Bipin K. Shah

Photophysical Study of Blue, Green, and Orange-Red Light-Emitting Carbazoles(1)

Simple synthetic procedures have been developed to prepare suitably substituted stable carbazoles B1-B3, G1-G3, and R1-R3. These compounds emit blue, green, and orange-red light, respectively, and show a red-shifted emission in the solid state relative to that in solution. The extent of the shift is highly dependent on the nature and the positions of the substituents. A red-shift as high as 120 nm can be achieved by a suitable substitution, especially by N-substitution of carbazole. The presence of a carbaldehyde or malononitrile group on the carbazole moiety is found to quench fluorescence severely in solution and in the solid state, as indicated by low fluorescence quantum yields of B1 (phi(F) approximately 0.03), B3 (phi(F) approximately 0.04), and G1-G3 (phi(F) approximately 0.04-0.15). However, the effect is not the same for the fluorescence lifetime (tau(F) approximately 1-5.69 ns). The rate constants of radiative and nonradiative deactivation of B1-R3 have been found to be in the range of 6.40 x 10(6) to 9.50 x 10(8) and 1.38 x 10(8) to 9.84 x 10(8), respectively. Lowering the temperature from 25 to -10 degrees C causes a small but distinct red-shift in the emissions and a systematic increase in the phi(F) values of the blue and green emitters. Solvatochromism and concentration-dependent emissions of the compounds are also discussed.

Solvent Dependant Optical Switching in Carbazole-Based Fluorescent Nanoparticles

Suitably substituted ethynylphenyl carbazoles (PBM and PPM) form stable fluorescent organic nanoparticles. The emission of the nanoparticles can be reversibly switched on/off in the blue-green and orange-red regions by a change in the ratio of the tetrahydrofuran/water system used in their preparation. The size of the nanoparticles was found to be dependent on the solvent ratio, and the emissions were significantly red-shifted compared to those of dilute solutions of PBM and PPM in tetrahydrofuran. This is attributed to the formation of intermolecular charge transfer complexes in the nanoparticle state. The application of the nanoparticles as a chemical vapor sensor has been suggested.

61.4: Anthanthrene Derivatives for Stable Blue-Emitting Organic Electroluminescent Devices

Anthanthrene derivatives have been synthesized as a new class of organic light emitting diode (OLED) blue emitters with a wide range of emission. The photo physical properties of the new emitter are reported. The EL device containing these new emitter produced EL efficiency of 3.0cd/A with CIE coordinates of 0.13 and 0.25. The luminance half life is more than 3500 hours with an initial luminance around 600 cd/m2.

Understanding the Mechanisms of Photodecarbonylation of the Photoprecursors of Higher Poly(acene)s

Polyacenes, linear poly(benzenoid) hydrocarbons, are significant compounds for various electronic applications. Smaller lower molecular weight members of the series (anthracene, tetracene, and pentacene) have been extensively investigated for their use as semiconductors, as active layers in organic field-effect transistors (OFETs) and in organic light-emitting diodes (OLEDs). Smaller HOMO-LUMO gaps might be achieved by synthesizing even higher poly(acene)s such as hexacene and heptacene. Nevertheless, an increased instability and insolubility associated with the increasing number of fused rings has become a synthetic challenge for chemists. We have recently developed a photochemical route that allows one to easily synthesize heptacene in rigid media from an α-diketone photoprecursor, thus opening a pathway to utilize these compounds in the construction of suitable devices. Photodecarbonylations of α-diketones are unusual reactions. In most instances, dione excited states become electron transfer acceptors. The decarbonylation is driven by the stability of the resulting aromatic hydrocarbon. We have studied the nature of the excited states involved in photodecarbonylation of the α-diketones and the structure of the carbon monoxide fragment(s). Results obtained on the mechanism of photodecarbonylation investigated using nanosecond flash photolysis and ultra-fast pump-probe techniques are discussed.

P-184: Photophysical Properties of Some Newly Synthesized Blue-Emitting Polyaromatic Hydrocarbons

Polycyclic aromatic hydrocarbons are important compounds for electronic applications. 7,16-Dihydroheptacene derivatives (1 – 3) and anthanthrene (4) and derivatives (5 – 9) were synthesized and their emission properties compared in solution (λmax = 426 – 467 nm, φF = 0.15 – 0.56, τF = 2.35 – 4.21 ns) and in the solid state (λmax = 420 – 468 nm, φF = 0.375 – 0.86). 1 – 9 are stable blue emitters, especially in the solid state, and are promising materials as the emissive layer in organic light emitting diodes (OLEDs).