Merocyanines based on 1,2-diphenyl-3,5-pyrazolidinedione

The acceptor strength of the 1,2-diphenyl-3,5-pyrazolidinedione residue is re-evaluated based on the spectral-fluorescent properties of donor–acceptor dyes. Low fluorescence of the title compounds is explained by the low-lying forbidden ππ*-state.

Effect of bulky substituents in the donor and acceptor terminal groups on solvatochromism of Brooker's merocyanine

Comparison of spectral properties of stilbazolium dyes in various solvents has exposed the influence of electrophilic and nucleophilic solvation on their solvatochromism.

Low-Temperature Effect on the Electronic Structure and Spectral-Fluorescent Properties of Highly Dipolar Merocyanines

Absorption and fluorescence spectra of a vinylogous series of reversely solvatochromic merocyanines based on benzimidazole and malononitrile have been studied in frozen ethanol solutions at 77 K. It is found that they possess negative thermochromism-in contrast to both positively solvatochromic merocyanines and negatively solvatochromic symmetrical ionic polymethines-and even stronger negative thermofluorochromism. It has been deduced from the spectral data that at low temperature their electronic structure becomes more dipolar, deviating substantially from the virtual ideal polymethine in both the ground and the excited states. At that, owing probably to the high polarity and ordering of frozen ethanol, the dipolarity of the studied merocyanines increases with the polymethine chain lengthening-the tendency not observed for them in common solvents. The conclusions, based on the spectral data analysis, have been verified by the (TD)DFT-PCM simulations of the dyes within the four-level scheme of electronic transitions.

Multi-component synthesis of fluorophores via catalytic generation of alkynoyl intermediates

This account summarizes recent developments of aggregation-induced emissive and emission solvatochromic fluorophores by multicomponent reactions. Key intermediates are catalytically generated alkynoyl derivatives that are directly transformed into luminophores in a one-pot fashion. The conciseness in combination with ready access to tailored chromophore libraries makes this synthetic methodological concept superior over classical multistep syntheses.

Expanded Theory of H- and J-Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer

The electronic excited states of molecular aggregates and their photophysical signatures have long fascinated spectroscopists and theoreticians alike since the advent of Frenkel exciton theory almost 90 years ago. The influence of molecular packing on basic optical probes like absorption and photoluminescence was originally worked out by Kasha for aggregates dominated by Coulombic intermolecular interactions, eventually leading to the classification of J- and H-aggregates. This review outlines advances made in understanding the relationship between aggregate structure and photophysics when vibronic coupling and intermolecular charge transfer are incorporated. An assortment of packing geometries is considered from the humble molecular dimer to more exotic structures including linear and bent aggregates, two-dimensional herringbone and "HJ" aggregates, and chiral aggregates. The interplay between long-range Coulomb coupling and short-range charge-transfer-mediated coupling strongly depends on the aggregate architecture leading to a wide array of photophysical behaviors.

UV-Vis absorption spectra and electronic structure of merocyanines in the gas phase

Gas-phase absorption spectra of a merocyanine vinylogous series have been studied for the first time. In vapour, their long-wavelength absorption bands were found to be considerably shifted hypsochromically, broader, more symmetrical, less intense, and their vinylene shift much smaller than even in low-polarity n-hexane. This indicates that in the gas phase their electronic structure closely approaches the nonpolar polyene limiting structure. The TDDFT calculations of the long-wavelength electronic transitions in the studied merocyanines in vacuo demonstrated good-to-excellent correlation - depending on the functional used - with the obtained experimental data. For comparison, the solvent effects was accounted for using the polarizable continuum model (PCM) with n-hexane and ethanol as low-polarity and high-polarity media, and compared with the UV-Vis spectral data in these solvents. In this case, the discrepancy between theory and experiment was much greater, increasing at that with the polymethine chain length.

Unraveling the structure and exciton coupling for multichromophoric merocyanine dye molecules

The relative orientation of chromophores is a key factor in determining the relationship between the structure and the functionality in molecular multichromophore ensembles. In the case of structurally flexible molecular systems in solution, the task to clarify the relevant effects of accessible chromophore orientations with spectroscopic observations is very demanding. In this study, we address this issue by investigating a series of differently connected multichromophoric systems composed of highly dipolar merocyanine dyes that are systematically varied in their substitution pattern and the number of chromophores attached to a bridging benzene ring. Combining electro-optical absorption (EOA) and UV/Vis spectroscopy with density functional theory (DFT) as well as exciton theory discloses conformational preferences and rationalizes the optical properties of the interacting chromophores. Our findings suggest for all multichromophoric systems there is a relative orientation of the chromophores which compensates for the individual dipole moments of the merocyanine dyes by pointing preferably in opposing directions. These orientations furthermore rationalize the observed spectral properties by partly excitonically-coupled subunits.

