Multicomponent and Domino Syntheses of AIE Chromophores - ACS

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Multicomponent and Domino Syntheses of AIE Chromophores Thomas J. J. Müller* Institut für Organische Chemie und Makromolekulare Chemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstrasse 1, D-40225 Düsseldorf, Germany *E-mail: [email protected]

Multicomponent and domino reactions are unique one-pot methodologies that enable the de novo formation of fluoro-phores in chromogenic reaction sequences. Particularly, by insertion-alkynylation or by alkynylation metal catalysis promises a catalytic entry into rapid, efficient and elegant formations of old and novel fluorogenic scaffolds. This chromophore concept also opened avenues to peculiar highly polar or highly polarizable solid state luminophores that show the characteristics of aggregation induced emission. The same synthetic concept also enables the diversity-oriented synthesis of solution and solid state blue-emissive molecular chromophores.

1. Introduction The quest for novel syntheses of functional π-electron systems (1) as constituting functional entities in molecular electronics (2, 3), in photo-electronic applications (4–10), in particular as organic light-emitting diodes (OLEDs) (11–14), dye-sensitized solar cells (DSSCs) (15, 16), and organic field effect transistors (OFETs) (17–19), and in sensing units in bio or environmental analytics (20–24) has become an increasingly important task for synthetic chemistry. In particular, an efficient and efficacious access to functional chromophores with specific photophysical properties remains a paramount challenge for organic and materials chemists. Ultimately and most advantageously, diversity-oriented syntheses (25–33) of functional molecules should occur in a one-pot fashion © 2016 American Chemical Society


by conceptually concatenating fundamental organic reactions (34–38). With this respect, multicomponent (34, 39–51) and domino reactions (35, 36, 52–54) represent elegant solutions to this quest and these conceptual approaches have become attractive approaches to biologically active molecules (55–58). By definition, in domino processes two or more bonds are formed without isolation of intermediates starting from one, two or more substrates, whereas in multicomponent processes more than two starting materials lead to the product that contains most of the employed atoms (34–51). Multicomponent reactions (MCR) represent a reactivity based concept (59), which can be conducted according to three different scenarios. For domino-type MCR all reagents, catalysts, solvents etc. must be present from the very beginning of the process. In sequential MCR the subsequent well-defined order of addition of components from step to step occurs, however, maintaining the reaction conditions identical. The last category is the consecutive MCR where the components are added stepwise and where conditions can be altered from step to step. Inevitably, all three scenarios promise high structural and functional diversity and for investigation of functional molecules with the immense explorative potential of MCR they have become a powerful synthetic tool. Inspired by our methodological work on consecutive MCR-syntheses of heterocycles initiated by Pd/Cu-catalyzed alkynylation (60–65) we have launched a program to illustrate diversity-oriented syntheses of chromophores by multicomponent and domino reactions one and half decades ago (32, 33). For the development of novel one-pot syntheses of fluorophores we have been following the idea that the MCR or domino process could act as the chromogenic event, i.e. the fluorophore of interest is formed by virtue of the one-pot process. Therefore, we named this concept chromophore approach (Scheme 1).

Scheme 1. Diversity-oriented fluorophore formation by an MCR or domino process based chromophore approach. (see color insert) In the vast field of luminescence, which has actively been developed over many decades, there are aspects that either remained challenging or even became known just recently. For instance, luminophores displaying large Stokes shifts and emission at short wavelength, i.e. blue luminescence, are particularly requested in OLED technologies (11–14, 66, 67). A hot topical field is the induction of emission and eventually also the enhancement of fluorescence upon aggregation. Aggregation induced emission (AIE), a phenomenon coined by Tang’s group (68–70), and aggregation induced enhanced emission (AIEE) first reported by Park’s group (71), have become particularly attractive and stimulating for all aspects of luminophore research. In this synopsis our contribution in the synthetic advancement of blue-emitters and AIE chromophores based upon 86

one-pot transformations initiated by modified Sonogashira alkynylation (72–74) are highlighted and summarized in a flashlight fashion.

2. Solid-State Emissive Chromophores by Domino Insertion-Coupling Sequences


After the discovery of the domino Sonogashira coupling-propargyl alcohol enone isomerization in 1999 (Scheme 2) (61, 74–76), we sought for an extension of this unusual detour of the Sonogashira alkynylation towards Negishi-type cyclic carbopalladation (77) towards 3-substituted benzo-furanones and indolones.

