Nonlinear optical response in organic semiconductors has been an attractive property for many practical applications. For frequency up-converted lasers, to date, conjugated polymers, fluorescent dyes ...and small organic molecules have been proposed but their performances have been severely limited due to the difficulty in simultaneously achieving strong nonlinear optical response and high performance optical gain. In this work, we show that structurally designed truxene-based star-shaped oligofluorenes exhibit strong structure–property relationships enabling enhanced nonlinear optical response with favorable optical gain performance. As the number of fluorene repeat units in each arm is increased from 3 to 6, these molecules demonstrate a two-photon absorption cross-section as high as 2200 GM, which is comparable to that of linear conjugated polymers. Tailored truxene oligomers with six fluorene units in each arm (T6) show two-photon absorption pumped amplified spontaneous emission with a threshold as low as 2.43 mJ cm −2 , which is better than that of the lowest reported threshold in organic semiconductors. Furthermore, we show a frequency up-converted laser using the newly designed and synthesized star-shaped oligomer T6 with a threshold as low as 3.1 mJ cm −2 , which is more than an order of magnitude lower than that of any conjugated polymer. Thus, these oligomers with enhanced nonlinear optical properties are highly attractive for bio-integrated applications such as photodynamic therapy and in vivo bio-sensing.
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a material that has become ubiquitous in the field of organic electronics. It is most commonly used as a hole transport layer ...(HTL) in optoelectronic devices and can be purchased commercially in various formulations with different properties. Whilst it is a most convenient material to work with, there are stability issues associated with PEDOT:PSS that are detrimental to device stability and these are due to the acidic nature of the PSS component. In this paper, we present a molecular, non-acidic alternative to PEDOT:PSS. The parent structure is composed of a quater(3,4-ethylenedioxythiophene) unit capped either side of the short chain with two pyridine units. This compound, termed (BEDOTPy)
2
, can be prepared chemically and electrochemically to give doped materials with a choice of counteranions. Further functionalisation
via
quaternisation at the nitrogen atoms allows for modification of solubility and film-forming properties. The conductivity of the doped samples can reach up to 3.75 S cm
1
. The materials are non-acidic and are therefore attractive alternatives to PEDOT:PSS for device applications. We demonstrate an OLED device using the compound (BEDOTPy-EtOH-I)
2
PF
6
as an HTL, and compare the device performance to one made with PEDOT:PSS. Due to the non-acidic nature of the molecular material, the corresponding OLED device does not show a drop in luminance over time, whereas a loss of performance is observed for the device containing PEDOT:PSS over a short period. These results are presented to introduce the parent compound (BEDOTPy)
2
as an attractive alternative to PEDOT:PSS, which can be easily modified chemically to provide a plethora of potential compounds with tunable properties.
Doped quater(3,4-ethylenedioxythiophene), capped either side of the chain with two pyridine units, is a promising alternative to PEDOT:PSS, as it removes the issue of acid-induced degradation of devices caused by PEDOT:PSS.
A family of star-shaped conjugated oligofluorene molecules based around a central benzene core is studied with the aim of identifying how changes in molecular structure can affect the laser ...performance of organic materials. As the oligofluorene arm length increases the optical transitions are found to move to longer wavelength, there is an increase in photoluminescence quantum yield and a corresponding reduction in the excitation density for amplified spontaneous emission. Distributed-feedback lasers based on these materials are tunable across 402-462 nm with lasing thresholds as low as 1.1 kW cm−2 and efficiencies as high as 6.6%. The laser performance is compared with that of family of star-shaped molecules with different core structure. This shows that a reduction in intermolecular interactions is very important to achieving high performance lasing in organic semiconductors.
Star‐shaped organic semiconductors with boron dipyrromethene (BODIPY) cores are color‐conversion materials for blue LED‐based visible light communications. The difference between 0 and 1 bits is ...resolved in colour‐converted data links at 100 Mb s−1. The BODIPY modulation bandwidth is eight times higher than that measured using conventional phosphor color converters.
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Star-shaped molecules are of growing interest as organic optoelectronic materials. Here a detailed study of their photophysics using fluorescence depolarisation is reported. Fluorescence ...depolarisation dynamics are studied in branched oligofluorene-truxene molecules with a truxene core and well-defined three-fold symmetry, and are compared with linear fluorene oligomers. An initial anisotropy value of 0.4 is observed which shows a two-exponential decay with time constants of 500 fs and 3-8 ps in addition to a long-lived component. The femtosecond component is attributed to exciton localisation on one branch of the molecule and its amplitude reduces when the excitation is tuned to the low energy tail of the absorption spectrum. The picosecond component shows a weak dependence on the excitation wavelength and is similar to the calculated rate of the resonant energy transfer of the localised exciton between the branches. These assignments are supported by density-functional theory calculations which show a disorder-induced splitting of the two degenerate excited states. Exciton localisation is much slower than previously reported in other branched molecules which suggests that efficient light-harvesting systems can be designed using oligofluorenes and truxenes as building blocks.
