Aggregation Of Chromophores Research Articles

A study of the spectral and luminescence manifestations of the aggregation of thymine chromophores in polythymidylic acid aqueous solutions at room temperature in the context of photochemical information recording using thymine

The luminescence and excitation spectra of polythymidylic acid aqueous solutions at room temperature were studied. In addition to the previously described band at 338 nm, two new bands at 320 and 350 nm were recorded at various excitation wavelengths. An examination of the excitation spectra that had not been studied previously, as well as their comparison with the differential absorption spectra previously recorded during photodimerization, allowed us to interpret the band at 320 nm as the band of noninteracting chromophores; the band at 338 nm as the band of the most photochemically active, densely packed stacking dimmers (exciton splitting of absorption band of ∼4000 cm−1); and the band at 350 nm as the band of photochemically inactive large stacking aggregates (n ≥ 10, exciton splitting of ∼8000 cm−1). The changes in the optical density of the polythymidylic acid aqueous solution at γ = 270 nm after successive irradiation of the solution with light at 279 + 302 and 248 nm were studied. The reasons for their incomplete reversibility are discussed.

Efficiently Site-Isolated Two-Photon Absorbing Dendrimer with Stilbazolium Chromophore

To prevent the chromophore aggregation, a dendritic stilbazolium salt-type chromophore was synthesized via base-catalyzed coupling reaction of trihydroxy-functionalized tetraphenylborate stilbazolium salt core with appropriate peripheral Fréchet-type dendrons G3-benzyl bromide. The casted solid film of the resulting two-photon absorbing dendrimer showed high fIuorescence efficiency and optical power limiting activity which were not observed from the bare chromophore without dendrons. This result is attributed to the site isolation effect of its dendritic structure. By introducing this chromophore we have successfully fabricated micropatterns with high resolution by two-photon polymerization.

Properties of Polyethylene Glycol Supported Tetraarylporphyrin in Aqueous Solution and Its Interaction with Liposomal Membranes

5,10,15,20-Tetrakis(4-hydroxyphenyl)porphyrin was functionalized by covalent attachment of poly(ethylene glycol) (PEG) chains of various molecular weights, 350, 2000, and 5000 Da. The properties of PEG-functionalized tetraarylporphyrins in aqueous solution and their interactions with liposomes have been studied. Electronic absorption spectroscopy, dynamic light scattering, atomic force microscopy, and fluorescence quenching were used to monitor aggregation of porphyrin chromophores and behavior of the attached PEG chains in the aqueous solution. The tendency for aggregation of porphyrin chromophores in aqueous solution and the efficiency of fluorescence quenching by KI decrease with increasing length of PEG chain linked to the porphyrin ring. The experimental results indicate that polymer clusters are present in aqueous solution of all pegylated porphyrins. The interactions between the pegylated porphyrins and phosphatidylcholine liposomes in the aqueous solution were studied using the fluorescence methods. The apparent binding constants of porphyrin chromophores to liposomes were determined. The degree of binding was found to be dependent on the molecular weight of the attached polymer.

Influence of Architecture, Concentration, and Thermal History on the Poling of Nonlinear Optical Chromophores in Block Copolymer Domains

Factors affecting the electric-field-induced poling of nonlinear optical chromophores in block copolymer domains were investigated by encapsulating the chromophores in a linear−diblock copolymer [poly(styrene-b-4-vinylpyridine)] and linear−dendritic (poly(methyl methacrylate)−dendron) block copolymer via hydrogen bonding. Temperature-dependent Fourier transform infrared spectroscopy and morphology evaluation by X-ray scattering and transmission electron microscopy were used with in situ second harmonic generation to correlate domain architectures, processing conditions such as thermal history, and chromophore concentrations with poling efficiency. Poling of chromophores encapsulated in the minority domain (spheres or cylinders) of a linear−diblock copolymer was inhibited by the increasing chromophore concentration within the domain and the chemical nature of the majority domain. Chromophore encapsulation in the majority domain produced the most favorable conditions for poling as measured by in situ second harmonic generation. Thermal annealing of the linear−diblock copolymer/chromophore composites resulted in chromophore aggregation with a corresponding decrease in nonlinear optical activity. The linear−dendron/chromophore system presented the most effective architecture for spatially dispersing chromophores. These findings suggest that while well-ordered phase-separated systems such as block copolymers enhance chromophore isolation over homopolymer systems, a more effective approach is to explore polymer chains end functionalized with chromophores.

