Fluorescent Red‐Emitting BODIPY Oligofluorene Star‐Shaped Molecules as a Color Converter Material for Visible Light Communications

Abstract

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.

Ultrafast Electronic Energy Transfer in an Orthogonal Molecular Dyad

“…Our investigations demonstrate that the different broadenings for the two absorption peaks of EET. The same theoretical approach used here would also allow detailed studies on EET in related molecular dyads of slightly larger size, 57 and of further ultrafast photophysical processes in molecular dyads that are for example composed of BOD as fluorophore and of diarylethenes as photoswitch. 58,59 This could elucidate the role of different underlying light-induced processes on a molecular level.…”

BODIPY star-shaped molecules as solid state colour converters for visible light communications

“…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 wavelengths.The 375 nm excitation is absorbed by the oligofluorene arms, and results in a fast energy transfer to the emitting state that is delocalised across the BODIPY core. The energy transfer is very fast and cannot be resolved by the TCSPC apparatus.…”

“…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.…”

Organic solar cells as high-speed data detectors for visible light communication

“…They offer relatively easy bandgap tuning through the visible which could allow a selective response to wavelengths relevant for VLC. Such receivers could therefore have great implications for the connectivity of future smart devices, from sensor networks to smart clothing, many of which could become selfpowered units as part of the 'Internet of Things'.Organic semiconductor materials have recently attracted much attention in the field of VLC [9][10][11]. They offer tremendous scope for simple processing and integration on different substrates, and have been used as colour converters to generate white light with a blue LED backlight.…”

“…Organic semiconductor materials have recently attracted much attention in the field of VLC [9][10][11]. They offer tremendous scope for simple processing and integration on different substrates, and have been used as colour converters to generate white light with a blue LED backlight.…”