63 research outputs found
Chromophores in molecular nanorings : when is a ring a ring?
The topology of a conjugated molecule plays a significant role in controlling both the electronic properties and the conformational manifold that the molecule may explore. Fully Ï-conjugated molecular nanorings are of particular interest, as their lowest electronic transition may be strongly suppressed as a result of symmetry constraints. In contrast, the simple Kasha model predicts an enhancement in the radiative rate for corresponding linear oligomers. Here we investigate such effects in linear and cyclic conjugated molecules containing between 6 and 42 butadiyne-linked porphyrin units (corresponding to 600 CâC bonds) as pure monodisperse oligomers. We demonstrate that as the diameter of the nanorings increases beyond âŒ10 nm, its electronic properties tend toward those of a similarly sized linear molecule as a result of excitation localization on a subsegment of the ring. However, significant differences persist in the nature of the emitting dipole polarization even beyond this limit, arising from variations in molecular curvature and conformation
One- to Two-Exciton Transitions in Perylene Bisimide Dimer Revealed by Two-Dimensional Electronic Spectroscopy
The excited-state energy levels of molecular dimers and aggregates play a critical role in their photophysical behavior and an understanding of the photodynamics in such structures is important for developing applications such as photovoltaics and optoelectronic devices. Here, exciton transitions in two different covalently bound PBI dimers are studied by two-dimensional electronic spectroscopy (2DES), a powerful spectroscopic method, providing the most complete picture of vibronic transitions in molecular systems. The data are accurately reproduced using the equation of motion-phase matching approach. The unambiguous presence of one-exciton to two-exciton transitions are captured in our results and described in terms of a molecular exciton energy level scheme based on the Kasha model. Furthermore, the results are supported by comparative measurements with the PBI monomer and another dimer in which the interchromophore distance is increased
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Design Principles for Two-Dimensional Molecular Aggregates Using Kasha's Model: Tunable Photophysics in Near and Short-Wave Infrared
Technologies
which utilize near-infrared (700 â 1000 nm) and short-wave infrared (1000 â
2000 nm) electromagnetic radiation have applications in deep-tissue imaging,
telecommunications and satellite telemetry due to low scattering and decreased
background signal in this spectral region. It is therefore necessary to develop
materials that absorb light efficiently beyond 1000 nm. Transition dipole
moment coupling (e.g. J-aggregation) allows for redshifted excitonic states and
provides a pathway to highly absorptive electronic states in the infrared. We present aggregates of two cyanine dyes whose
absorption peaks redshift dramatically upon aggregation in water from ~800
nm to 1000 nm and 1050 nm respectively with sheet-like morphologies and high
molar absorptivities (e ~ 105 M-1cm-1). We use Frenkel exciton theory to extend
Kashaâs model for J and H aggregation and describe the excitonic states of
2-dimensional aggregates whose slip is controlled by steric hindrance in the
assembled structure. A consequence of the increased dimensionality is the
phenomenon of an intermediate âI-aggregateâ, one which redshifts yet displays
spectral signatures of band-edge dark states akin to an H-aggregate. We
distinguish between H-, I- and J-aggregates by showing the relative position of
the bright (absorptive) state within the density of states using temperature
dependent spectroscopy. I-aggregates hold potential for applications as charge
injection moieties for semiconductors and donors for energy transfer in NIR and
SWIR. Our results can be used to better design chromophores with predictable
and tunable aggregation with new photophysical properties
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Generalized Kasha's Model: T-Dependent Spectroscopy Reveals Short-Range Structures of 2D Excitonic Systems
Generalized Kasha's Scheme for Classifying Two-Dimensional Excitonic Molecular Aggregates: Temperature Dependent Absorption Peak Frequency Shift
We propose a generalized theoretical framework for classifying
two-dimensional (2D) excitonic molecular aggregates based on an analysis of
temperature dependent spectra. In addition to the monomer-aggregate absorption
peak shift, which defines the conventional J- and H-aggregates, we incorporate
the peak shift associated with increasing temperature as a measure to
characterize the exciton band structure. First we show that there is a
one-to-one correspondence between the monomer-aggregate and the T-dependent
peak shifts for Kasha's well-established model of 1D aggregates, where
J-aggregates exhibit further redshift upon increasing temperature and
H-aggregates exhibit further blueshift. On the contrary, 2D aggregate
structures are capable of supporting the two other combinations: blueshifting
J-aggregates and redshifting H-aggregates, owing to their more complex exciton
band structures. Secondly, using spectral lineshape theory, the T-dependent
shift is associated with the relative abundance of states on each side of the
bright state. We further establish that the density of states can be connected
to the microscopic packing condition leading to these four classes of
aggregates by separately considering the short and long-range contribution to
the excitonic couplings. In particular the T-dependent shift is shown to be an
unambiguous signature for the sign of net short-range couplings: Aggregates
with net negative (positive) short-range couplings redshift (blueshift) with
increasing temperature. Lastly, comparison with experiments shows that our
theory can be utilized to quantitatively account for the observed but
previously unexplained T-dependent absorption lineshapes. Thus, our work
provides a firm ground for elucidating the structure-function relationships for
molecular aggregates and is fully compatible with existing experimental and
theoretical structure characterization tools.Comment: 29 pages, 4 figure
Deoxyribonucleic Acid Encoded and Size-Defined Ï-Stacking of Perylene Diimides.
