Suresh Das

Role of Molecular Packing in Determining Solid-State Optical Properties of π-Conjugated Materials

The optical properties of π-conjugated organic molecules in their solid state are critically important in determining performance efficiencies of optoelectronic devices such as organic light-emitting diodes and organic thin-film transistors. This Perspective discusses some recent systematic explorations aimed toward arriving at an understanding of the role that molecular packing plays in determining these properties.

Halogenated Squaraine Dyes as Potential Photochemotherapeutic Agents. Synthesis and Study of Photophysical Properties and Quantum Efficiencies of Singlet Oxygen Generation

Abstract— We describe the synthesis and photophysical studies, including measurements of quantum yields of triplet excited states and singlet oxygen generation of bis(3,5‐dibromo‐2,4,6‐trihydroxyphenyl)squaraine (2) and bis(3,5‐diiodo‐2,4,6‐trihydroxyphenyl)squaraine (3). These dyes exist in solution in the protonated, neutral, single and double depro‐tonated forms, depending on pH. The pKa values of these dyes were found to be relatively lower than those of the parent bis(2,4,6‐trihydroxyphenyl)squaraine (1). Only the single deprotonated forms (Sq) of 2 and 3 showed measurable fluorescence. In microheterogeneous media such as in the presence of β‐cyclodextrin, cetyltrunethylammonium bromide and polyvinylpyrrolidone), bathochromic shifts in the absorption and emission spectra of Sq were observed with a substantial enhancement in their fluorescence yields. Triplet excited states are the main transient intermediates obtained upon 532 nm laser excitation of the various forms of 2 and 3 in methanol. These triplets have lifetimes in the range from 0.061 to 132 μs. The triplet quantum yields of double deprotonated forms are low (φT = <0.01), whereas the neutral and Sq−forms of 2 (φr = 0.12 and 0.22) and 3 (φT= 0.24 and 0.5), respectively, exhibited significant triplet yields. Quantum yields of singlet oxygen generation by Sq−forms of 2 and 3 were determined in methanol and were found to be 0.13 and 0.47, respectively, which are in good agreement with the triplet yields obtained in these systems.

Ultrafast photochemical events associated with the photosensitization properties of a squaraine dye

Abstract The photosensitization properties of a squaraine dye are elucidated by means of transient absorption and emission spectroscopy techniques. The charge injection from excited dye into the conduction band of TiO2 semiconductor occurs with the laser pulse duration of 18 ps (k > 5 × 1010 s−1). The quick recombination between the dye radical cation and the injected charge (k = 3.7 × 109 s−1) is a major limiting factor in controlling the efficiency of sensitization.

Photocatalytic degradation of waste water pollutants. Titanium dioxidemediated oxidation of a textile dye, Acid Blue 40

The photocatalyzed degradation of Acid Blue 40, a textile dye, in aqueous suspension of TiO2 has been studied by monitoring the change in dye concentration and formation of carbon dioxide as a function of irradiation time. Mechanistic studies suggest that in addition to the reaction of the hydroxyl and superoxide radicals with the dye, electron transfer from the photoexcited TiO2 to the dye forms a significant pathway in the photodegradation process.

Supergelation via purely aromatic π-π driven self-assembly of pseudodiscotic oxadiazole mesogens.

A series of highly luminescent oxadiazole-based stilbene molecules (OXD4, OXD8, OXD10, and OXD12) exhibiting interesting enantiotropic liquid crystalline and gelation properties have been synthesized and characterized. The molecules possessing longer alkyl substituents, OXD10 and OXD12, possess a pseudodisc shape and are capable of behaving as supergelators in nonpolar solvents, forming self-standing gels with very high thermal and mechanical stability. Notably the self-assembly of these molecules, which do not possess any hydrogen-bonding motifs normally observed in most reported supergelators, is driven purely by π-stacking interactions of the constituent molecules. The d-spacing ratios estimated from XRD analysis of OXD derivatives possessing longer alkyl chains show that the molecules are arranged in a columnar fashion in the mesogens and the self-assembled nanofibers formed in the gelation process.

Trigonal 1,3,4-oxadiazole-based blue emitting liquid crystals and gels.