Scheme 2. Mechanistic rationale of the domino Sonogashira coupling-propargyl alcohol enone isomerization. As seminal examples the 3-propenylidene benzofuranone 1 and the 3-propynylidene indolone 2 were obtained in the sense of an insertioncoupling(-isomerization), however interestingly, with E-configuration of the alkenylidene/alkynylidene moiety (Scheme 3) (78). This proof of principle paved the way to novel domino sequences that were elaborated in different directions, some of them also towards novel fluorophores (vide infra). Also the newly formed extended Michael system of indolone 2 encouraged us later to scout new consecutive three-component syntheses to push-pull 3-amino alkenylidene indolones (vide infra). All this prompted us to revisit the unusual stereodivergent propynylidene formation from a methodological perspective. Variation of both the 3-substituted propynoyl ortho-iodo anilides 3 and alkynes 4 gave rise to the formation of alkynylidene indolones 5 in moderate to excellent yield, however, with varying E/Z selectivities (Scheme 4) (79). Quantum chemical modelling indicates a stereomutation on the stage of the vinyl-Pd-species that form upon insertion, depending on the nature of the terminal propynoyl substituent and the alkynyl reaction partner. A postcoupling treatment of the isolated stereochemically enriched enynes 5 with amine base excludes the a posteriori equilibration of the stereoisomers. 87


Scheme 3. First examples of domino insertion-coupling(-isomerization) syntheses of E-configured alkenylidene benzofuranone 1 and alkynylidene indolone 2.

Scheme 4. Domino insertion-coupling synthesis of alkynylidene indolones 5.

Investigation of the electronic properties of alkynylidene indolones 5 revealed longest wave length absorption maxima in dichloromethane appearing in a range between 351 and 485 nm with extinction coefficients of 11300-75400 L mol-1 cm-1. Most remarkably, none of these chromophores luminesces in solution, however, all N-substituted derivatives (R1 ≠ H) display quite intense emission in the solid state upon irradiation with UV light (λexc = 365 nm). Depending on the substitution pattern the emission maxima in the solid state are found between 533 and 635 nm (Figure 1), where the most pronounced red shift can be identified for p-aminophenyl substituted derivatives, i.e. for push-pull substitution. This finding additionally supports the highly polar character of the vibrationally relaxed excited state of the alkynylidene indolone fluorophores. 88


Figure 1. Normalized absoprtion (solid lines) and emission spectra (dashed lines) of phenyl alkynylidene indolone 5a (left) and p-N,N-dibutylphenyl alkynylidene indolone 5b (right) (recorded at T = 298 K, λexc = λabs). Performing the domino insertion-coupling reaction with 3-substituted propynoyl ortho-iodo anilides 3 or 3-substituted propynoyl ortho-iodo phenolates 6 with 1-(hetero)aryl propargyl allylethers 7 furnished spiro-indolones 8 or spiro-benzofurans 9 quite efficiently in the sense of a insertion-couplingisomerization-Diels-Alder sequence (Scheme 5) (8, 80).

Scheme 5. Domino insertion-coupling-isomerization-Diels-Alder synthesis of spiro-indolones 8 or spiro-benzofurans 9. Based upon the product analysis this unusual hetero-domino process can be rationalized as follows. Oxidative addition of the substrates 3 or 6 with the palladium(0) complex furnishes after insertion of the tethered alkynoyl moiety a benzoanellated species 10 that readily undergoes alkynylation with the in situ formed copper acetylide 11 to give after reductive elimination the alkynylidene 89


