ACENES AND ACENEQUINONES FOR OPTICS AND ORGANIC ELECTRONICS

Acenes have been explored by a number of research groups in the field of organic electronics with a particular emphasis on transistor materials. This group has been actively studying acene‐based organic semiconductors for more than a decade using a crystal engineering approach and has developed acene derivatives for applications in field‐effect transistors, light‐emitting diodes, and photovoltaics. In addition to organic electronics, crystal engineering has important applications in a number of other fields, quite notably in the design of metal‐organic frameworks. Chapters 2 and 3 of this dissertation focus on applying crystal engineering to the synthesis of acene derivatives for use as solid‐state, long‐wavelength fluorescent organic dyes in the field of biomedical imaging. More specifically, this work studied the synthesis and properties of dioxolane‐functionalized pentacenes and hexacenes. One of these pentacene derivatives has already been demonstrated in biomedical imaging which may lead to improved treatment of tuberculosis. The dioxolane‐functionalized hexacene is still under evaluation for bioimaging applications. Chapters 4 and 5 focus on crystal engineering in relation to organic electronics. Chapter 4 deals with fine‐tuning of crystal packing and demonstrated that small differences in molecular structure can result in significant changes to the solid‐state structure which affects semiconductor properties. Finally, chapter 5 studies the use of singlet fission in photovoltaics and demonstrated that this process does occur in a solar cell incorporating a hexacene derivative. Pentadithiophenes were also synthesized for singlet fission photovoltaics, but they have yet to be studied further

Geminate and nongeminate recombination of triplet excitons formed by singlet fission.

We report the simultaneous observation of geminate and nongeminate triplet-triplet annihilation in a solution-processable small molecule TIPS-tetracene undergoing singlet exciton fission. Using optically detected magnetic resonance, we identify recombination of triplet pairs directly following singlet fission, as well as recombination of triplet excitons undergoing bimolecular triplet-triplet annihilation. We show that the two processes give rise to distinct magnetic resonance spectra, and estimate the interaction between geminate triplet excitons to be 60 neV.EPSRC [grant no. EP/J017361/1 and EP/G060738/1]. E. Oppenheimer Foundation and St. Catherine's College, Cambridge. NSF [CMMI- 1255494].This is the author accepted manuscript. The final version is available at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.238701

Identification of a triplet pair intermediate in singlet exciton fission in solution.

Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley-Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spin-singlet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS]--tetracene we find rapid (<100 ps) formation of excimers and a slower (∼ 10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endothermic singlet fission process.H.L.S was supported by the Winton Programme for the Physics of Sustainability and A.J.M received funding from the Engineering and Physical Sciences Research Council.This is the accepted manuscript. The final version is available at http://www.pnas.org/content/112/25/7656.abstract

Synthesis and Optical Properties of Dioxolane-Functionalized Hexacenes and Heptacenes

The synthesis of dioxolane-functionalized hexacenes and heptacenes is reported. While heptacenes were too reactive to be successfully isolated, hexacenes showed higher stability and characteristic long-wavelength fluorescence both in solution and in the solid state as crystalline powders

Synthesis and Optical Properties of Dioxolane-Functionalized Hexacenes and Heptacenes

The synthesis of dioxolane-functionalized hexacenes and heptacenes is reported. While heptacenes were too reactive to be successfully isolated, hexacenes showed higher stability and characteristic long-wavelength fluorescence both in solution and in the solid state as crystalline powders

Synthesis and Optical Properties of Dioxolane-Functionalized Hexacenes and Heptacenes

The synthesis of dioxolane-functionalized hexacenes and heptacenes is reported. While heptacenes were too reactive to be successfully isolated, hexacenes showed higher stability and characteristic long-wavelength fluorescence both in solution and in the solid state as crystalline powders

Geminate and nongeminate recombination of triplet excitons formed by singlet fission

We report the simultaneous observation of geminate and nongeminate triplet-triplet annihilation in a solution-processable small molecule TIPS-tetracene undergoing singlet exciton fission. Using optically detected magnetic resonance, we identify recombination of triplet pairs directly following singlet fission, as well as recombination of triplet excitons undergoing bimolecular triplet-triplet annihilation. We show that the two processes give rise to distinct magnetic resonance spectra, and estimate the interaction between geminate triplet excitons to be 60 neV

