7 research outputs found
Fast Triplet Formation via Singlet Exciton Fission in a Covalent Perylenediimide-Ī²-apocarotene Dyad Aggregate
A covalent dyad was synthesized in
which perylene-3,4,:9:10-bisĀ(dicarboximide) (PDI) is linked to Ī²-apocarotene
(Car) using a biphenyl spacer. The dyad is monomeric in toluene and
forms a solution aggregate in methylcyclohexane (MCH). Using femtosecond
transient absorption (fsTA) spectroscopy, the monomeric dyad and its
aggregates were studied both in solution and in thin films. In toluene,
photoexcitation at 530 nm preferentially excites PDI, and the dyad
undergoes charge separation in Ļ = 1.7 ps and recombination
in Ļ = 1.6 ns. In MCH and in thin solid films, 530 nm excitation
of the PDI-Car aggregate also results in charge transfer that competes
with energy transfer from <sup>1</sup>*PDI to Car and with an additional
process, rapid Car triplet formation in <50 ps. Car triplet formation
is only observed in the aggregated PDI-Car dyad and is attributed
to singlet exciton fission (SF) within the aggregated PDI, followed
by rapid triplet energy transfer from <sup>3</sup>*PDI to the carotenoid.
SF from Ī²-apocarotene aggregation is ruled out by direct excitation
of Car films at 414 nm, where no triplet formation is observed. Time-resolved
electron paramagnetic resonance measurements on aggregated PDI-Car
show the formation of <sup>3</sup>*Car with a spin-polarization pattern
that rules out radical-pair intersystem crossing as the mechanism
of triplet formation as well
Picosecond Control of Photogenerated Radical Pair Lifetimes Using a Stable Third Radical
Photoinduced electron transfer reactions
in organic donorāacceptor
systems leading to long-lived radical ion pairs (RPs) have attracted
broad interest for their potential applications in fields as diverse
as solar energy conversion and spintronics. We present the photophysics
and spin dynamics of an electron donor ā electron acceptor
ā stable radical system consisting of a <i>meta</i>-phenylenediamine (mPD) donor covalently linked to a 4-aminonaphthalene-1,8-dicarboximide
(ANI) electron-accepting chromophore as well as an Ī±,Ī³-bisdiphenylene-Ī²-phenylallyl
(BDPA) stable radical. Selective photoexcitation of ANI produces the
BDPAāmPD<sup>+ā¢</sup>āANI<sup>āā¢</sup> triradical in which the mPD<sup>+ā¢</sup>āANI<sup>āā¢</sup> RP spins are strongly exchange coupled. The presence of BDPA is
found to greatly increase the RP intersystem crossing rate from the
initially photogenerated BDPAā<sup>1</sup>(mPD<sup>+ā¢</sup>āANI<sup>āā¢</sup>) to BDPAā<sup>3</sup>(mPD<sup>+ā¢</sup>āANI<sup>āā¢</sup>),
resulting in accelerated RP recombination via the triplet channel
to produce BDPAāmPDā<sup>3*</sup>ANI as compared to
a reference molecule lacking the BDPA radical. The RP recombination
rates observed are much faster than those previously reported for
weakly coupled triradical systems. Time-resolved EPR spectroscopy
shows that this process is also associated with strong spin polarization
of the stable radical. Overall, these results show that RP intersystem
crossing rates can be strongly influenced by stable radicals nearby
strongly coupled RP systems, making it possible to use a third spin
to control RP lifetimes down to a picosecond time scale
Photogenerated Quartet State Formation in a Compact Ring-Fused Perylene-Nitroxide
We report on a novel small organic
molecule comprising a perylene
chromophore fused to a six-membered ring containing a persistent nitroxide
radical to give a perylene-nitroxide, or <b>PerNO</b><sup>ā¢</sup>. This molecule is a robust, compact molecule in which the radical
is closely bound to the chromophore but separated by saturated carbon
atoms, thus limiting the electronic coupling between the chromophore
and radical. We present both ultrafast transient absorption experiments
and time-resolved EPR (TREPR) studies to probe the spin dynamics of
photoexcited <b>PerNO</b><sup><b>ā¢</b></sup> and
utilize X-ray crystallography to probe the molecular structure and
stacking motifs of <b>PerNO</b><sup><b>ā¢</b></sup> in the solid state. The ability to control both the structure and
electronic properties of molecules having multiple spins as well as
the possibility of assembling ordered solid state materials from them
is important for implementing effective molecule-based spintronics
Electron Hopping and Charge Separation within a Naphthalene-1,4:5,8-bis(dicarboximide) Chiral Covalent Organic Cage
We present the stereoselective synthesis
of a chiral covalent organic
cage consisting of three redox-active naphthalene-1,4:5,8-bisĀ(dicarboximide)
(NDI) units by dynamic imine chemistry. Single crystal X-ray diffraction
analysis shows that hostāguest interactions and racemic cocrystallization
allow for controlling the solid state structure. Electronic interactions
between the NDI units probed by absorption and circular dichroism
spectroscopies, electrochemistry and theoretical calculations are
shown to be weak. Photoexcitation of NDI leads to intracage charge
separation with a longer lifetime than observed in the corresponding
