594 research outputs found
Photoinduced Radical Cations Enable Anti‐Kasha Emission in a Pyrene‐Based Azacationic Ladder Polymer
No full textConjugated ladder polymers are a unique class of macromolecules, characterized by their rigid and thermally stable structures. This work presents the synthesis and characterization of a pyrene‐based azacationic ladder polymer (polymer A). Spectroscopic analysis points to the generation of radical cationic units during photoreduction, while cationic species are retained in the polymer backbone, thereby enabling in situ n‐type doping. Unconventional anti‐Kasha emission, with a maximum at 490 nm, appears to originate from these radical species in solution. In toluene, the lower dipole moment of the solvent leads to dual emission: anti‐Kasha emission from radical cations and an S1 → S0 transition from polycationic units at 780 nm. This interpretation is supported by density functional theory/time‐dependent density‐functional theory calculations, which indicates that the large energy gap between the D3 and D2 states of the radical cationic units may inhibit internal conversion, allowing anti‐Kasha behavior. Despite their potential reactivity, the cationic and radical cationic species remain stable in solution in the dark for over 110 h. To the authors’ best knowledge, polymer A is the only ladder‐type conjugated polymer reported to exhibit anti‐Kasha emission together with light‐induced n‐type doping behavior
Unusual Non-Kasha Photophysical Behavior of Aggregates of Push–Pull Donor–Acceptor Chromophores
No full textUnlike the Frenkel exciton model,
the essential state model (ESM)
for aggregates of donor–acceptor (DA) chromophores accounts
self-consistently for the effect of the reaction field on the ground
and excited states of the individual chromophores, an effect that
can lead to divergences from Kasha’s H- and J-aggregate classification
schemes for highly polarizable molecules. In this work, two slip-stack
dimer geometries of DA chromophores are considered, one with parallel
permanent dipole moments (Geometry I) and the other with antiparallel
dipole moments (Geometry II). Based on the ESM, it is shown that in
both GI and GII dimers the aggregation-induced spectral shifts characteristic
of H- and J-aggregates agree exactly with the Kasha/Frenkel model,
but only in the cyanine limit, where both the ground and excited states
of an isolated chromophore consist of equal admixtures of the neutral
and charge-separated diabatic states D0A0 and
D+A–. The agreement is also limited to
the perturbation regime where intermolecular interactions are relatively
weak. However, in the polyene limit, where the ground state of the
monomer is dominated by the D0A0 component,
the agreement with the Kasha/Frenkel model breaks down. In this limit,
GI dimers exhibit exaggerated spectral shifts, as much as twice as
large as the Kasha shifts, while GII aggregates exhibit shifts which
can be much smaller than the Kasha shifts. Hence, in the latter case,
one can obtain practically unshifted H- and J-aggregates. When vibronic
coupling is incorporated into the ESM via a Holstein-like Hamiltonian,
it is further revealed that the vibronic signatures derived from the
Frenkel exciton theory carry over to the ESM in the weak coupling
regime, providing an unambiguous means for determining H- and J-aggregation.