indolone or benzofuranone 12. The electron withdrawing lactame/lactone functionality leads to a remote activation of the propargylic position in 12 to undergo a base catalyzed propargyl-allene isomerization furnishing the allene derivative 13. This vinyl allene with electron deficient substitution is particularly suited for a concluding Diels-Alder reaction with inverse electron demand. The tethered allyl ether on the allenyl system sets the stage for the intramolecular (4+2)-cycloaddition furnishing the spiro-indolones 8 or spiro-benzofurans 9. All spiro-compounds share structurally rigidified trans-cis-configured 1,3-butadienes with (hetero)aryl substitution representing the longest wavelength absorbing chromophore units with intense absorption bands between 327 to 398 nm. Upon excitation the model trans,cis-1,4-diphenyl butadiene does not luminesce, but rather returns to the electronic ground state by a conformational twisting and efficient internal conversion (81). However, all spirocyclic 1-(hetero)aryl-4-(hetero)aryl butadienes 8 and 9 display intense luminescence with large Stokes shifts in solution (4300 to 9600 cm-1, emission maxima range from 433 to 545 nm) and in the solid state (Figure 2). Time-correlated single photon counting (TCSPC) measurements in solution give luminescence lifetimes between 0.26 and 4.97 ns. While the structural nature of spiro-indolones 8 and spiro-benzofuranones 9 only affects absorption maxima to a minor extent, dimethyl substitution considerably increases the fluorescence quantum yield for spiro-benzofuranones 9, but only marginally for spiro-indolones 8. However, the dimethyl substitution has no influence on the emission wavelength of spiro-indolones 8.

Figure 2. Representative blue (8a), green (8b) and orange (8c) solid state fluorescent spiro-indolones 8 (λexc = 370 nm). (see color insert) The synthetic concept was extended to the preparation of several luminescent bichromophores 14 (82). Besides the fluorogenic 1,4-diaryl substituted trans-cis-configured 1,3-butadiene moiety the N-dansyl fragment was introduced at the diversity position of R1, additionally anthryl substitution at propargylic position furnished a further type of bichromophore (Figure 3). In comparison to the spiro-cyclic E,Z-fixed 1,4-diaryl butadiene displaying relatively sharp 90


absorption maxima at 350 nm, N-dansyl derivatives absorb at 360 nm. The emission spectra reveal two maxima, an intense one at 510 nm und a blue shifted shoulder at 470 nm. As a consequence N-dansyl derivatives apparently luminesce with turquoise luminescence, whereas the spiro-fixed E,Z-fixed 1,4-diphenyl butadiene chromophore emits intensive blue light (Figure 4) (80).

Figure 3. Selected N-dansyl- and anthryl-substituted spiro-indolones 14 with bichromophore emission characteristics (determined in CH2Cl2 at T = 298 K; a[Lcm-1mol-1]; bDetermined with quinine sulfate as a standard (0.1 m H2SO4), Φf = 0.54; cStokes shift Δṽ = ṽmax,abs - ṽmax,em; dDetermined with coumarin 153 as a standard in ethanol, Φf = 0.38).

Figure 4. Comparison of a blue fluorescent spiro-indolone (left), a turquoise emissive N-dansyl-spiro-indolone (center), and a yellow luminescent anthryl-substituted spiro-indolone (right) (λexc = 370 nm). (see color insert) 91


A comparison of the fluorescence quantum yields Φf supports an intramolecular interaction between the 1,4-diaryl butadiene chromophore and the dansyl chromophore in the excited state and deactivation by partial intramolecular energy transfer is very likely due to sufficient spectral overlap. Particularly interesting are anthryl-substituted spiro-indolone bichromophores that show a significant deviation of the absorption and emission behavior of the other series. Three distinct maxima and a shoulder can be clearly assigned to spiro-fixed E,Z-fixed 1,4-diphenyl butadiene and anthracene typical chromophores in the absorption spectra. However, the emission spectra of anthryl derivatives do not reveal anthracene typical behavior but rather emission from an expanded π-system (Figure 5). Indeed two emission maxima between 525 and 560 nm can be identified and upon eyesight the solutions are yellow luminescent.

Figure 5. Normalized absorption (solid line) and emission (dashed line) spectra of the anthryl-spiro-indolone bichromophore 14c (recorded in CH2Cl2 at T = 298 K).

3. Solid-State Emissive Chromophores by Multicomponent Insertion-Coupling-Addition Sequences The alkynylidene indolones 5 accessible by domino insertion-coupling sequence represent expanded Michael-type systems and the mild reaction conditions of their generation set the stage for devising consecutive threecomponent syntheses as already successfully demonstrated for ynones and amines to give enaminones in the sense of a one-pot coupling-addition reaction (83). Upon reaction of 3-substituted propynoyl ortho-iodo anilides 3 and alkynes 4 under the conditions of the insertion-coupling sequence the intermediary formed alkynylidene indolones 5 were directly reacted with primary and secondary amines 15 to furnish 4-aminopropenylidene indolones 16 in good to excellent yields (Scheme 6) (84). Most remarkably, the Michael-type addition proceeds in a highly stereoconvergent fashion, presumably via the intermediacy of zwitterion 17, which allows a thermodynamic equilibration due to delocalization of the enolate finally establishing the observed E,E-configuration of newly formed 92


conjugated diene. The class of 4-aminopropenylidene indolones 16 formed by the chromogenic three-component process can be considered as push-pull butadienes on the basis of the indolone scaffold. Interestingly, all representatives display intense orange to red luminescence in the solid state (crystal, amorphous or film); surprisingly, in solution the same chromophores are nonemissive (Figure 6).