The photochemistry of some main chain liquid crystalline 4,4′-stilbene dicarboxylate polyesters

Several aspects of the photochemistry and photophysics of four main chain liquid crystalline polyesters with a rigid trans-stilbene 4,4'-dicarboxylate mesogen as chromophore and flexible spacer groups are reported. The three polymers with the longest 'spacer' groups are liquid crystalline at room temperature, two have smectic phases. Chromophore aggregation has a dramatic effect on the photophysics and photochemistry of these polymers. Each of the polymers in poor solvents or as films has greatly perturbed UV-Vis absorption and fluorescence spectra due to aggregation of the stilbene chromophore. These effects are more pronounced upon annealing above the glass transition temperature, T(g), and in the mesophase. Film fluorescence is excitation wavelength dependent and is suppressed at elevated temperatures. The stilbene 'environment' in both films and solution is clearly heterogeneous and energy transfer processes relatively slow. The dominant photochemical reaction upon direct excitation above 300 nm is 2 + 2 photocycloaddition rendering polymer films insoluble. No significant trans-to-cis photoisomerization can be detected upon initial irradiation of the polymer films. There is evidence for the formation of aldehyde and carboxylate functionality upon irradiation in the presence of air. Loss of the aggregate UV-Vis absorption and fluorescence occurs during irradiation. Difference UV-Vis spectra of irradiated films suggest preferential initial consumption of dimeric aggregates. Loss of stilbene UV-Vis absorption upon irradiation above 300 nm can be partly photoreversed upon subsequent 254 nm irradiation. The rate of stilbene chromophore loss from films increased significantly above Tg and in the smectic phase above room temperature.

Twisted π-Electron System Electrooptic Chromophores. Structural and Electronic Consequences of Relaxing Twist-Inducing Nonbonded Repulsions

The synthesis, structural and spectroscopic characterization, and nonlinear optical response properties of a “slightly” twisted zwitterionic 4-quinopyran electrooptic chromophore FMC, 2-{4-[1-(2-propylheptyl)-1H-pyridine-4-ylidene]cyclohexa-2,5-dienylidene}malononitrile, are reported. X-ray diffraction data and density functional theory (DFT) minimized geometries confirm that deletion of the four o-, o′-, o′′-, and o′′′-methyl groups in the parent chromophore TMC-2, 2-{4-[3,5-dimethyl-1-(2-propylheptyl)-1H-pyridin-4-ylidene]-3,5-dimethylcyclohexa-2,5-dienylidene}malononitrile, relaxes the arene−arene twist angle from 89.6 to 9.0°. These geometrical changes result in a significantly increased contribution of the quinoidal structure to the molecular ground state of FMC (versus TMC-2), reduced solvatochromic shifts in the optical spectra, and a diminished electric-field-induced second-harmonic (EFISH) generation derived molecular hyperpolarizability (μβ = −2340 × 10−48 esu of DFMC, the dendrimer derivative of FMC, vs −24000 × 10−48 esu of TMC-2) in CH2Cl2 at 1907 nm. Pulsed field gradient spin–echo (PGSE) NMR spectroscopy and EFISH indicate that the levels of FMC aggregation in solution are comparable to those of TMC-2 (monomers and dimers) in CH2Cl2 solution. B3LYP and INDO/S computation of chromophore molecular structure, aggregation, and hyperpolarizability trends are in good agreement with experiment.

Theoretical Investigations of the Effects of J-Aggregation on the Linear and Nonlinear Optical Properties ofE-4-(4-Dimethylaminostyryl)-1-methylpyridinium [DAMS+

J-type aggregation of organic chromophores into inorganic host matrixes provides a useful route toward materials showing strong second-order nonlinear optical (NLO) response. The increased NLO response of J-aggregates is related to the peculiar arrangement of the NLO-phores into the host matrix, which produces the appearance of a narrow and intense band in the material electronic absorption spectrum, red-shifted with respect to the main absorption band of the isolated NLO-phore. A theoretical investigation, based on DFT, TDDFT, and ZINDO calculations on the relationship between the structural features of various [DAMS+] ([DAMS+] = E-4-(4-dimethylaminostyryl)-1-methylpyridinium) dimeric or oligomeric aggregates and their linear and nonlinear optical properties shows that the appearance of a new red-shifted absorption band, typical of J-aggregation, is associated with interchromophoric transitions of charge-transfer character, due to the splitting of HOMO and LUMO levels. The intensity of this latter band i...