Star-shaped molecules consisting of a 1,3,4-oxadiazole core derivatized with alkoxy-substituted phenyl ethynylenes, FD12 (dodecyl) and FD16 (hexadecyl) were synthesized. These molecules exhibited enantiotropic columnar mesophases over a wide temperature range, with the liquid crystalline phases exhibiting strong blue fluorescence. On cooling, FD12 transformed into a transparent glass at room temperature wherein the liquid crystalline texture was retained. The glassy film remained stable over a period of one year and exhibited blue luminescence with an absolute quantum yield of 26%. The oxadiazole derivatives formed stable luminescent gels in decane and study of their morphology by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicated formation of interlocked network of self-assembled fibers. X-ray diffraction (XRD) analysis of the xerogel of these derivatives indicated oblique columnar ordering of the molecules within the fibers. The length of the alkyl substituent was observed to have a significant effect on the absorption and fluorescence properties of the gels, which was attributable to the role of the alkyl substituents in controlling the nature of the molecular packing within the self -assembled fibers of the gels.

Fluorescence enhancement of bis(2,4,6-trihydroxyphenyl)squaraine anion by 2 : 1 host–guest complexation with β-cyclodextrin

β-Cyclodextrin (β-CD) forms a 2: 1 inclusion complex with the anion of bis(2,4,6-trihydroxyphenyl)squaraine dye (Sq), in aqueous solutions, bringing about a significant enhancement in the fluorescence yield (ca. 90-fold) and the stability of the dye anion. Apart from the effect of the hydrophobic environment and the decrease in rotational freedom brought about by β-CD encapsulation, intramolecular hydrogen bonding between the OH groups on the phenyl rings and the oxygen atoms in the central cyclobutane ring in the anion guest molecule can significantly contribute to the stiffening of the molecule in the excited state. In the absence of β-CD, intermolecular hydrogen bonding with solvent molecules disrupts the intramolecular hydrogen-bonding processes, bringing about a decrease in the fluorescence yield of the anion.

Tuning Microstructures in Organogels: Gelation and Spectroscopic Properties of Mono- and Bis-cholesterol-Linked Diphenylbutadiene Derivatives†

The gelation and photophysical properties of mono- and bis-cholesterol derivatives linked to diphenylbutadiene have been investigated. Scanning electron microscopy of xerogels of the monocholesterol derivatives indicated that these molecules self-assemble into 3D networks consisting of helically twisted fibers. In contrast, the morphology of xerogels of the bis-cholesterol derivatives indicated agglomerated spheres. In concentrated solutions (> 10(-4) M), the self-assembled superstructure of the monocholesterol derivatives consists of helically twisted fibers whereas that of the bis-cholesterol derivatives indicated clustered spheres. An investigation of spectroscopic properties suggests that the morphology of the superstructures formed in these systems may be correlated to the nature of the molecular aggregates formed. Absorption and emission spectral studies as a function of concentration and temperature suggested the formation of predominantly J-type aggregates in the monocholesterol and H-type aggregates in the bis-cholesterol derivatives. It is proposed that the slipped stack arrangement within the J aggregates of the monocholesterol derivatives resulted in the formation of helically twisted fibers, whereas the cofacial arrangement in the H aggregates of the bis-cholesterol derivatives could prevent such fiber formation, resulting in the formation of the agglomerated spheres.

Observation of a non-conventional Horner–Wadsworth–Emmons olefination product and the effect of the lateral ethyl substitution on the solid state fluorescence

Donor-acceptor-substituted diphenylbutadienes, namely 1-(p-methoxyphenyl)-4-(p-cyanophenyl)buta-1E,3E-diene (MCB) and 1-(p-N,N-dimethylaminophenyl)-4-(p-cyanophenyl)buta-1E,3E-diene (ACB) exhibit fluorescence properties in their solid that are distinctly different from their fluorescence in solution. The red-shifted absorption and emission spectra observed in the solid state are attributed to the formation of J aggregates. Preparation of these derivatives via a Horner–Wadsworth–Emmons reaction, wherein the phosphonate obtained by treating p-cyanobenzyl bromide with triethyl phosphite was condensed with corresponding cinnamaldehydes, also yielded the non-conventional olefination products 1-(p-methoxyphenyl)-4-(p-cyanophenyl)-4-(ethyl)buta-1E,3E-diene (MCBE) and 1-(p-N,N-dimethylaminophenyl)-4-(p-cyanophenyl)-4-(ethyl)buta-1E,3E-diene (ACBE), which bear an ethyl group substituent on their butadiene chain. The formation of these products suggests a base-catalyzed 1,3-migration of an ethyl group from an oxygen center to the benzylic position in the initially formed phosphonate. The presence of the ethyl group in an otherwise planar molecule was observed to significantly hinder aggregation in the solid state, resulting in molecule-like fluorescence even in their bulk state.

Light induced generation of stable blue phase in photoresponsive diphenylbutadiene based mesogen

Light induced isothermal phase transition leading from smectic A* phase to a stable blue phase was achieved via photoisomerization of a chiral diphenylbutadiene based mesogen.