Supramolecularly Cross-Linked Nanogel by Merocyanine Pendent Copolymer

Directional dipole-dipole interaction mediated antiparallel dimerization of merocyanine dye (MD) has been explored for maneuvering supramolecular assembly of MD-conjugated flexible macromolecules leading to a cross-linked nanogel. The MD-functionalized copolymer was synthesized by a newly developed organocatalytic transesterification strategy for postpolymerization functionalization of poly(pentafluorophenyl acrylate) (polyPFPA)-based reactive copolymer. Presence of ∼35% pendant MD attached to a coil-like polymer chain leads to spontaneous formation of highly emitting cross-linked nanogel with efficient container property and appreciable stability in toluene owing to strong dimerization propensity among the MD. Considering the significance of MD in the context of nonlinear optics and photovoltaics, these results not only enrich the toolbox for engineering macromolecular assembly, but also open up new possibilities for future organic materials.

Synthesis, electronic structure and spectral fluorescent properties of vinylogous merocyanines derived from 1,3-dialkyl-benzimidazole and malononitrile

A vinylogous series of merocyanines were synthesized with 1,3-dibutyl-benzimidazole and malononitrile residues as the donor and acceptor terminal groups. These dyes do not comprise carbonyl groups, which are prone to the strong specific solvation by polar solvents up to hydrogen bond formation, and nevertheless they possess distinct reversed solvatochromism, i.e. their molecules have very high dipolarity. At that, they are soluble in a wide range of solvents from n-hexane to ethanol and do not aggregate readily. They were studied thoroughly by UV/Vis, fluorescence, IR, and NMR spectroscopy methods. Their structure and spectral properties in the ground and excited fluorescent states were modelled at the DFT level both in vacuum and in solvents of various polarities by using the PCM solvent field simulation. The calculations were performed using several hybrid functionals (B3LYP, CAM-B3LYP, and wB97XD) and the split-valence 6-31G (d,p) basis set.

Design, synthesis and photophysical studies of styryl-based push–pull fluorophores with remarkable solvatofluorochromism

A library of 20 styryl-based push–pull dyes derived from 6-amino substituted benzothiazoles were prepared by an efficient and practical synthetic route from low-cost starting materials.

Controlling photophysics of styrylnaphthalimides through TICT, fluorescence and E,Z-photoisomerization interplay

The steady-state and time-resolved spectral properties of styrylnaphthalimides are studied in various solvents.

Hydroxybenzo[b]quinolizinium Ions: Water-Soluble and Solvatochromic Photoacids

It is shown by photometric and fluorimetric analysis, along with supporting theoretical calculations, that hydroxy-substituted benzo[b]quinolizinium derivatives display the characteristic features of organic photoacids. Specifically, the experimental and theoretical results confirm the strong acidity of these compounds in the excited state (pK_a* < 0). The combination of the prototropic properties of 8- and 9-hydroxybenzo[b]quinolizinium with the particular solvent-solute interactions of the excited acid and its conjugate base leads to a pronounced fluorosolvatochromism, hence the emission maxima shift from 468 nm (8-hydroxybenzo[b]quinolizinium) or 460 nm (9-hydroxybenzo[b]quinolizinium) in CH₃CN to 507 and 553 nm in DMF, respectively. This novel type of photoacid represents several features that may be used for applications as water-soluble fluorescent probes or as a source for the photoinduced supply of acidity.

Supramolecular control over the structural organization of a second-order NLO-active organogelator

A study of the structural parameters which govern the supramolecular organization of an organogelator built from the Disperse Red moiety is proposed. In particular, the key balance between intermolecular H-bonding and/or π-π interactions is addressed by comparing the effect of a secondary amide vs. an ester linker within the molecular structure. Solution ¹H-NMR studies show the superiority of the former interaction in promoting the nanostructuring process, allowing it to reach a gel state in toluene. The nanostructures obtained from both the amide and the ester derivatives were also studied in the solid state. In particular, the use of second-harmonic generation microscopy demonstrates that an anisotropic organization of the material can even be observed in the case of the ester derivative, which demonstrates the efficiency of the tris(alkoxy)benzene unit in directing the self-assembly process, independently of additional H-bond interactions.