Chromophore aggregation and photoinduced dichroism in sol–gel films

Photoinduced dichroism (PID) and UV–visible spectroscopic studies have been performed in silica-based thin films prepared by the sol–gel technique, containing Disperse Red 1 (DR1) as trans–cis photoisomerizable chromophore and carbazole as a plasticizer. The absorbances parallel and perpendicular to the induced anisotropy axis, A∥ and A⊥, have been monitored during illumination. Some of the samples show an increase of the average absorbance T0=(A∥+2A⊥)/3 over the initial value. This effect can be interpreted by the presence of aggregates of DR1 molecules, breaking down during illumination and increasing the absorption at the probe wavelength. The formation of the aggregates is evidenced by a blue-shift in the absorption spectra. The carbazole apolar group acts as spacers between the DR1 molecules, effectively avoiding the formation of aggregates. Model equations have been proposed to describe the kinetics of the system and the initial PID data have been fitted by using a simple phenomenological model.

Aggregates of Naphthalene Chromophores in Poly(vinylalcohol)-graft-poly(vinylnaphthalene) Pseudomicelles

A novel amphiphilic grafted copolymer, poly(vinylalcohol)-graft-poly(vinylnaphthalene), was synthesized and studied spectroscopically. The unusual photophysical properties of its aqueous solutions were observed for the first time and are attributed to the aggregation of the naphthalene chromophores in the bulk of polymer pseudomicelles. The enormous red shift of the fluorescence spectra is interpreted in terms of the confinement effect. The interpretation is supported by model calculations, taking into account the mixing of Frenkel excitons and charge-transfer states of chromophore clusters in densely packed polymer domains. Gradual funneling of the excitation energy between aggregates of different sizes is discussed in the context of the measured fluorescence depolarization spectra.

Sequential Self-Organization of Silver(I) Layered Materials with Strong SHG by J Aggregation and Intercalation of Organic Nonlinear Optical Chromophores through Mechanochemical Synthesis

The inorganic−organic hybrid layered material of stoichiometry [DAMS+][Ag5I6-], characterized by a strong SHG activity, has been prepared mechanochemically by room-temperature ball-milling a mixture of AgI and [DAMS+][I-], eventually followed by a moderate heating. By means of electronic absorption spectra, XRPD, and second harmonic generation (SHG) experiments we proved that the intralayer ordering of the [DAMS+] nonlinear optical chromophores into J-aggregates precedes the interlayer one, on the whole producing, by a sequential process, the “macropolarity” responsible for the strong SHG. The mechanochemical approach was shown to be of general use, as it was successfully applied to the synthesis of the analogous [DAMS+][Cu5I6-], [DAZOP+][Ag5I6-], and [DAES+][Ag5I6-] materials.

Aggregation: A new mechanism of relaxation of polar order in electro-optic polymers

Second order nonlinear optical properties of poled thin films of a guest–host system consisting in a charge transfer chromophore dissolved in poly bis(phenol A carbonate) matrix were studied by optical second harmonic generation and as function of chromophore content. The chromophores were oriented by the corona poling technique. The optical microscopy observations evidence a new mechanism of relaxation, which arises from the accelerated chromophore aggregation due to the heating which facilitates their diffusion.

Colloidal Sphere Formation, H-Aggregation, and Photoresponsive Properties of an Amphiphilic Random Copolymer Bearing Branched Azo Side Chains

