29 research outputs found
Solvent-Assisted Self-Assembly of Fullerene into Single-Crystal Ultrathin Microribbons as Highly Sensitive UVâVisible Photodetectors
The size, shape, and crystallinity of organic nanostructures play an important role in their physical properties and are mainly determined by the self-assembling kinetics of molecular components often involving the solvent conditions. Here, we reported a kinetically controlled self-assembly of C<sub>60</sub> assisted by the solvent carbon bisulfide (CS<sub>2</sub>) into single-crystal ultrathin microribbons of 2C<sub>60</sub>·3CS<sub>2</sub>, upon mixing the poor solvent isopropyl alcohol with a C<sub>60</sub>/CS<sub>2</sub> stock solution. Surface energy calculations reveal that these microribbons represent a kinetically favored high-energy state as compared with the thermodynamically stable shape of prismatic rods. High-resolution transmission electron microscopy observations clarify that association of CS<sub>2</sub> at the nucleation stage helps to guide and rigidify the formation of ÏâÏ stacking 1D chains of C<sub>60</sub> through the surrounding CS<sub>2</sub> cage-like structures, which further act as glue, boosting lateral assembly of as-formed 1D chains into untrathin 2D microribbon single crystals. Precise control over the thickness, width, and length of 2C<sub>60</sub>·3CS<sub>2</sub> microribbons was achieved by manipulation of the growth kinetics through adjusting the solvent conditions. Upon heating to 120 °C, sublimation of CS<sub>2</sub> components results in fcc C<sub>60</sub> microribbons. We found that both microribbons of solvated monoclinic 2C<sub>60</sub>·3CS<sub>2</sub> and pure fcc C<sub>60</sub> exhibit highly sensitive photoconductivity properties with a spectral response range covering UV to visible. The highest on/off ratio of two-terminal photodetectors based on single ribbons reaches around 250, while the responsitivity is about 75.3 A W<sup>â1</sup> in the UV region and 90.4 A W<sup>â1</sup> in the visible region
Donor-Linked Di(perylene bisimide)s: Arrays Exhibiting Fast Electron Transfer for Photosynthesis Mimics
The first example of donor-linked
diÂ(perylene bisimide)Âs is reported. UVâvis absorption spectra
of these newly synthesized dyads showed intense absorption across
the entire visible region, demonstrating their excellent light-harvesting
activities. The severe fluorescence quenching event probed by steady-state
fluorescence spectroscopy and the free-energy calculations suggested
the possibility of electron transfer (ET) in these arrays upon photoexcitation.
Further femtosecond transient absorption spectra clarified that the
fluorescence quenching was due to fast intramolecular ET. The rate
of the charge separation (CS) was found to be as high as 10<sup>12</sup> s<sup>â1</sup> in CH<sub>2</sub>Cl<sub>2</sub>. It was suggested
that the large ET driving forces, strong donorâacceptor electronic
coupling, and relatively small reorganization energy of diPBI accounted
for the rapid ET process in a synergic manner. The fate of the generated
radical ion pair depended on the solvent used. Rapid charge recombination
to ground state occurred for the dyads in polar CH<sub>2</sub>Cl<sub>2</sub> and for diPBI-TPA in nonpolar toluene. However, sufficient <sup>3</sup>diPBI* population was attained via efficient spinâorbit
coupled intersystem crossing from the charge-separated state for diPBI-PdTPP
in toluene. These photophysical properties are interpreted as the
cooperation between thermodynamic feasibility and kinetic manipulation
Naphthalenediimide-Benzothiadiazole Copolymer Semiconductors: Rational Molecular Design for Air-Stable Ambipolar Charge Transport
Rational
design of air-stable ambipolar polymeric semiconductors
was achieved by covalently connecting naphthalenediimide (NDI) units
with benzothiadiazole (BZ) through thiophene (T) linkers, namely, <b>PNDI-</b><i><b>mT</b></i><b>(BZ)</b><i><b>mT</b></i> (<i>m</i> = 1, 2), in which well-coplanar <i><b>mT</b></i><b>(BZ)</b><i><b>mT</b></i> moieties as a whole act as donors rather than acceptors reported
in previous studies. Decreasing the number of thiophene linkers from <i>m</i> = 2 to 1 lowers both LUMO and HOMO energy levels. As a
result, the carriers in organic thin film transistors (OTFTs) could
be switched from unipolar <i>p</i>-channel only to ambipolar
transport. In ambient conditions, <b>PNDI-</b><i><b>2T</b></i><b>(BZ)</b><i><b>2T</b></i> presents an average hole mobility of 0.07 ± 0.02 cm<sup>2</sup> V<sup>â1</sup> s<sup>â1</sup>, while <b>PNDI-</b><i><b>T</b></i><b>(BZ)</b><i><b>T</b></i> exhibits balanced ambipolar charge transport in a bottom-gate/top-contact