In the strong coupling limit, the divergences from the Kasha model
are even more pronounced with the emergence of strongly blue-shifted
J-aggregates with enhanced vibronic coupling. Moreover, when the intermolecular
coupling V becomes equal to −2η, where
η is the monomer’s diabatic excitation energy (with η
> 0), the dimer behaves as if no vibronic coupling were present
at
all. In other words, the strong vibronic progressions characteristic
of a monomer in the polyene limit vanish when V =
−2η. Applications are made to a merocyanine dimer, which
abides by the GII configuration
(Specially Arranged Invited Paper) THE COMPETITION BETWEEN BIPROTONIC PHOTOTAUTOMERISM, EXIMER FORMATION, AND PROTON TUNNELING IN DNA BASE-PAIRS
No full textAuthor Institution: Institute of Molecular Biophysics, Florida State UniversityThe competition between the rapid excitation phenomena proton-tunneling, molecular excimer formation, and biprotonic phototautomerism is discussed with special reference to the radiation chemistry and photochemistry of DNA and its component bases. Watson and Crick in their earliest consideration pointed out the possible genetic effect of tautomerization of the bases in the primary structure of DNA. Recent discussions by Lowdin, followed by electronic structure calculations on GC base pairs by Rein and Harris, Rein and Ladik, et al., have emphasized proton-tunneling as a quantum-mechanical mechanism involved in tautomerization. In contrast, studies of luminescence of DNA and polynucleotide analogs have suggested excimer formation as a preliminary to photochemical change. New experimental studies in our laboratory, carried out by C. A. Taylor, M. Ashraf El-Bayoumi and M. Kasha, have established the existence of a new phenomenon in N-heterocyclic base-pairs under excitation. It was shown that a cooperative transfer of two protons involved in hydrogen-bonding of the base-pair occurs in 7-azaindole as a model compound. Tautomenzation of the bases simultaneously occurred. Excimer formation was excluded. A temperature-dependence study in a continuation of the research by El-Bayoumi excludes proton-tunneling as a significant phenomenon in this case. The existence of four typical potential models covering possible tautomerization cases in natural base-pairs is discussed in relation to an extension of these results to the DNA photomutation problem. Theoretical calculations recently carried out by Horowitz, Evleth, and Kasha (Santa Cruz) on the variation of excited state energies of the normal and tautomerized N-perturbed indoles from 7-azaindole to purine to adenine support the biprotonic phototautomerization as a relevant excitation step in DNA base-pair excited state chemistry. Recent ab initio theoretical work by Clementi, et al., confirms the double-minimum potential model for cooperative transfer of two protons in doubly-hydrogen bonded systems, as envisaged in the biprotonic phototautomerism
Prospects for Gene lntrogression from Hordeum bulbosum L. into Barley (H. vulgare L.).
No full textHybridizations between Hordeum vulgare L. (cultivated barley) and H. bulbosum L. have been performed over many years with two aims. First, the production of doubled haploid barley cultivars (Kasha and Kao 1970); second, the transfer of desirable traits such as resistance to fungal and viral pathogens from the wild species into barley. Apart from the report of an occasional recombinant (Xu and Kasha 1992; Pickering et al 1994), successful gene introgression has been hampered by several barriers. In this report we will describe recent progress in overcoming these barriers
Intermolecular Charge Transfer in H- and J‑Aggregates of Donor–Acceptor–Donor Chromophores: The Curious Case of Bithiophene-DPP
No full textA vibronic exciton model is developed to describe low-energy
electronic
excitations in slip-stack aggregates of donor–acceptor–donor
(DAD) chromophores with substantial overlap between the neighboring
donor and acceptor fragments. In such stacks, J- and H-aggregate behavior
is driven by intermolecular charge transfer (ICT) and not Coulomb
coupling as is assumed in the Kasha model. In-phase (out-of-phase)
intermolecular charge transfer integrals result in J-aggregate (H-aggregate)
behavior, as unambiguously determined by the vibronic spectral signatures.
Interestingly, both J- and H-aggregates are red-shifted, in contrast
to the predictions of the Kasha model. Simulated spectral line shapes
agree well with recent experiments on two derivatives of the bithiophene
diketopyrrolopyrrole chromophore, 2T-DPP-2T, with different terminal
groups
THEORY OF MOLECULAR LUMINESCENCE
No full textAuthor Institution: Institute of Molecular Biophysics, The Florida State Universit
Classification of Electronic Levels in Complex Molecules
No full textAuthor Institution: Ryerson Physical Laboratory, University of ChicagoPresentations without an abstract printed in the proceedings do not have an abstract (image or text) in the Knowledge Bank record
Intercombinations in Molecular Electronic Spectra
No full textAuthor Institution: Florida State UniversityPresentations without an abstract printed in the proceedings do not have an abstract (image or text) in the Knowledge Bank record
British Columbia's Indigenous Territories by Stage of Treaty Negotiation
No full textIntegrated Environmental PlanningIndigenous groups in BC are at various stages of negotiation with the provincial government to establish treaty agreements. There is a significant amount of overlap between the traditional territories of many of theses nations and treaty groups. One objective of this project was to create a web map that would help examine these areas. The project also aimed to identify if an how geographic location is affecting or has affected the treaty negotiations of Indigenous groups located in British Columbia. Some Indigenous groups have withdrawn from the treaty process, and a visualization of these groups was created to help to determine why this is happening
- …