Fluorescence of carbonyl-containing intraionic polymethines

Electronic structure and spectral-fluorescent properties of four related indole-based intraionic polymethines are discussed. They all comprise at least one carbonyl group in the acceptor part of their molecule but the effects of the carbonyls upon their UV/Vis and fluorescence spectra depend substantially on its position within the polymethine chromophore. At that, solvation of the carbonyls with highly electrophilic protic solvents can, as a function of dye structure, cause both a rise and decrease of fluorescence quantum yield of a dye or have no tangible effect at all. To get insight into the regularities of such behaviour, the dyes were examined closely using both their absorption and fluorescence spectral data and the (TD) DFT quantum chemical simulation.

Promoting Spontaneous Second Harmonic Generation through Organogelation

An organogelator based on the Disperse Red nonlinear optical chromophore was synthesized according to a simple and efficient three-step procedure. The supramolecular gel organization leads to xerogels which display a spontaneous second harmonic generation (SHG) response without any need for preprocessing, and this SHG activity appears to be stable over several months. These findings, based on an intrinsic structural approach, are supported by favorable intermolecular supramolecular interactions, which promote a locally non-centrosymmetric NLO-active organization. This is in sharp contrast with most materials designed for SHG purposes, which generally require the use of expensive or heavy-to-handle external techniques for managing the dipoles' alignment.

The unusual solvatochromism and solvatofluorochromism of longwave absorbing and emitting barbiturate merocyanine dyes

Spectral-fluorescent properties of a series of merocyanine dyes comprising the barbituric acid residue as the electron-accepting terminal group are investigated in comparison with those of their N,N-methylated analogues in media of various polarity. It is revealed that in polar aprotic solvents the electronic absorption spectra of the studied compounds are influenced dramatically by the formation of hydrogen bonds between the NH-groups of barbituric residue and solvent molecules. An effect of such nucleophilic solvation on the electronic structure of the studied dyes is analysed using both the spectral data obtained and the DFT/B3LYP quantum chemical simulation. It is found also, that solvation has comparatively weak influence on the shape and position of the fluorescence bands of the studied merocyanines while the fluorescence quantum yield changes substantially in solvents of various polarity.

Multicomponent syntheses of functional chromophores

Multicomponent reactions are a valuable tool for the synthesis of functional π-electron systems. Two different approaches can be taken into account for accessing the target structures. In the more conventional scaffold approach an already existing chromophore is coupled with other components to give a complex functional π-system. Here, electronically monotonous components can also be introduced, which may exert synergistic electronic effects within the novel compound. The more demanding chromophore concept generates a complete π-electron system and a scaffold concurrently. The latter approach is particularly stimulating for methodologists since π-systems might be accessible from simple starting materials. This review encompasses the advances in the preparation of functional π-electron systems via multicomponent processes during the past few years, based both on the scaffold and chromophore concepts. Besides the synthetic strategies the most important properties, i.e. redox potentials, absorption and emission maxima or fluorescence quantum yields, of the synthesized molecules are highlighted.

Ratiometric Imaging Using a Single Dye Enables Simultaneous Visualization of Rac1 and Cdc42 Activation

Biosensors that report endogenous protein activity in vivo can be based on environment-sensing fluorescent dyes. The dyes can be attached to reagents that bind selectively to a specific conformation of the targeted protein, such that binding leads to a fluorescence change. Dyes that are sufficiently bright for use at low, nonperturbing intracellular concentrations typically undergo changes in intensity rather than the shifts in excitation or emission maxima that would enable precise quantitation through ratiometric imaging. We report here mero199, an environment-sensing dye that undergoes a 33 nm solvent-dependent shift in excitation. The dye was used to generate a ratiometric biosensor of Cdc42 (CRIB199) without the need for additional fluorophores. CRIB199 was used in the same cell with a FRET sensor of Rac1 activation to simultaneously observe Cdc42 and Rac1 activity in cellular protrusions, indicating that Rac1 but not Cdc42 activity was reduced during tail retraction, and specific protrusions had reduced Cdc42 activity. A novel program (EdgeProps) used to correlate localized activation with cell edge dynamics indicated that Rac1 was specifically reduced during retraction.

The role of the dipolar neighborhood on the relaxation dynamics of multichromophoric merocyanines

The interactions of neighboring, highly dipolar merocyanine dyes lead to stabilization of the intramolecular charge-transfer state.

A new class of spirocyclic photochromes reacting with light of both UV and visible ranges

Deep coloration of weakly colored spirocyclic derivatives (R = NMe₂, NEt₂, and OMe) can be achieved by irradiation with light of any wavelength between 254 and 642 nm.

Scope of negative solvatochromism and solvatofluorochromism of merocyanines

Can negatively solvatochromic merocyanines retain their generic characteristics in the full range of solvent polarities including low-polarity alkanes? To find the answer, the solvatochromism of a vinylogous series of tailored highly dipolar merocyanines was thoroughly investigated both experimentally and theoretically.