Scheme 6. Three-component insertion-coupling-addition synthesis of 4-aminopropenylidene indolones 16. A closer inspection of the X-ray structure of the representative compound 16b reveals that the essentially coplanarily arranged push-pull chromophore is additionally stabilized by a parallel π-stacking alignment of the aryl substituents with interplanar centroid distances of 3.6 Å (Figure 7). The moderate positive absorption solvochromicity in solution underlines a relatively low polar electronic ground state with some charge-transfer of S0-S1-transition character (85). However, in the solid state spectra redshifted absorption maxima (492-502 nm) indicate J-aggregation (86) of the push-pull chromophores. Most remarkably, all dyes 16 display intense orange red fluorescence with large Stokes shifts (Δṽ ~2600 cm-1) and sharp emission maxima between 622 and 644 nm. This peculiar effect of the appearance of narrow red shifted aggregation induced emission bands (68–70) additionally rationalizes by Davydov splitting in the solid state (87, 88). Later this concept was extended with L-amino acid esters 18 as amino component to give film luminescent indolone merocyanines 19 containing L-amino acid esters as donors (Scheme 7) (89). The occurrence of mixtures of diastereomers as a result of the Michael-type addition of L-alanine ethyl ester and L-leucine methyl ester prompted us to study the energetics of the chromogenic event by computational methods. Assuming a stepwise Michael addition (see also Scheme 6) of the L-alanine methyl ester 18a the elusive allenol intermediate 20a should be formed from the zwitterion 17a (Figure 8). According to PM3 93


computations with the implemented solvation energy model SM5.4/P the final, irreversible 1,5-H-shift represents the thermodynamic driving force in the sense of an allenol-enone tautomerism. Although the isomer E,Z-19a is slightly more stable than the isomer E,E-19a the computed transition state energy difference ΔΔG‡(TS20a–E,E-19a – TS20–E,Z-19a) suggests that the formation of product E,E-19a proceeds by kinetic control.

Figure 6. Normalized absorption (solid line) and emission spectra (dashed line) of a drop-casted film of 4-aminopropenylidene indolone 16a.

Figure 7. X-ray structure analysis of 4-aminopropenylidene indolone 16b with a selected angle, torsional angle and interplanar distance (the grey shaded plane on the right indicates the chromophore plane). 94


Scheme 7. Three-component insertion-coupling-addition synthesis of 4-amino acid ester substituted prop-3-enylidene indolones 19.

Figure 8. Computed reaction profile for the terminal 1,5-H-shift of the elusive allenol intermediate 20a to form the diastereomers E,E-19a and E,Z-19a. The 4-amino acid substituted prop-3-enylidene indolones 19 are also nonemissive in solution as found for the derivatives 16, however, they display intense orange to red luminescence in amorphous films with sharp emission bands in a range from 579 to 631 nm with Stokes shifts range between 4600 and 5600 cm–1. A further extension of the consecutive three-component insertion-couplingaddition sequence to the synthesis of triene push-pull systems was achieved by employing enamines, such as Fischer’s base (21), as nucleophiles after the formation of the alkynylidene indolone intermediate (90). Most remarkably, a mechanistic dichotomy was observed to proceed with excellent selectivity furnishing 1-styryleth-2-enylidene indolones 22 in good to excellent yields as violet solids with a metallic luster for electron rich 3-substituted propynoyl ortho-iodo anilides 3 (R1 = Me) (Scheme 8). However, for electron deficient 3-substituted propynoyl ortho-iodo anilides 3 (R1 = Tos) under the same conditions selectively 4-(1,3,3-trimethylindolin-2-ylidene)but-2-en-1-ylidene indolones 23 were formed in good to excellent yields as bluish black solids with a metallic luster (Scheme 9). 95


Scheme 8. Three-component insertion-coupling-addition synthesis of 1-styryleth-2-enylidene indolones 22.