Negative Isotope Effect on Field-Effect Hole Transport in Fully Substituted 13 C-Rubrene

International audiencehave revealed the simultaneous presence of both high room temperature hole mobility above 10 cm 2 V −1 s −1[5] and, crucially , a negative temperature exponent for the mobility, i.e., mobility increases as temperature decreases down to about 180 K. [6] These observations and others, including mobility anisotropy [7,8] and a robust Hall effect, [9,10] are strong evidence for band-like transport in rubrene (band-width ≈0.5 eV), [11] where many features of classical band transport are observed. However, the estimated carrier mean free paths remain on the order of the unit cell dimension (≈1 nm), thus precluding the typical band picture in conventional semiconductors. The precise nature of band-like transport in organic semiconductor crystals is still being explored. [12] Currently, a consistent picture of charge transport in high mobility crystals is offered by the dynamic disorder model (DDM, also known as transient localization), in which the carrier transient localization length l loc and the propagation rate ω are determined by a competition between intermolecular electronic coupling and charge-phonon interaction ; in this scenario the carrier mobility μ can be evaluated as [13] µ ω = B loc 2 e k T l (1) Isotopic substitution is a useful method to study the influence of nuclear motion on the kinetics of charge transport in semiconductors. However, in organic semiconductors, no observable isotope effect on field-effect mobility has been reported. To understand the charge transport mechanism in rubrene, the benchmark organic semiconductor, crystals of fully isotopically substituted rubrene, 13 C-rubrene (13 C 42 H 28), are synthesized and characterized. Vapor-grown 13 C-rubrene single crystals have the same crystal structure and quality as native rubrene crystals (i.e., rubrene with a natural abundance of carbon isotopes). The characteristic transport signatures of rubrene, including room temperature hole mobility over 10 cm 2 V −1 s −1 , intrinsic band-like transport, and clear Hall behavior in the accumulation layer of air-gap transistors, are also observed for 13 C-rubrene crystals. The field-effect mobility distributions based on 74 rubrene and 13 C-rubrene devices, respectively, reveal that 13 C isotopic substitution produces a 13% reduction in the hole mobility of rubrene. The origin of the negative isotope effect is linked to the redshift of vibrational frequencies after 13 C-substitution, as demonstrated by computer simulations based on the transient localization (dynamic disorder) scenario. Overall, the data and analysis provide an important benchmark for ongoing efforts to understand transport in ordered organic semiconductors

Microfluidic Generation of Droplets with a High Loading of Nanoparticles

Microfluidic approaches for controlled generation of colloidal clusters, for example, via encapsulation of colloidal particles in droplets, have been used for the synthesis of functional materials including drug delivery carriers. Most of the studies, however, use a low concentration of an original colloidal suspension (<10 wt %). Here we demonstrate microfluidic approaches for directly making droplets with moderate (10–25 wt %) and high (>60 wt %) particle concentrations. Three types of microfluidic devices, PDMS flow-focusing, PDMS T-junction, and microcapillary devices, are investigated for direct encapsulation of a high concentration of polystyrene (PS) nanoparticles in droplets. In particular, it is shown that PDMS devices fabricated by soft lithography can generate droplets from a 25 wt % PS suspension, whereas microcapillary devices made from glass capillary tubes are able to produce droplets from a 67 wt % PS nanoparticle suspension. When the PS concentration is between 0.6 and 25 wt %, the size of the droplets is found to change with the oil-to-water flow rate ratio and is independent of the concentration of particles in the initial suspensions. Drop sizes from ∼12 to 40 μm are made using flow rate ratios <i>Q</i>_oil/<i>Q</i>_water from 20 to 1, respectively, with either of the PDMS devices. However, clogging occurs in PDMS devices at high PS concentrations (>25 wt %) arising from interactions between the PS colloids and the surface of PDMS devices. Glass microcapillary devices, on the other hand, are resistant to clogging and can produce droplets continuously even when the concentration of PS nanoparticles reaches 67 wt %. We believe that our findings indicate useful approaches and guidelines for the controlled generation of emulsions filled with a high loading of nanoparticles, which are useful for drug delivery applications