This work studied the colloidal sphere formation, H-aggregation, and photoresponsive properties of an amphiphilic random copolymer functionalized with branched azo side chains (POAPB6P-AC). The colloidal spheres were prepared through gradual hydrophobic association of the polymer chains in THF−H2O media, which was induced by a continuous increase in the water content. The forming process and morphology of the colloidal spheres were characterized by DLS, SLS, and TEM. UV−vis spectroscopy was used to study the azo chromophore H-aggregation and structure variation in the colloid formation process by measuring the parameters such as λmax, the trans-to-cis photoisomerization rate, and the isomerization degree at the photostationary state. The results indicate that the colloidal sphere formation undergoes several different stages as the water content increases. The polymeric chains start to associate at the critical water content (CWC). CWC is related with the initial concentration of the polymer in THF and is estimated to be 23 vol % when the initial concentration is 1.0 mg/mL. When the water content increases beyond the CWC, more and more polymer chains change from “isolated” single chains to clusters of associated chains. When the water content reaches 37%, almost all polymer chains are involved in the clusters. As the water content further increases, the clusters experience a collapse caused by the strong hydrophobic interaction, which is indicated by a dramatic size contraction detected from the DLS measurements. When the water content reaches about 60 vol %, the azo chromophores of POAPB6P-AC start to form H-aggregates, which is indicated by a significant blue shift (from 360 to 339 nm) of the UV−vis maximum absorption band. The structure variation can also be detected from the photoisomerization behavior of the systems. When the water content changes from 50 to 60 vol %, both the trans-to-cis photoisomerization rate and the isomerization degree at the photostationary state decrease dramatically. Finally, when the water content increases from 95 to 100 vol %, the isomerization rate and degree significantly decrease again. In this stage, THF in the microphase is almost completely replaced by H2O, and colloidal structure is “frozen” because of the free volume decrease and chain entanglement. The above observations give insight into the occurrence of H-aggregation in colloids of amphiphilic azo polymers for the first time. The understanding can lead to further development in expanding functionalities of the photoresponsive colloidal spheres through manipulation of the chromophore aggregation.

Nanostructuration of Phenylenevinylenediimide-Bridged Silsesquioxane: From Electroluminescent Molecular J-Aggregates to Photoresponsive Polymeric H-Aggregates

A new approach to control molecular aggregation of pi-conjugated chromophores in the solid state has been investigated. Our strategy was to use a modifiable bulky fragment which should induce a J-aggregation and offer the possibility to reach an H-aggregation upon its chemical modification by lateral slip of pi-conjugated molecules. The chosen fragment for that purpose was the hydrolyzable triethoxysilane function (Si(OEt)3). Our objective was to design and synthesize electroluminescent or solar cell hybrid organic-inorganic materials by the sol-gel process applied to a bifunctionalized silane. With this intention, the synthesis of the sol-gel processable phenylenevinylenediimide silsesquioxane 6 was accomplished and the study of spin-coated thin films of the pure silane precursor subjected or not to the sol-gel process has been carried out. Optical properties of 6 are consistent with the formation of J-aggregates in the solid state due to the steric hindrance introduced by the triethoxysilane units. Conversely, the spectroscopic behavior observed for the hybrid film 6F is attributed to an H-aggregation corresponding to a "card pack" orientation of the distyrylbenzeneimide chromophores in the compressed silicate network. Morevover, 6 and 6F also exhibited different electronic behaviors: light-emitting diodes exhibited high brightness with the native precursor 6 and almost no light output with the sol-gel processed silsesquioxane 6F. Photovoltaic cells showed the opposite behavior with low photocurrent generation in the precursor case and higher photocurrents with the sol-gel processed layers. These results provide a deeper understanding of the present self-assembly process that is strongly governed by the molecular packing of the oligosiloxane precursor.

Novel dendritic light-emitting materials containing polyhedral oligomeric silsesquioxanes core