monomeric NDI and dimeric NDI cyclophane imines. The EPR spectrum
of the singly reduced cage shows that the electron is localized on
a single NDI unit at ambient temperatures and transitions to rapid
hopping among all three NDI units upon heating to 350 K. Dynamic covalent
chemistry thus promises rapid access to covalent organic cages with
well-defined architectures to study charge accumulation and electron
transport phenomena
Spin-Selective Photoreduction of a Stable Radical within a Covalent DonorāAcceptorāRadical Triad
Controlling
spināspin interactions in multispin molecular
assemblies is important for developing new approaches to quantum information
processing. In this work, a covalent electron donorāacceptorāradical
triad is used to probe spin-selective reduction of the stable radical
to its diamagnetic anion. The molecule consists of a perylene electron
donor chromophore (D) bound to a pyromellitimide acceptor (A), which
is, in turn, linked to a stable Ī±,Ī³-bisdiphenylene-Ī²-phenylallyl
radical (R<sup>ā¢</sup>) to produce D-A-R<sup>ā¢</sup>. Selective photoexcitation of D within D-A-R<sup>ā¢</sup> results
in ultrafast electron transfer to form the D<sup>+ā¢</sup>-A<sup>āā¢</sup>-R<sup>ā¢</sup> triradical, where D<sup>+ā¢</sup>-A<sup>āā¢</sup> is a singlet spin-correlated
radical pair (SCRP), in which both SCRP spins are uncorrelated relative
to the R<sup>ā¢</sup> spin. Subsequent ultrafast electron transfer
within the triradical forms D<sup>+ā¢</sup>-A-R<sup>ā</sup>, but its yield is controlled by spin statistics of the uncorrelated
A<sup>āā¢</sup>-R<sup>ā¢</sup> radical pair, where
the initial charge separation yields a 3:1 statistical mixture of
D<sup>+ā¢</sup>-<sup>3</sup>(A<sup>āā¢</sup>-R<sup>ā¢</sup>) and D<sup>+ā¢</sup>-<sup>1</sup>(A<sup>āā¢</sup>-R<sup>ā¢</sup>), and subsequent reduction of R<sup>ā¢</sup> only occurs in D<sup>+ā¢</sup>-<sup>1</sup>(A<sup>āā¢</sup>-R<sup>ā¢</sup>). These findings inform the design of multispin
systems to transfer spin coherence between molecules targeting quantum
information processing using the agency of SCRPs
Positionally Defined, Binary Semiconductor Nanoparticles Synthesized by Scanning Probe Block Copolymer Lithography
We report the first method for synthesizing binary semiconductor
materials by scanning probe block copolymer lithography (SPBCL) in
desired locations on a surface. In this work, we utilize SPBCL to
create polymer features containing a desired amount of Cd<sup>2+</sup>, which is defined by the feature volume. When they are subsequently
reacted in H<sub>2</sub>S in the vapor phase, a single CdS nanoparticle
is formed in each block copolymer (BCP) feature. The CdS nanoparticles
were shown to be both crystalline and luminescent. Importantly, the
CdS nanoparticle sizes can be tuned since their diameters depend on
the volume of the originally deposited BCP feature
Marked Consequences of Systematic Oligothiophene Catenation in Thieno[3,4ā<i>c</i>]pyrrole-4,6-dione and Bithiopheneimide Photovoltaic Copolymers
As
effective building blocks for high-mobility transistor polymers,
oligothiophenes are receiving attention for polymer solar cells (PSCs)
because the resulting polymers can effectively suppress charge recombination.
Here we investigate two series of in-chain donorāacceptor copolymers, <b>PTPDnT</b> and <b>PBTInT</b>, based on thienoĀ[3,4-<i>c</i>]Āpyrrole-4,6-dione (<b>TPD</b>) or bithiopheneimide
(<b>BTI</b>) as electron acceptor units, respectively, and oligothiophenes
(<b>nT</b>s) as donor counits, for high-performance PSCs. Intramolecular
SĀ·Ā·Ā·O interaction leads to more planar <b>TPD</b> polymer backbones, however backbone torsion yields greater open-circuit
voltages for <b>BTI</b> polymers. Thiophene addition progressively
raises polymer HOMOs but marginally affects their band gaps. FT-Raman
spectra indicate that <b>PTPDnT</b> and <b>PBTInT</b> conjugation
lengths scale with <b>nT</b> catenation up to <i>n</i> = 3 and then saturate for longer oligomer. Furthermore, the effects
of oligothiophene alkylation position are explored, revealing that
the alkylation pattern greatly affects film morphology and PSC performance.
The <b>3T</b> with āoutwardā alkylation in <b>PTPD3T</b> and <b>PBTI3T</b> affords optimal Ļ-conjugation,
close stacking, long-range order, and high hole mobilities (0.1 cm<sup>2</sup>/(V s)). These characteristics contribute to the exceptional
ā¼80% fill factors for <b>PTPD3T</b>-based PSCs with PCE
= 7.7%. The results demonstrate that <b>3T</b> is the optimal
donor unit among <b>nT</b>s (<i>n</i> = 1ā4)
for photovoltaic polymers. Grazing incidence wide-angle X-ray scattering,
transmission electron microscopy, and time-resolved microwave conductivity
measurements reveal that the terthiophene-based <b>PTPD3T</b> blend maintains high crystallinity with appreciable local mobility
and long charge carrier lifetime. These results provide fundamental
materials structure-device performance correlations and suggest guidelines
for designing oligothiophene-based polymers with optimal thiophene
catenation and appropriate alkylation pattern to maximize PSC performance