Scheme 9. Three-component insertion-coupling-addition synthesis 4-(1,3,3-trimethylindolin-2-ylidene)but-2-en-1-ylidene indolones 23.

This mechanistic bifurcation was studied computationally and the inspection of the Mulliken charge on the relevant zwitterionic intermediate 24 indicated that the methyl substitution leads to a less stable enolate part that rapid collapses with the iminium moiety in the sense of a 1,4-dipolar cyclization to give the cyclobutene 25. Conrotatory ring opening directly leads to the 1-styryleth-2-enylidene indolone 22. Conversely, tosyl substitution ensures a higher stability and, therefore, a longer persistence to allow for a 1,7-proton transfer to the allenol 26, which tautomerizes to the extended conjugated triene 23 via 1,5-sigmatropic H-shift (Scheme 10). The photophysical characteristics of 1-styryleth-2-enylidene indolones 22 are similar to the 4-aminopropenylidene indolones 16 and 19, showing deep red colored solutions and and film formation with intense, broad unstructured absorption bands between 510 and 522 nm (dichloromethane) and between 519 and 532 nm (films), respectively, as a consequence of J-aggregation (86). As a consequence aggregation induced emission is indicated by deep red luminescence of both amorphous films and dyes in the solid state with sharp bands appearing between 644 and 665 nm. As for the related indolones 16 and 19, the emission of the indolones 22 is completely quenched in solution. 96


Scheme 10. Mechanistic rationale of the dichotomy furnishing the 1-styryleth-2-enylidene indolones 22 and 4-(1,3,3-trimethylindolin-2-ylidene)but2-en-1-ylidene indolones 23. Likewise 4-(1,3,3-trimethylindolin-2-ylidene)but-2-en-1-ylidene indo-lones 23 are dark blue to black solids with broad unstructured longest wave length absorption bands in solution in a range from 577 to 597 nm with high molar extinction coefficients. J-aggregation red shifts the absorption bands of the films to 599-623 nm. Upon eyesight the films of the merocyanines 23 did not display emission, in accordance with the energy gap law (91).

4. Solid-State Emissive Push-Pull Chromophores by Multicomponent Coupling-Addition Sequences Sonogashira coupling of acid chlorides and terminal alkynes under modified conditions (72, 73), i.e. employing only the stoichiometrically necessary equivalent of tertiary amine for scavenging the hydrochloric acid, opened new avenues to consecutive multicomponent syntheses of heterocycles as a consequence of the mild reaction conditions of this versatile entry to alkynones (60–65). Encouraged by Michael and Michael type additions of amines to ynones (83) and the previously discussed intermediate alkynylidene indolones (84, 89) and the addition of Fischer’s base to alkynylidene indolones (90) we conceived the Michael addition of Fischer’s base and in situ generated S,N-ketene acetals as a novel, efficient way of accessing push-pull chromophores in a one-pot fashion (92). The stepwise nature of the Michael type addition of enamines to ynones additionally opens mechanistic dichotomies imposed by steric and electronic effects (vide supra for alkynylidene indolone). Indeed, we observed three different scenarios of merocyanine formation depending on the ynone in conjunction with the employed nucleophile. 97


In the consecutive three-component coupling-addition sequence of aroyl chlorides 27 and (trimethylsilyl)acetylene (28) and Fischer’s base (21) or the in situ formed S,N-ketene acetal from 29 push-pull butadienes 30 were generated with moderate to good yield (Scheme 11). However, upon changing the alkyne coupling partner to aryl acetylenes 31 the resulting ynones open bifurcating pathways to the addition products. With dimethyl benzothiazolium iodide (29), which generates a reactive S,N-ketene acetal upon deprotonation, push-pull butadienes 32 were formed in moderate to good yield (Scheme 12), whereas with Fischer’s base (21) 2-styryl substituted push-pull ethylenes 33 are formed in good to excellent yield (Scheme 13). The mechanististic rationalization is similar to the related alkynylidene indolones, however, in these cases the stability of the zwitterionic intermediate is governed by stabilization of the iminium species, i.e. the S,N-ketene acetal forms a more stable iminium ion than Fischer’s base.

Scheme 11. Three-component coupling-addition synthesis of push-pull butadienes 30.

Scheme 12. Three-component coupling-addition synthesis of push-pull butadienes 32.

