A New Molecular Design Based on Thermally Activated Delayed Fluorescence for Highly Efficient Organic Light Emitting Diodes.

Two benzoylpyridine-carbazole based fluorescence materials DCBPy and DTCBPy, bearing two carbazolyl and 4-(t-butyl)carbazolyl groups, respectively, at the meta and ortho carbons of the benzoyl ring, were synthesized. These molecules show very small ΔEST of 0.03 and 0.04 eV and transient PL characteristics indicating that they are thermally activated delayed fluorescence (TADF) materials. In addition, they show extremely different photoluminescent quantum yields in solution and in the solid state: in cyclohexane the value are 14 and 36%, but in the thin films, the value increase to 88.0 and 91.4%, respectively. The OLEDs using DCBPy and DTCBPy as dopants emit blue and green light with EQEs of 24.0 and 27.2%, respectively, and with low efficiency roll-off at practical brightness level. The crystal structure of DTCBPy reveals a substantial interaction between the ortho donor (carbazolyl) and acceptor (4-pyridylcarbonyl) unit. This interaction between donor and acceptor substituents likely play a key role to achieve very small ΔEST with high photoluminescence quantum yield.

Quantifying the photothermal efficiency of gold nanoparticles using tryptophan as an in situ fluorescent thermometer.

The photothermal efficiencies, denoting the efficiency of transducing incident light to heat, of gold nanoparticles of different diameters (∅ = 22-86 nm) were quantified upon exposure at 532 nm. The fluorescence of tryptophan at 300-450 nm upon 280 nm excitation serves as an in situ fluorescent thermometer to illustrate the evolution of the average temperature change in the heating volume of the nanoparticle solution. The fluorescence intensity decreases as the temperature increases, having a linear gradient of 2.05% fluorescence decrease per degree Celsius increment from 20 to 45 °C. The presence of gold nanoparticles at the nM level does not perturb the temperature-dependent fluorescence of tryptophan in terms of fluorescence contour and temperature response. The heating volume was defined by overlapping the collimated 532 nm laser (∅ = 0.83 mm) for exciting the nanoparticles and the 280 nm continuous-wave beam (∅ = 0.81 mm) for exciting tryptophan in a 2 mm × 2 mm square tube, and the fluorescence was collected perpendicularly to the collinear alignment. This method has satisfactory reproducibility and a sufficient temperature detectivity of 0.2 °C. The profiles of the average temperature evolution of the mixtures containing nanoparticles and tryptophan were derived from the evolution of fluorescence and analyzed using collective energy balancing. The relative photothermal efficiencies for different sizes of gold nanoparticles with respect to the 22 nm nanoparticle agree with those predicted using Mie theory. The employment of tryptophan as a fluorescent thermometer not only provides an in situ tool to monitor the photothermal effect of nanostructures but is also applicable to thermal imaging in biological applications.