Funder: University of CambridgeNatural photosystems use protein scaffolds to control intermolecular interactions that enable exciton flow, charge generation, and long-range charge separation. In contrast, there is limited structural control in current organic electronic devices such as OLEDs and solar cells. We report here the DNA-encoded assembly of Ï-conjugated perylene diimides (PDIs) with deterministic control over the number of electronically coupled molecules. The PDIs are integrated within DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side, and specification of intermolecular DNA hybridization. In this way, we have developed a "toolbox" for construction of any stacking sequence of these semiconducting molecules. We have discovered that we need to use a full hierarchy of interactions: DNA guides the semiconductors into specified close proximity, hydrophobic-hydrophilic differentiation drives aggregation of the semiconductor moieties, and local geometry and electrostatic interactions define intermolecular positioning. As a result, the PDIs pack to give substantial intermolecular Ï wave function overlap, leading to an evolution of singlet excited states from localized excitons in the PDI monomer to excimers with wave functions delocalized over all five PDIs in the pentamer. This is accompanied by a change in the dominant triplet forming mechanism from localized spin-orbit charge transfer mediated intersystem crossing for the monomer toward a delocalized excimer process for the pentamer. Our modular DNA-based assembly reveals real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision.ERC Horizon 2020 (grant agreement No 670405 and No 803326)
EPSRC Tier-2 capital grant EP/P020259/1.
Winton Advanced Research Programme for the Physics of Sustainability. Simons Foundation (Grant 601946).
Swedish research council, VetenskapsrÄdet 2018-0023
Deoxyribonucleic Acid Encoded and Size-Defined Ï-Stacking of Perylene Diimides
Natural photosystems use protein scaffolds to control intermolecular interactions that enable exciton flow, charge generation, and long-range charge separation. In contrast, there is limited structural control in current organic electronic devices such as OLEDs and solar cells. We report here the DNA-encoded assembly of Ï-conjugated perylene diimides (PDIs) with deterministic control over the number of electronically coupled molecules. The PDIs are integrated within DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side, and specification of intermolecular DNA hybridization. In this way, we have developed a âtoolboxâ for construction of any stacking sequence of these semiconducting molecules. We have discovered that we need to use a full hierarchy of interactions: DNA guides the semiconductors into specified close proximity, hydrophobicâhydrophilic differentiation drives aggregation of the semiconductor moieties, and local geometry and electrostatic interactions define intermolecular positioning. As a result, the PDIs pack to give substantial intermolecular Ï wave function overlap, leading to an evolution of singlet excited states from localized excitons in the PDI monomer to excimers with wave functions delocalized over all five PDIs in the pentamer. This is accompanied by a change in the dominant triplet forming mechanism from localized spinâorbit charge transfer mediated intersystem crossing for the monomer toward a delocalized excimer process for the pentamer. Our modular DNA-based assembly reveals real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision
Structure Dependence of Kinetic and Thermodynamic Parameters in Singlet Fission Processes
Singlet fissionâwhereby one absorbed photon generates two coupled triplet excitonsâis a key process for increasing the efficiency of optoelectronic devices by overcoming the ShockleyâQueisser limit. A crucial parameter is the rate of dissociation of the coupled triplets, as this limits the number of free triplets subsequently available for harvesting and ultimately the overall efficiency of the device. Here we present an analysis of the thermodynamic and kinetic parameters for this process in parallel and herringbone dimers measured by electron paramagnetic resonance spectroscopy in coevaporated films of pentacene in p-terphenyl. The rate of dissociation is higher for parallel dimers than for their herringbone counterparts, as is the rate of recombination to the ground state. DFT calculations, which provide the magnitude of the electronic coupling as well as the distribution of molecular orbitals for each geometry, suggest that weaker triplet coupling in the parallel dimer is the driving force for faster dissociation. Conversely, localization of the molecular orbitals and a stronger tripletâtriplet interaction result in slower dissociation and recombination. The identification and understanding of how the intermolecular geometry promotes efficient triplet dissociation provide the basis for control of triplet coupling and thereby the optimization of one important parameter of device performance
Optical Anisotropy and Momentum-Dependent Excitons in Dibenzopentacene Single Crystals
High-quality single crystals of the organic semiconductor (1,2;8,9)-dibenzopentacene were grown via physical vapor transport. The crystal structureâunknown beforeâwas determined by single-crystal X-ray diffraction; polarization-dependent optical absorption measurements display a large anisotropy in the ac plane of the crystals. The overall Davydov splitting is âŒ110 meV, which is slightly lower than that in the close relative pentacene (120 meV). Momentum-dependent electron energy-loss spectroscopy measurements show a clear exciton dispersion of the Davydov components. An analysis of the dispersion using a simple 1D model indicates smaller electron- and hole-transfer integrals in dibenzopentacene as compared to pentacene. The spectral weight distribution of the excitation spectra is strongly momentum-dependent and demonstrates a strong momentum-dependent admixture of Frenkel excitons, charge-transfer excitons, and vibrational modes
Oblique Packing and Tunable Excitonic Coupling in DNA-Templated Squaraine Rotaxane Dimer Aggregates
When molecules are aggregated such that their excited states form delocalized excitons, their spatial arrangement, or packing, can be coarsely controlled by templating and finely controlled by chemical substitution; however, challenges remain in controlling their packing on intermediate length scales. Here, we use an approach based on mechanically interlocked molecules to promote an elusive oblique packing arrangement in a series of three squaraine rotaxane dimers. We template the squaraine rotaxane dimers using DNA and observe two excitonically split bands of near-equal intensity in their absorption spectra â a distinct signature of oblique packing, validated by theoretical modeling of the experimental results. Additional fine control of packing is demonstrated by fluorinating the macrocycle of the rotaxane, which promotes denser packing and stronger excitonic interactions
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