A new series of star-like light-emitting materials (POSS1, POSS2 and POSS3) were synthesized by hydrosilylation of the polyhedral oligomeric silsesquioxane with 4,4′-bis(4-(di-(4-methylphenyl)amino)styryl)-2-(hexan-1-yloxy)-5-(10-undecen-1-yloxy)benzene (C1), 4,4′-bis[(E)-2-( N-ethylcarbazoyl)ethenyl]-2-(hexan-1-yloxy)-5-(10-undecen-1-yloxy)benzene (C2), and Iridium(III) bis(2-phenylpyridine-C 2- N′) (13-teradecenyl acetonate) (C3), respectively. All synthesized materials are soluble in common organic solvents, such as chloroform, toluene and 1,2-dichloroethane, and exhibit good film-forming properties. Therefore, they can be used to fabricate devices by spin-coating. The aggregation of peripheral chromophores can be prevented by the rigid POSS core. A double-layer, light-emitting diode with the configuration of indium–tin oxide/poly(ethylene 3,4-dioxythiophene)/POSS1/Ca/Al was fabricated using POSS1 as the active layer. The device emitted green light with a maximum brightness of 115 cd/m 2 and a current yield of 0.07 cd/A. When the light-emitting layer is blended with 0.8% electron-transport material, 2-(4′- tert-butylphenyl)-5-(4′-diphenyl)-1,3,4-oxadiazole, the maximum brightness and current yield of the device can reach 1469 cd/m 2 and 0.8 cd/A, respectively. The performance of a POSS2 device with the same device structure can reach 1102 cd/m 2 and 0.88 cd/A. POSS3 is a kind of triplet material and blended with 4,4- N,N′-dicarbazole-biphenyl as host material, to fabricate the light-emitting device. The maximum brightness and current yield reach 1008 cd/m 2 and 1.04 cd/A, respectively. Blending with 1,3,5-tris(2- N-phenylbenzimi dazolyl)benzene as the hole-blocking material yields a maximum brightness and current yield of 1172 cd/m 2 and 3.99 cd/A.

A Tetrameric Sugar-Based Azobenzene That Gels Water at Various pH Values and in the Presence of Salts

A novel low-molecular mass tetrameric sugar derivative containing azobenzene core, 1, showed pronounced hydrogelation at micromolar concentration. Based on this observation, four related azobenzene based tetrameric sugar derivatives, 4-7, and three tetrameric sugar derivatives with a bis-terephthalamide core, 9-11, were also synthesized. However, none of these closely related analogues of the compound 1 showed effective gelation. The gel formed from 1 was characterized extensively using melting temperature analysis, UV-vis, FT-IR, circular dichroism spectroscopy, and scanning electron microscopy. The resultant gel exhibited impressive tolerance to the pH variation of the aqueous phase and gelated water in the pH range of 4-10. While UV-vis and CD spectroscopy indicated that pronounced aggregation of the azobenzene chromophores in 1 was responsible for gelation, FT-IR studies showed that hydrogen bonding is also a contributing factor in the gelation process. The melting of gel was found to depend on the pH of the aqueous medium in which gel was formed. The gel showed considerable photostability to UV irradiation, indicating tight intermolecular packing inside the gelated state that rendered azobenzene groups in the resultant aggregate refractory to photoisomerization. The electron micrographs of the aqueous gels of 1 showed the existence of spongy globular aggregates in such gelated materials. Addition of salts to the aqueous medium led to a delay in the gelation process and also caused remarkable morphological changes in the microstructure of the gel.

Stable second‐order NLO semi‐IPN system based on bipyridine‐containing polyimide and alkoxysilane dye

AbstractAn organosoluble polyimide based on bipyridyl moiety and an alkoxysilane dye have been developed for second‐order non‐linear optics (NLOs). This bipyridine‐containing polyimide exhibits a glass transition temperature of 254°C and a degradation temperature of 400°C. An NLO‐active semi‐interpenetrating network (IPN) system was prepared by blending the polyimide with the alkoxysilane dye via in situ sol‐gel process of alkoxysilane. The selection of this bipyridine‐containing polyimide as the polymeric matrices provides improved solubility and thermal stability, and most importantly enhanced intermolecular interactions. No aggregation of the NLO chromophores in the polyimide matrices was observed through morphology and NLO studies. Under the limitation of chromophore degradation at elevated temperatures, the pristine poled/cured alkoxysilane dye exhibits poorer long‐term stability. By introducing the polyimide upon a silica network by the semi‐IPN system, randomization of the oriented chromophores can be effectively suppressed. Using in situ contact poling, the r33 coefficients of 2.2–17.0 pm/V were obtained for the optically clear semi‐IPN NLO materials. Excellent temporal stability (100°C) was also achieved for these semi‐IPN materials. Copyright © 2005 John Wiley & Sons, Ltd.