device architecture, the average electron and hole mobilities was
0.05 ± 0.02 (<i>Ό</i><sub>e</sub>) and 0.1 ±
0.03 (<i>ÎŒ</i><sub>h</sub>) cm<sup>2</sup> V<sup>â1</sup> s<sup>â1</sup>, respectively. Moreover, OTFTs based on both
polymer show good air-stability with negligible changes after stored
in ambient over 3 months
Controlled Self-Assembly of Organic Microcrystals for Laser Applications
The
small organic molecule <i>p</i>-distyrylbenzene (DSB)
has been controllably prepared into one-dimensional microwires (1D-MWs)
and 2D rhombic microdisks (2D-RMDs) by modulating the growth kinetics
in the process of morphology growth. These as-prepared organic microcrystals,
1D-MWs and 2D-RMDs, exhibit a shape-dependent microcavity effect in
that the single 1D-MW can act as a Fabry-PeÌrot (FP) mode lasing
resonator while the individual 2D-RMD functions as the whispering-gallery-mode
(WGM) microcavity. Moreover, as compared with the 1D FP resonators,
there exists a higher quality factor (<i>Q</i>) in the WGM
lasing resonator under the identical optical path length. Significantly,
the lasing threshold, <i>E</i><sub>th</sub> = 1.02 ÎŒJ/cm<sup>2</sup>, of 2D-RMDs is much lower than <i>E</i><sub>th</sub> = 2.57 ÎŒJ/cm<sup>2</sup> of 1D-MWs. Our demonstration can
give the direction for the development of the organic solid-state
microlasers
Self-Assembly of Perylenediimide Nanobelts and Their Size-Tunable Exciton Dynamic Properties
Upon the oxidation of perylenediimide dianion precursors, controlled release of neutral units paves the way for the solution-phase self-assembly of nanobelts via synergistic ÏâÏ stacking and hydrogen-bonding interactions. The obtained belt size has been regulated through adjusting the precursor supersaturation. This controlled synthesis also offers us an opportunity to explore size-tunable exciton dynamics features in the nanobelt, in which the competitive evolution to H-like exciton or excimer is found to be in strong relevance to the molecular packing and crystal size
Photoreactions of Porphyrins Initiated by Deep Ultraviolet Single Photons
The
newly built 177 nm all-solid-state deep ultraviolet (DUV) laser
photoionization mass spectrometer finds a unique advantage to identify
porphyrins that bear ionization energies close to 7.0 eV. We observed
dramatic selectivity of tetraphenylporphyrins (TPPs) pertaining to
varied photochemical processes initiated by the DUV laser excitation.
Single-photon ionization was found dominant for 2H-TPP resulting in
a fragmentation-free mass spectrum; photoinduced dehydrogenation was
observed for zinc TPP, but both dehydrogenation and demetalation are
noted for copper TPP. Along with first-principle calculations, we
demonstrate how the photoinduced reactions vary with residual energies
of photoionization, highest occupied molecular orbitalâlowest
unoccupied molecular orbital gaps, donorâacceptor orbital overlaps,
single-step barriers, and whether or not there is a major process
of structural rearrangement. It is demonstrated that the rotation
of benzene ring under proper laser radiation prompts dehydrogenation
process; also, metallo-TPPs do not support direct demetalation, but
it is selectively accomplishable along with dehydrogenation and successive
hydrogenation processes. These findings not only provide insights
into the hydrogen atom transfer in porphyrins initiated by ultraviolet
laser but also suggest promising applications of the DUV laser in
designed synthesis and chemical modification of porphyrins
Engineering of Interfacial Electron Transfer from DonorâAcceptor Type Organic Semiconductor to ZnO Nanorod for Visible-Light Detection
Interfacial electron transfer (IET) plays a key role
in photoactive
organic/inorganic hybrid nanomaterials and remains elusive with regard
to interfacial energy level alignment. In this study, we prepared
hybrid ZnO nanorods by grafting n-type perylene bisimide (PBI) derivatives
bearing carboxylic acid groups at nitrogen positions. No evidence
in terms of direct electron transfer from PBI to ZnO can be observed
in PBI/ZnO hybrids. In sharp contrast, incorporation of electron-rich
oligothiophene (<i>n</i>T, <i>n</i> = 1, 2) moieties
into PBI core at bay positions resulted in a highly efficient cascade
IET in <i>n</i>T-PBI/ZnO (<i>n</i> = 1, 2) hybrid
nanorods, which was initiated by photoinduced electron transfer (PET)
from <i>n</i>T (<i>n</i> = 1, 2) to PBI and then
followed by charge shifting from PBI anion to ZnO across the interface.
