Number:
SC0016288
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Physical Chemistry Letters
Additional Journal Information:
Journal Volume: 7; Journal Issue: 16; Journal ID: ISSN 1948-7185
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 14 SOLAR ENERGY

Citation Formats

Abraham, Baxter, Nieto-Pescador, Jesus, and Gundlach, Lars. Ultrafast Relaxation Dynamics of Photoexcited Zinc-Porphyrin: Electronic-Vibrational Coupling. United States: N. p., 2016. Web. doi:10.1021/acs.jpclett.6b01439.
Abraham, Baxter, Nieto-Pescador, Jesus, & Gundlach, Lars. Ultrafast Relaxation Dynamics of Photoexcited Zinc-Porphyrin: Electronic-Vibrational Coupling. United States. doi:10.1021/acs.jpclett.6b01439.
Abraham, Baxter, Nieto-Pescador, Jesus, and Gundlach, Lars. Tue . "Ultrafast Relaxation Dynamics of Photoexcited Zinc-Porphyrin: Electronic-Vibrational Coupling". United States. doi:10.1021/acs.jpclett.6b01439. https://www.osti.gov/servlets/purl/1345807.
@article{osti_1345807,
title = {Ultrafast Relaxation Dynamics of Photoexcited Zinc-Porphyrin: Electronic-Vibrational Coupling},
author = {Abraham, Baxter and Nieto-Pescador, Jesus and Gundlach, Lars},
abstractNote = {Cyclic tetrapyrroles are the active core of compounds with crucial roles in living systems, such as hemoglobin and chlorophyll, and in technology as photocatalysts and light absorbers for solar energy conversion. Zinc-tetraphenylporphyrin (Zn-TPP) is a prototypical cyclic tetrapyrrole that has been intensely studied in past decades. Because of its importance for photochemical processes the optical properties are of particular interest, and, accordingly, numerous studies have focused on light absorption and excited-state dynamics of Zn-TPP. Relaxation after photoexcitation in the Soret band involves internal conversion that is preceded by an ultrafast process. This relaxation process has been observed by several groups. Until now, it has not been established if it involves a higher lying ”dark” state or vibrational relaxation in the excited S2 state. Here we combine high time resolution electronic and vibrational spectroscopy to show that this process constitutes vibrational relaxation in the anharmonic 2 potential.},
doi = {10.1021/acs.jpclett.6b01439},
journal = {Journal of Physical Chemistry Letters},
number = 16,
volume = 7,
place = {United States},
year = {Tue Aug 02 00:00:00 EDT 2016},
month = {Tue Aug 02 00:00:00 EDT 2016}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record

Citation Metrics:
Cited by: 4works
Citation information provided by
Web of Science

