A simulation-led strategy enacts two-electron transfer between an intramolecular singlet fission chromophore (tetracyanomethylene quinoidal bithiopehene with β,β'-solubilizing groups) and multielectron acceptor (anthraquinone). The thermodynamic plausibility of multielectron transfer from a double-triplet state and the absorption spectra of electron transfer (ET) products were predicted using quantum chemical simulations. These predictions are consistent with experimental observations of reduced lifetimes in time-resolved fluorescence spectroscopy, changes in transmission profile, and appearance of new absorption bands in transient absorption spectroscopy, all of which support multi-ET in the QOT2/AQ mixture. The analysis suggests 2ET is favored over 1ET by a 2.5:1 ratio.
A new
series of donor–acceptor ladder-type molecules were
synthesized via Scholl reaction. These molecules contain up to 25-fused
rings but still show good air stability and good solubility. The ring-fusing
reaction is found to be sensitive to the nature of the side-chain
in donor units. The molecular conformations were investigated by 2D
NMR and DFT calculations. The photophysical properties were investigated
and an intense intramolecular charge-transfer was observed. All of
the molecules exhibit two-photon absorption (TPA) activity, and their
TPA cross-section shows a linear relationship with increasing conjugation
length of the thienoacene-PDI derivatives.
New
light harvesting organic conjugated polymers containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b;3,4-b′]dithiophene(BDT) donor
groups and thiophene with various electron-withdrawing acceptor groups
were investigated. Also investigated was poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), which is one of the most efficient
photovoltaic conjugated polymers. In this study, the steady state
absorption, steady state emission, ultrafast fluorescent decay dynamics,
and nonlinear optical properties of these light harvesting conjugated
polymers were probed in solution. All of the conjugated polymers investigated
have significant absorption over much of the visible spectrum due
to small band gaps due to low lying LUMO energies created by the electron-withdrawing
groups. Fluorescence upconversion studies on the conjugated polymers
showed short decay dynamics for conjugated polymers with strong electron-withdrawing
groups. Two-photon absorption spectroscopy showed large two-photon
absorption cross sections for the conjugated polymers with strong
electron-withdrawing acceptors. Fluorescence anisotropy decay studies
showed contributions from both hopping and a coherent energy migration
process for some of the polymers. The polymers were investigated for
their photovoltaic efficiency and correlated with both the steady-state
and time-resolved dynamics of the investigated donor–acceptor
polymers.
State-of-the-art
femtosecond spectroscopies and quantum-chemical
methods were used to investigate the excited-state dynamics of D−π–A+ (C1) and D−π–A+–π–D (C2) methylpyridinium (acceptor
unit, A) derivatives bearing dibutylamino groups as strong electron
donors (D) and bithiophenes as highly effective π-rich spacers.
The absorption spectra of C1 and C2 are
broad and shifted to the red side of the visible spectral range. A
significant negative solvatochromism was observed for the absorption
bands of the investigated salts with increasing solvent polarity that
was rationalized in terms of the change in electron density upon excitation.
The absorption spectra of C2 are red-shifted with respect
to those of C1, whereas the emission bands of the two
compounds overlap, suggesting a localization of the excitation on
just one branch of the quadrupolar compound, which becomes the fluorescent
portion. This is in agreement with our quantum-mechanical calculations,
which predict that the symmetry of C2 is broken in the
relaxed S1 geometry. Excited-state symmetry breaking was
observed in all of the investigated solvents regardless of their polarity.
Femtosecond transient absorption and fluorescence up-conversion measurements
revealed that the excited-state dynamics of C1 is essentially
dominated by solvent relaxation, whereas in the case of C2, two distinct excited singlet states were detected in polar solvents,
where an intramolecular charge-transfer (ICT) state is efficiently
produced. The main photoinduced decay pathway of both compounds was
found to be internal conversion in all of the investigated media.
High two-photon-absorption cross sections of 500 and 1400 GM for C1 and C2, respectively, were obtained by means
of femtosecond-resolved two-photon excited fluorescence measurements,
thus demonstrating the enhancement in the nonlinear optical properties
of the quadrupolar compound over its dipolar counterpart, in agreement
with the more efficient ICT observed in the case of C2.
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