224 research outputs found
Experimental Evidence of Direct Exchange Interaction Mediating Intramolecular Singlet Fission in Weakly-Coupled Dimers
The electronic interaction between an optically active singlet state
() and a dark state of singlet multiplicity, known as correlated
triplet pair (), plays a crucial role in the effective transformation
from to during intramolecular singlet fission (iSF). This
process is understood through mechanisms such as direct exchange coupling and
incoherent processes that involve super-exchange coupling through
charge-transfer states. However, most insights into these mechanisms are
derived from theoretical studies due to the difficulties in obtaining
experimental evidence. In this study, we investigate the excited-state
interactions between and in spiro-conjugated iSF sensitizers
by employing transient two-dimensional electronic spectroscopy. This approach
allows us to focus on the early stages of the conversion from to
. Upon optical excitation, a superposition of and is
created, which gradually transitions to favor within the
characteristic time frames of iSF. The observed high-order signals indicate
circular repopulation dynamic that effectively reinitiates the iSF process from
higher energy electronic states. Our findings, supported by
semi-quantum-mechanical simulations of the experimental data, suggest the
presence of a direct iSF mechanism in the dimers, facilitated by weak
non-adiabatic coupling between and . This experiment provides
new insights into the equilibrium between the two electronic states, a
phenomenon previously understood primarily through theoretical models.Comment: 26 pages, 4 Figure
Karyotypic conservatism in samples of Characidium cf. zebra (Teleostei, Characiformes, Crenuchidae): Physical mapping of ribosomal genes and natural triploidy
Basic and molecular cytogenetic analyses were performed in specimens of Characidium cf. zebra from five collection sites located throughout the Tietê, Paranapanema and Paraguay river basins. The diploid number in specimens from all samples was 2n = 50 with a karyotype composed of 32 metacentric and 18 submetacentric chromosomes in both males and females. Constitutive heterochromatin was present at the centromeric regions of all chromosomes and pair 23, had additional interstitial heterochromatic blocks on its long arms. The nucleolar organizer regions (NORs) were located on the long arms of pair 23, while the 5S rDNA sites were detected in different chromosomes among the studied samples. One specimen from the Alambari river was a natural triploid and had two extra chromosomes, resulting in 2n = 77. The remarkable karyotypic similarity among the specimens of C. cf. zebra suggests a close evolutionary relationship. On the other hand, the distinct patterns of 5S rDNA distribution may be the result of gene flow constraints during their evolutionary history
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