A series of copolymers containing the benzo1,2- d :4,5- d ′bis(thiazole) (BBT) unit has been designed and synthesised with bisthienyl-diketopyrrolopyrrole (DPP), dithienopyrrole (DTP), ...benzothiadiazole (BT), benzodithiophene (BDT) or 4,4′-dialkoxybithiazole (BTz) comonomers. The resulting polymers possess a conjugation pathway that is orthogonal to the more usual substitution pathway through the 2,6-positions of the BBT unit, facilitating intramolecular non-covalent interactions between strategically placed heteroatoms of neighbouring monomer units. Such interactions enable a control over the degree of planarity through altering their number and strength, in turn allowing for tuning of the band gap. The resulting 4,8-BBT materials gave enhanced mobility in p-type organic field-effect transistors of up to 2.16 × 10 −2 cm 2 V −1 s −1 for pDPP2ThBBT and good solar cell performance of up to 4.45% power conversion efficiency for pBT2ThBBT .
Two novel tetrathiafulvalene (TTF) containing compounds 1 and 2 have been synthesised via a four-fold Stille coupling between a tetrabromo-dithienoTTF 5 and stannylated thiophene 6 or thiazole 4. The ...optical and electrochemical properties of compounds 1 and 2 have been measured by UV-vis spectroscopy and cyclic voltammetry and the results compared with density functional theory (DFT) calculations to confirm the observed properties. Organic field effect transistor (OFET) devices fabricated from 1 and 2 demonstrated that the substitution of thiophene units for thiazoles was found to increase the observed charge transport, which is attributed to induced planarity through S-N interactions of adjacent thiazole nitrogen atoms and TTF sulfur atoms and better packing in the bulk.
We report electrochemical studies, spectroscopy, and electrogenerated chemiluminescence (ECL) of four monodisperse star-shaped truxene core−oligofluorene compounds (T1−T4). All oligomers produced ...stable radical anions and radical cations and showed blue ECL by ion annihilation with an intensity that could be seen with the naked eye. ECL spectra showed that all ECL emissions were at the same position as the fluorescence emission, except for T1, the compound with the shortest fluorene arms that produced some longer wavelength emission in addition to that seen in the fluorescence spectrum. When tetra-n-butylammonium oxalate was used as a coreactant for T1, the emission was much weaker than that in ion annihilation with the same long-wavelength emission observed, making it unlikely that this emission can be ascribed to excimer formation. The ECL intensity of T4 was about 80% of the common blue ECL emitter, 9,10-diphenylanthracene (DPA), under similar conditions.
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Star-shaped conjugated systems with varying oligofluorene arm length and substitution patterns of the central BODIPY core have been synthesised, leading to two families of compounds, T-B1-T-B4 and ...Y-B1-Y-B4, with T- and Y-shaped motifs, respectively. Thermal stability, cyclic voltammetry, absorption and photoluminescence spectroscopy of each member of these two families were studied in order to determine their suitability as emissive materials in photonic applications.
The synthesis of two 4‐dendralene compounds incorporating thiophene‐(p‐nitrophenyl) donor–acceptor units is presented. The dendralenes adopt two different conformers in solution and solid state and ...the transformation between the structures can be controlled by light and heat. The electron‐donating components of the dendralenes are represented by bromothienyl (in 13) and ethylenedioxythiophene(EDOT)‐thienyl (in 15) end‐groups. The most facile transformation involves the isomerisation of donor–acceptor conjugated systems (a conformers) into structures in which only the thiophenes are conjugated (b conformers), and this process is driven by ambient light. The structures of the two conformers of compound 13 are confirmed by single‐crystal X‐ray diffraction studies and the structural changes in both compounds have been monitored by 1H NMR spectroscopy and absorption studies. The transformations were found to be first‐order processes with rate constants of k=0.0027 s−1 and k=0.00022 s−1 for 13 and 15, respectively. Density functional theory calculations at the B3LYP/6‐31G* level give credence to the proposed mechanism for the a→b conversion, which involves photoinduced intramolecular charge transfer (ICT) as the key step. The EDOT derivative (15) can be polymerised by electrochemical oxidation and a combination of cyclic voltammetry and UV/Vis spectroelectrochemical experiments indicate that the a conformer can be trapped and stabilised in the solid state.
Let′s isomerise! Photoresponsive and thermally induced conformational changes of two 4‐dendralene compounds incorporating thiophene–(p‐nitrostyryl) donor–acceptor units have been studied (see figure). The most facile transformation involves the isomerisation of donor–acceptor conjugated systems into structures in which only the thiophenes are conjugated, and this process is driven by ambient light.
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