Ravi M. Adhikari

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.

Carbazole donor-carbazole linker-based compounds: preparation, photophysical properties, and formation of fluorescent nanoparticles.

A new class of highly soluble and stable compounds (1-4) has been synthesized and characterized. Compounds 1, 2, and 3 have 3,6-disubstituted carbazole as electron-donor-linked through the 2,7-positions of N-substituted carbazole with variably substituted phenyl acetylenes as electron acceptors. Compound 4 has two identical donors linked through the 2,7-position of N-substituted carbazole. These compounds absorb from UV to visible region and emit intensely from blue to green. The effect on the photophysical properties of these compounds by changing acceptors while keeping the donor and linker constant has been studied. A study of solvent effects on their photophysical properties has shown that an increase in polarity of the solvent causes a reduction of fluorescence quantum yields. With a decrease in temperature, these compounds showed increased emission intensity accompanied by a red shift of emission. Edge excitation red shift showed as much as 128 nm for compound 2 in isopropanol. Solid-state fluorescence quantum yields vary from 0.27 to 0.68 for these compounds. The fluorescence lifetimes of these compounds are solvent dependent. Compound 2 showed remarkable change in emission with concentration. Compounds 2 and 3 form highly stable fluorescent organic nanoparticles (FONs) in tetrahydrofuran/water mixtures. The general and specific solvent effect on the emission properties of these compounds are investigated by Lipert-Mataga plots. The potentiality of these compounds for use in dye-sensitized solar cells and in organic light-emitting diodes is under study in our laboratories.

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.

Aromatic Fumaronitrile Core-Based Donor−Linker−Acceptor−Linker−Donor (D-π-A-π-D) Compounds: Synthesis and Photophysical Properties

A new class of aromatic fumaronitrile core-based compounds with different donors and linkers has been synthesized and well characterized. Compounds 1 and 2 have indole and 2-phenylindole groups as electron donors, respectively. Compounds 3 and 4 have a diphenylamino group as the electron donor, and compound 5 has a 3,6-di-tert-butylcarbazole group as an electron donor. These compounds absorb in the blue-to-green region and emit in the blue-to-red region depending on the electron donor, linker, and solvents. The quantum yields of fluorescence of these compounds in solution are measured and found to be moderate, but in solid states, they are high. These compounds display strong emission solvatochromism that is reflected by a large shift in their fluorescence emission maxima on changing the solvents. This change is accompanied by a successive decrease in fluorescence intensity. The fluorescence lifetimes of these compounds are measured in different solvent and found to vary from <1 to 7 ns. Optical switching of these compounds with solvents, concentration, and excitation energy have been studied. The correlation between the functional group and optical properties has been established to some extent. The ability of these compounds to function as colorimetric and luminescence pH sensors is demonstrated with color changes and luminescence switching upon the addition of trifluoroacetic acid. The potentiality of these compounds for application in optoelectronics has been optically assessed.

Unusual photophysical properties of substituted carbazoles.

Photophysical properties of a new class of fluorescent, stable carbazoles (B1-B3, G1-G3, and R1-R2) as a function of excitation wavelength in matrixes and solution are reported. The emission maxima of B1 and B3 and G1-G3 show red shifts (6-16 nm) and substantially increased fluorescence quantum yields with a decrease in temperature from 25 to -10 degrees C. Considerable edge excitation red shift has been observed in B2. The emission of G1 shows both specific and general effects of solvent while intermolecular excitation energy transfer occurs from the naphthyl to the carbazolyl moiety in G3. Lippert-Mataga plots confirm the existence of multiple emitting states in each of these compounds.