Save / Share:
  • The ground- and excited-state optical spectra of two U[sup IV] bis(porphyrin) sandwich complexes reveal features characteristic of porphyrins held within van der Waals contact. The photoexcited complexes exhibit ultrafast (approximately 1 ps) deactivation to the ground electronic state in a multistep process involving intersystem crossing into the triplet manifold and subsequent relaxation via ligand-field (f,f) excited state(s). This ultrafast and highly energetic electronic deactivation is followed by complex time-dependent spectral changes on the picosecond time scale that are ascribed to vibrational relaxation in the ground electronic state. 22 refs., 5 figs.
  • We study, by means of a Monte Carlo simulator, the hot phonon effect on the relaxation dynamics in photoexcited graphene and its quantitative impact as compared with considering an equilibrium phonon distribution. Our multi-particle approach indicates that neglecting the hot phonon effect significantly underestimates the relaxation times in photoexcited graphene. The hot phonon effect is more important for a higher energy of the excitation pulse and photocarrier densities between 1 and 3 × 10{sup 12 }cm{sup −2}. Acoustic intervalley phonons play a non-negligible role, and emitted phonons with wavelengths limited up by a maximum (determined by the carrier concentration) induce a slower carriermore » cooling rate. Intrinsic phonon heating is damped in graphene on a substrate due to the additional cooling pathways, with the hot phonon effect showing a strong inverse dependence with the carrier density.« less
  • We perform time- and angle-resolved photoemission spectroscopy (trARPES) on optimally doped Bi 2Sr 2CaCu 2O 8+δ (BSCCO-2212) using sufficient energy resolution (9 meV) to resolve the k-dependent near-nodal gap structure on time scales where the concept of an electronic pseudotemperature is a useful quantity, i.e., after electronic thermalization has occurred. We study the ultrafast evolution of this gap structure, uncovering a very rich landscape of decay rates as a function of angle, temperature, and energy. We explicitly focus on the quasiparticle states at the gap edge as well as on the spectral weight inside the gap that “fills” the gap—understoodmore » as an interaction, or self-energy effect—and we also make high resolution measurements of the nodal states, enabling a direct and accurate measurement of the electronic temperature (or pseudotemperature) of the electrons in the system. Rather than the standard method of interpreting these results using individual quasiparticle scattering rates that vary significantly as a function of angle, temperature, and energy, we show that the entire landscape of relaxations can be understood by modeling the system as following a nonequilibrium, electronic pseudotemperature that controls all electrons in the zone. Furthermore, this model has zero free parameters, as we obtain the crucial information of the SC gap Δ and the gap-filling strength Γ TDoS by connecting to static ARPES measurements. The quantitative and qualitative agreement between data and model suggests that the critical parameters and interactions of the system, including the pairing interactions, follow parametrically from the electronic pseudotemperature. In conclusion, we expect that this concept will be relevant for understanding the ultrafast response of a great variety of electronic materials, even though the electronic pseudotemperature may not be directly measurable.« less
  • We perform time- and angle-resolved photoemission spectroscopy (trARPES) on optimally doped Bi 2Sr 2CaCu 2O 8+δ (BSCCO-2212) using sufficient energy resolution (9 meV) to resolve the k-dependent near-nodal gap structure on time scales where the concept of an electronic pseudotemperature is a useful quantity, i.e., after electronic thermalization has occurred. We study the ultrafast evolution of this gap structure, uncovering a very rich landscape of decay rates as a function of angle, temperature, and energy. We explicitly focus on the quasiparticle states at the gap edge as well as on the spectral weight inside the gap that “fills” the gap—understoodmore » as an interaction, or self-energy effect—and we also make high resolution measurements of the nodal states, enabling a direct and accurate measurement of the electronic temperature (or pseudotemperature) of the electrons in the system. Rather than the standard method of interpreting these results using individual quasiparticle scattering rates that vary significantly as a function of angle, temperature, and energy, we show that the entire landscape of relaxations can be understood by modeling the system as following a nonequilibrium, electronic pseudotemperature that controls all electrons in the zone. Furthermore, this model has zero free parameters, as we obtain the crucial information of the SC gap Δ and the gap-filling strength Γ TDoS by connecting to static ARPES measurements. The quantitative and qualitative agreement between data and model suggests that the critical parameters and interactions of the system, including the pairing interactions, follow parametrically from the electronic pseudotemperature. In conclusion, we expect that this concept will be relevant for understanding the ultrafast response of a great variety of electronic materials, even though the electronic pseudotemperature may not be directly measurable.« less
  • A variety of porphyrin arrays connected together with different linkage were devised for possible applications to molecular optoelectronic devices such as wires, logic gates, and artificial light-harvesting arrays, etc. It has been relatively well established that the light signal transmission in these molecular assemblies is based on exciton migration process, which possibly gives rise to the structural changes during the exciton delocalization process. Zinc(II) 5,15-di(3,5-di-tert-butylphenyl)porphyrin (Z1), its directly meso,meso-linked porphyrin dimer (Z2), trimer (Z3), and tetramer (Z4) were synthesized with the goal to elucidate the relationship between exciton migration and structural change upon photoexcitation. One of the most important factorsmore » in structural changes for these porphyrin arrays is mainly determined by the dihedral angle between adjacent porphyrin moieties. For a systematic approach toward the investigation of the exciton coupling dynamics influenced by the relative orientation between neighboring porphyrin molecules, various time-resolved spectroscopic techniques such as fluorescence decay and transient absorption measurements with different polarization in pump/probe beams have been utilized. The steady-state excitation anisotropy spectra of Z2, Z3, and Z4 porphyrin arrays show that the photoexcitation of the high-energy exciton Soret band induces a large angle change between absorption and emission dipoles in contrast with the photoexcitation of the low-energy exciton split Soret and Q-bands. In the order of Z1, Z2, Z3, and Z4, their S{sub 1} states decay faster because of the increasing energy dissipation processes into a larger number of accessible states. In contrast, the rotational diffusion rates become slower in the same order because the overall molecular shape is elongated along the long axis of the molecular arrays, which experiences a large displacement of solvent molecules in rotational diffusion motion. Ultrafast fluorescence decay measurements show that the S{sub 2} {r_arrow} S{sub 1} internal conversion process occurs in less than 1 ps in Z2, Z3, and Z4 due to the existence of exciton split band as a ladder-type deactivation channel, while this process is relatively slow in Z1 ({approximately}1.6 ps). Femtosecond transient absorption experiments with magic angle and different polarization in probe beam were performed to find the relationship between energy relaxation and anisotropy dynamics upon photoexcitation. The internal conversion in Z2, Z3, and Z4 is likely to be accompanied by the incoherent energy hopping processes occurring in less than {approximately}200 fs judging from a large change in the anisotropy value in the transient absorption decay. In addition, the decay components with approximately 8 ps time constant were observed in both fluorescence up-conversion and femtosecond transient absorption decays. These components are believed to arise from the conformational change in the excited states, because the dihedral angle distribution in these arrays was estimated to be 90{degree} {+-} 20{degree} at ambient temperature from the AM1 calculation.« less