Integration of Cyanine, Merocyanine and Styryl Dye Motifs with Synthetic Bacteriochlorins

Understanding the effects of substituents on spectral properties is essential for the rational design of tailored bacteriochlorins for light-harvesting and other applications. Toward this goal, three new bacteriochlorins containing previously unexplored conjugating substituents have been prepared and characterized. The conjugating substituents include two positively charged species, 2-(N-ethyl 2-quinolinium)vinyl- (B-1) and 2-(N-ethyl 4-pyridinium)vinyl- (B-2), and a neutral group, acroleinyl- (B-3); the charged species resemble cyanine (or styryl) dye motifs whereas the neutral unit resembles a merocyanine dye motif. The three bacteriochlorins are examined by static and time-resolved absorption and emission spectroscopy and density functional theoretical calculations. B-1 and B-2 have Qy absorption bathochromically shifted well into the NIR region (822 and 852 nm), farther than B-3 (793 nm) and other 3,13-disubstituted bacteriochlorins studied previously. B-1 and B-2 have broad Qy absorption and fluorescence features with large peak separation (Stokes shift), low fluorescence yields, and shortened S1 (Qy ) excited-state lifetimes (~700 ps and ~100 ps). More typical spectra and S1 lifetime (~2.3 ns) are found for B-3. The combined photophysical and molecular-orbital characteristics suggest the altered spectra and enhanced nonradiative S1 decay of B-1 and B-2 derive from excited-state configurations in which electron density is shifted between the macrocycle and the substituents.

Hypercondensation of an amino acid: synthesis and characterization of a black glycine polymer

A granular material was obtained by thermal polymerization of glycine at 200 °C. It has been named "thermomelanoid" because of its strikingly deep-black color. The polymerization process is mainly a dehydration condensation leading to conventional amide bonds, and also CC double bonds that are formed from CO and CH2 groups ("hypercondensation"). Spectroscopic data, in particular from (13) C and (15) N solid-state cross-polarization magic angle spinning (CP-MAS) NMR spectra, suggest that the black color is due to (cross-)conjugated CC, CO, and NH groups. Small glycine peptides, especially triglycine, appear to be key intermediates in the formation of the thermomelanoid. This has been concluded by comparing the thermal behavior of glyn homopeptides (n=2-6) and glycine. The glycine polymerization was accompanied by the formation of small amounts of byproducts. Notably, a few percent of alanine and aspartic acid could be detected in the polymer. By using (13) C-labeled glycine, it was shown that these two amino acids formed through a common pathway, namely CαCα bond formation between glycine molecules. The thermomelanoid is hydrolyzed by strong acids and bases at room temperature, forming brown solutions.

Effect of Concentration on Isomerization of Rhodanine Derivatives of Merocyanine Dyes in Polar Solvents

Rhodanine derivatives of merocyanine dyes with residues of 1,3,3-trimethyl-3H-indole and 3-ethylbenzothiazoline have been found to possess two molecular forms in diluted solutions of polar solvents such as dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidinone. The first molecular form was observed to prevail at low concentrations of the dyes, normally up to 10−5 M. The second one prevails at higher concentrations and is displayed through a new band in the electronic absorption spectrum, which is red-shifted with respect to the absorption band of the first form. No similar effect was found for these dyes by use of nonpolar solvents or upon alkyl-substitution of the molecules at nitrogen atom in the rhodanine moiety. We assign the above two forms to different molecular isomers and the analogous spectral changes were shown to take place by light or heat influence which correspond to a typical isomerization effect for the related merocyanine dyes. It is discussed that the isomer transformation is facilitated by the increased mobility of the proton bonded to the nitrogen atom of the rhodanine moiety in the polar environment and the increased amount of dye-dye collisions.

One-pot three-component synthesis and photophysical characteristics of novel triene merocyanines

Novel triene merocyanines, i.e. 1-styryleth-2-enylidene and 4-(1,3,3-trimethylindolin-2-ylidene)but-2-en-1-ylideneindolones are obtained in good to excellent yields in a consecutive three-component insertion Sonogashira coupling-addition sequence. The selectivity of either series is remarkable and has its origin in the stepwise character of the terminal addition step as shown by extensive computations on the DFT level. All merocyanines display intense absorption bands in solution and the film spectra indicate J-aggregation. While 1-styryleth-2-enylideneindolones show an intense deep red emission in films, 4-(1,3,3-trimethylindolin-2-ylidene)but-2-en-1-ylideneindolones are essentially nonemissive in films or in the solid state. TD-DFT computations rationalize the charge-transfer nature of the characteristic broad long-wavelength absorptions bands.