High performance UVâvis photodetectors based on <i>n</i>T-PBI/ZnO (<i>n</i> = 1, 2) hybrids have been fabricated
and show responsivity of 21.2 and 12.4 A/W and an on/off ratio as
high as 537 and 403, whereas that based on PBI/ZnO shows little visible-light
response. Our results suggest that donorâacceptor type compounds
can be used for constructing photoactive hybrid nanomaterials, in
which efficient cascade IET modifies interfacial electronic structure
and helps extend the spectral response range
Tunable Morphology of the Self-Assembled Organic Microcrystals for the Efficient Laser Optical Resonator by Molecular Modulation
Organic
single-crystalline micro/nanostructures can effectively
generate and carry photons due to their smooth morphologies, high
photoluminescence quantum efficiency, and minimized defects density
and therefore are potentially ideal building blocks for the optical
circuits in the next generation of miniaturized optoelectronics. However,
the tailor-made organic molecules can be generally obtained by organic
synthesis, ensuring that the organic molecules aggregate in a specific
form and generate micro/nanostructures with desirable morphology and
therefore act as the efficient laser optical resonator remains a great
challenge. Here, the molecular modulation of the morphology on the
laser optical resonator properties has been investigated through the
preparation of the elongated hexagonal microplates (PHMs) and the
rectangular microplates (ORMs), respectively, from two model isomeric
organic molecules of 1,4-bisÂ(4-methylstyryl)Âbenzene (<i>p</i>-MSB) and 1,4-bisÂ(2-methylstyryl)Âbenzene (<i>o</i>-MSB).
Significantly, fluorescence resonance phenomenon was only observed
in the individual ORM other than the PHM. It indicates that the rectangular
resonators possess better light-confinement property over the elongated
hexagonal resonators. More importantly, optically pumped lasing action
was observed in the <i>o</i>-MSB rectangular morphology
microplates resonator with a high <i>Q</i> â 1500
above a threshold of âŒ540 nJ/cm<sup>2</sup>. The excellent
optical properties of these microstructures are associated with the
morphology, which can be precisely modulated by the organic molecular
structure. These self-assembled organic microplates with different
morphologies can contribute to the distinct functionality of photonics
elements in the integrated optical circuits at micro/nanoscale
Epitaxial Self-assembly of Binary Molecular Components into Branched Nanowire Heterostructures for Photonic Applications
We
report a sequential epitaxial growth to prepare organic branched
nanowire heterostructures (BNwHs) consisting of a microribbon trunk
of 1,4-dimethoxy-2,5-diÂ[4âČ-(cyano)Âstyryl]Âbenzene (COPV) with
multiple nanowire branches of 2,4,5-triphenylimidazole (TPI) in a
one-pot solution synthesis. The synthesis involves a seeded-growth
process, where COPV microribbons are grown first as a trunk followed
by a seeded-growth of TPI nanowire branches at the pregrown trunk
surfaces. Selected area electron diffraction characterizations reveal
that multiple hydrogen-bonding interactions between TPI and COPV components
play an essential role in the epitaxial growth as a result of the
structural matching between COPV and TPI crystals. A multichannel
optical router was successfully realized on the basis of the passive
waveguides of COPV green photoluminescence (PL) along TPI nanowire
branches in a single organic BNwH
Exceptional Intersystem Crossing in Di(perylene bisimide)s: A Structural Platform toward Photosensitizers for Singlet Oxygen Generation
Photosensitized reactions of molecular oxygen have found far-reaching applications in various fields, and the development of new photosensitizer compounds is of crucial importance. We here describe a new class of triply linked bay-fused diperylene bisimides (DiPBIs) which exhibited several unique features, rendering them a new structural platform for the development of highly efficient and photostable photosensitizers. (i) The extended Ï-conjugation shifts its absorption into the bodyâs therapeutic window. (ii) The nonplanarity of the distorted cores enhances the spinâorbit coupled intersystem crossing. (iii) The long-lasting high-energy T<sub>1</sub> state facilitates singlet oxygen generation via energy-transfer reaction between T<sub>1</sub> and ground-state oxygen