Scheme 13. Three-component coupling-addition synthesis of 2-styryl substituted push-pull ethylenes 33. 98


All three types of push-pull chromophores reveal interesting photophysical properties. The merocyanines 30 are luminescent in solution and the closer inspection of the solvent polarity effects reveals positive absorption and emission solvatochromism of merocyanine 30a according to Reichardt’s ET(30) (93) and Lippert-Mataga (94, 95) plots (Figure 9). The empirical linear free-enthalpy relationship (LFER) indicates a lower dipole moment for the electronic ground state and a minor charge transfer character of the excited Franck-Condon state upon S0-S1 transition (96). Supported by the Lippert-Mataga analysis the vibrationally relaxed S1 state possesses a high dipole moment with enhanced charge-transfer character. A small shift in solvent polarity from methyl cyclohexane to benzonitrile causes a red shift of 860 cm-1 in the emission.

Figure 9. Absorption and emission solvochromicity of of merocyanine 30a (top: plot of the longest wave length absorption (black) and shortest wave length emission (red) maxima against Reichardt’s solvatochromicity parameters ET(30) (r2abs = 0.93, r2em = 0.97); bottom: Lippert-Mataga plot of the Stokes shifts against polarity parameters (r2abs = 0.90). 99


While merocyanines 30 are luminescent both in solution and in the solid state, the push-pull butadienes 32 and the 2-styryl substituted push-pull ethylenes 33 are nonluminescent in solution. However, all three types of merocyanines reveal pronounced luminescence in the crystalline solid-state (Figure 10). In addition, push-pull butadienes 30 and 32 possess film forming properties and also in the amorphous solid state red-shifted absorption bands and intense sharp even further red-shifted emission maxima can be detected (Figure 11).

Figure 10. Solid state (left) and solution (in dichloromethane, right) luminescence of merocyanines 30a, 32a, and 33a (λexc = 366 nm). (see color insert)

Figure 11. Normalized absorption (black) and emission spectra (red) of a drop-casted film of merocyanine 32a and its emission of crystalline powder (λexc = 366 nm). (see color insert) Both H-aggregate (97) and J-aggregate formation (98, 99) of merocyanines have been described by UV/vis-spectroscopic studies. The X-ray structure analysis of compound 32b (Figure 12), crystallizing in the monoclinic space group C2/c, reveals further insight into the observed J-aggregation of the merocyanine molecules due to the anti-parallel alignment and self-organization by π-π-stacking of the molecules and thereby enhancing the polar microenvironment. 100


Figure 12. Crystal packing of closest parallel molecules of compound 32b (d = 3.52 Å within dimer, and d = 3.44 Å between dimers) in a stack (top) and in the lattice (bottom).

5. Blue Emissive Heterocyclic Chromophores by Multicomponent Coupling-Addition-Cyclocondensation Sequences For OLED applications red and green emissive fluorophores are predominantly reported (100, 101), since charge-transfer excited-states typically exhibit a narrower band gap than normal locally excited states. Inevitably blue, non- charge-transfer emitters usually possess a wider band gap (102–104). Therefore, novel diversity-oriented accesses to molecular blue emitters are highly desirable. Based upon the catalytic generation of alkynones as an entry to consecutive multicomponent syntheses of heterocycles (60–65) we have accessed several classes of blue emissive systems. 101


5.1. Three- and Four-Component Syntheses of Blue Emissive Pyrazoles by Coupling-Addition-Cyclocondensation Sequences Pyrazoles and imidazoles have received considerable interest due to unique electronic and optical properties and their use as optical brighteners, as UV stabilizers, and as constituents in photoinduced electron transfer systems (105–114). Based on the concept of catalytic generation of alkynones as an entry to consecutive multicomponent syntheses of heterocycles (60–65) we disclosed an efficient regioselective three-component synthesis of highly fluorescent 1,3,5-substituted pyrazoles 37 from acid chlorides 34, terminal alkynes 35, and hydrazines 36 (Scheme 14) (115). Absorption (λmax,abs between 260 and 385 nm) and emission (λmax,em between 320 and 380 nm) properties of pyrazoles 37 are strongly affected by substitution pattern. Most derivatives are intensely blue to green fluorescent with fluorescence quantum yields Φf between

Multicomponent and Domino Syntheses of AIE Chromophores - ACS

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