The fused, bicyclic molecule, 2,1,3‐Benzothiadiazole (BT), has become a key ingredient in the design of new organic semiconductors for light emission and energy harvesting applications. Here, the synthesis is reported of a series of trigonal, star‐shaped compounds comprising a truxene core and three quater‐dialkylfluorene arms into each of which a BT unit is inserted sequentially at each possible position (T4BT‐A to T4BT‐E). Analysis of the resulting electronic properties shows that as a consequence of conjugative coupling to the core and the resulting symmetry there are three distinct locations for the BT unit and the influence that these locations have on light emission and other spectroscopic characteristics is discussed. The systematic variation in photophysical properties for the different structural isomers helps to clarify the influence of BT unit addition to 9,9‐dialkylfluorene chains. It also helps to establish a design template for the construction of donor‐acceptor conjugated materials with targeted properties. For T4BT‐E with a BT unit at the terminal position of each arm, the photoluminescence quantum efficiency is significantly reduced and no amplified spontaneous emission is observed under typical pumping conditions. Theoretical calculations assist in understanding the variation in behaviors among the T4BT‐X family of compounds, especially in relation to their photoluminescence decay times and the Raman scattering intensities of their dominant BT‐unit‐centred molecular vibrations.

at high effi ciency and brightness, but the long excited state lifetime of the phosphors restricts modulation frequency and hence the transmission rate. [ 2 ] The photoluminescence (PL) lifetime typical of the phosphor materials used is on the microsecond timescale, which limits the system bandwidth to a few megahertz (MHz). [ 5,6 ] There is currently a strong need for alternative materials for fast color converters, which can combine high illumination performance with short PL lifetimes. This combination of properties implies that the key fi gure of merit for color converter materials is a short natural radiative lifetime.Organic semiconductors offer many potential advantages as a novel color converter for VLC due to their visible band gaps, short radiative lifetime, and high photoluminescence quantum yield (PLQY). They have been applied as a color converter in vacuum-evaporated OLEDs [ 7 ] and we recently demonstrated the use of a yellow emitting conjugated polymer as a fast additive color converter for VLC. [ 8 ] To achieve high-quality sources for good color rendering, suitable materials with green or red PL are also needed to mix with the blue emission from the LED.In this paper, we explore the potential of star-shaped organic semiconductors based on boron dipyrromethene (BODIPY) cores as red-conversion material for LEDs in VLC applications. We selected BODIPY as the core of the molecules because it is a red emitter of known stability. [ 10 ] We investigate the infl uence of different substitution patterns of the BODIPY core with oligofl uorene arms, leading to different shapes of the molecules (T-and Y-shaped). In particular, we study the photophysical properties of these materials, measure their modulation bandwidth, and demonstrate their potential for VLC by achieving a data rate ten times higher than measured with a conventional phosphor color converter. BODIPY (4,4-difuoro-4-bora-3a,4a-diaza-s-indacene)-based organic semiconductors have attracted signifi cant attention due to their electronic and optical properties with narrow absorption and emission bands, high PLQY, and good thermal stability. [ 9 ] Furthermore, they are solution-processable unlike previous red organic color converter materials. [ 7 ] They have been applied in many fi elds such as biology, [ 11 ] medicine, [ 12 ] solar energy, [ 13 ] and lasers [ 10 ] . BODIPYs can have red emission with lifetimes in the range of a few nanoseconds and can strongly absorb in the range of the GaN LEDs electroluminescence (≈450 nm) making them suitable candidates to be used in three-component additive coloration for VLC.

Copyright © and Moral Rights are retained by the author(s) and/ or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This item cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder(s). The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders.This document is the author's post-print version, incorporating any revisions agreed during the peer-review process. Some differences between the published version and this version CURVE is the Institutional Repository for Coventry University may remain and you are advised to consult the published version if you wish to cite from it. They are broadband visible emitters whose emission can be tuned by changing the chemical structure, and can have high photoluminescent quantum yields (PLQY) of up to 90% in undiluted films 5,9,10 . This combined with simple and low cost solution processing techniques make them attractive materials for optoelectronic devices.An emerging application area for organic semiconductors is in the field of visible light communications (VLC).Increasing demand for wireless communications has driven research into improving data transmission concepts 3The oligofluorene arms have a substituent effect on the BODIPY core and therefore the increase in the number of arms influences the photophysics of the material.Solid state colour converters were spin-coated from solution to make thin films on quartz substrates for the photophysical studies. Films of thickness ca. 100 nm were deposited from solutions at concentrations of 10 and 20 mg/ml for photophysical and communications measurements respectively. These were spin-coated at 1500 rpm for 60 seconds. Figure 1 (bottom) shows the absorption and PL spectra of the films for each molecule. The absorption spectra show peaks at 450 and 620 nm for Y1; 350, 473 and 626 nm for Y2; 350, 477 and 625nm for Y3; and 366, 480 and 625nm for Y4. The absorption band below 400 nm is attributed to the fluorene arms and increases in intensity with arm length. Y1 has a peak in absorption at 450 nm matching well the emission of the blue LEDs used for lighting, and this feature shifts slightly to longer wavelengths for longer arms as seen in Figure 1. The peak emission wavelengths were in the red region of the spectrum, 663, 679, 682, 682 nm for Y1, Y2, Y3and Y4 respectively. The red PL emission comes from an extended conjugation across the BODIPY core and adjacent fluorene units as has been described previously 24 . The bathochromic shift from Y1 to Y2 is also evident in solution 23,24 and indicates that there is a further delocalisation of the excited state across the BODIPY and neighbouring fluorene units.Time-resolved fluorescence measurements were conducted using the time correlated single photon counting (TCSPC) technique, exciting the materials at 375 nm and detecting at the corresponding peak PL...

We report a study of blends of semiconducting polymers as saturated red color converters to replace commercial phosphors in hybrid Light emitting diodes (LEDs) for visible light communication (VLC). By blending two star-shaped organic semiconductor molecules, we found a near complete energy transfer (>90% efficiency) from the green-emitting truxene-cored compound T4BT-B to the red-emitting boron dipyrromethene (BODIPY) cored materials. Furthermore, we have demonstrated the capability of these materials as fast color converters for VLC by measuring their intrinsic optical modulation bandwidth and data rate. The measured 3 dB modulation bandwidth of blends (∼55 MHz) is more than 10 times higher than commercially available LED phosphors and also higher than the red-emitting BODIPY color converter alone in solution. The data rate achieved with this blend is 20 times higher than measured with a commercially available phosphor based color converter

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