Low temperature relaxations and effects on poling efficiencies of dendronized nonlinear optical side-chain polymers

Low temperature relaxations in a dendronized nonlinear optical (NLO) side-chain polymer were found to take place at more than 20 °C below the glass transition temperature. Relaxations of localized mobilities, removed from long range relaxations responsible for chromophore aggregation, are shown to offer new gateways for optimized acentric ordering of the chromophores. Supreme electro-optical (EO) activity was achieved by electrical poling close to the critical temperatures of localized mobilities identified as dendronized NLO side-chain relaxations. This study features, in particular, one new instrumental approach to relaxation studies of thin spin coated NLO polymer films; the shear-modulation force microscopy (SM-FM) method. Originating from scanning force microscopy (SFM), the SM-FM method grants access to the detection of low temperature relaxations in constrained thin NLO films not obtainable by conventional means.

Dendronized Perylene Diimide Emitters: Synthesis, Luminescence, and Electron and Energy Transfer Studies

Aggregation of chromophores in the solid state commonly causes undesirable red shifts in the emission spectra and/or emission quenching. To overcome this problem, we have prepared soluble perylenetetracarboxidiimide dyes in which the chromophores are effectively shielded by polyphenylene dendrimers attached in the bay positions. Models show that attachment of the shielding units in the bay position should provide more efficient shielding than attaching them via the imide moieties. The dendrimers possess excellent film-forming properties due to alkyl substituents on their peripheries. The lack of a red shift in emission upon going from solution to the solid state indicates the dendrons suppress interaction of the emissive cores, leading to pure red-orange emission. Single-layer LEDs produce red-orange emission with relatively low efficiency especially for the higher generation dendrons, which is attributed to poor charge conduction. LEDs using blends of the dendrimers and the undendronized dye as a model c...

Cross-Linkable Zwitterionic Polyimides with High Electrooptic Coefficients at Telecommunication Wavelengths

A series of nonlinear optical (NLO) polyimides were prepared by grafting 4−16 wt % zwitterionic pyridinium tricyanoquinonedimethane chromophore with a large negative hyperpolarizability (β0 = −590 × 10-30 esu) onto the host polyimides through an ester bond formation. To enhance the temporal stability of the poled NLO polymers, a new thermally reactive group, i.e., 5-aminobenzocylcobutenone, was introduced into the NLO polymer systems to effect the cross-linking during the thermal poling process. The resulting NLO polyimides showed good solubility in polar aprotic solvents such as N,N-dimethylformamide and could form uniform films by casting or spin coating without any noticeable chromophore aggregation or phase separation as examined under an optical microscope. These NLO polyimides had glass transition temperatures in the range of 170−213 °C and were thermally stable up to 270 °C. The electrical resistivity of NLO polyimides was found to be on the order of 1015 Ω·cm at 80 °C and dependent on the applied voltages. As the temperature increased, the resistivity dropped monotonically to 2.4 × 1011 Ω·cm at 200 °C. With a 10 wt % chromophore loading (or a number density of 15 × 1019 cm-3) in the polymer, the electrooptic (EO) coefficient (r33 at 1550 nm) reached 45 pm/V. No substantial decrease in EO coefficients was observed after the test cell was kept in the dark at 85 °C under nitrogen after 1200 h nor when the test cell was irradiated with 1550 nm light (50 μW/∼4 mm2) for 10 h at ambient temperature in air.

Chiroptical Properties of Terthiophene Chromophores Dispersed in Oriented and Unoriented Polyethylene Films

AbstractSummary: Two new chiral terthiophene chromophores II and III were prepared with 99% enantiomeric excess. Chiroptical properties of these dyes dispersed in ultra high molecular weight polyethylene (UHMWPE) films were determined and compared with the same properties in solution. In the solid state, the optical activity strongly depends on the interaction mechanisms within small crystalline aggregates of chromophores. The film morphology and chromophore dispersion were also investigated by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The good correlation between chromophore aggregation and chiroptical activity of the binary films promotes circular dichroism (CD) as an effective technique for monitoring the phase dispersion behaviour of dichroic dyes into polymer matrices. By mechanical stretching of the film a linearly dichroic orientation of the chromophores is obtained which results in a high degree of linear dichroism. The influence of the uniaxial orientation of terthiophene molecules along the drawing direction of UHMWPE on the chiroptical properties of the films, and the possible application of the oriented devices as linear polarizers are discussed.Absorption and CD spectra of unoriented UHMWPEII film at different rotation angles θ.magnified imageAbsorption and CD spectra of unoriented UHMWPEII film at different rotation angles θ.