Helicity-Dependent Regiodifferentiation in the Excited-State Quenching and Chiroptical Properties of Inward/Outward Helical Coumarins

A turn-on fluorescence probe for imaging tyrosinase at the wound site in broken tail of zebrafish

Tyrosinase (TYR) is a copper-containing oxidase that affects the synthesis of melanin in the human body, which is regulate to the pigmentation of the skin. Nevertheless, abnormal expression of TYR can lead to albinism, vitiligo and other skin diseases. Excessive accumulation of TYR is a marker of melanoma cancer and an important factor leading to pigmentation during wound healing, freckles and browning of fruits and vegetables. Efficient tracking of TYR is of significance for studying its pathophysiological mechanism. Herein, we synthesized a benzindole-based fluorescent probe Pro-OH to detect TYR in living cells and zebrafish. The probe displayed a high selectivity and sensitivity in distinguishing TYR from other analytes with the low detection limit of 1.024 U/mL. Importantly, Pro-OH was successfully used to imagine TYR at the wound site of broken tail of zebrafish.

Strongly emitting, centrosymmetric, ladder-type bis-coumarins with crankshaft architecture

Quadrupolar bis-coumarins bearing dialkylamino groups, prepared by a double Pechmann reaction and subsequent oxidation, strongly emit yellow-orange light. Comparison with non-substituted analogs reveals that, the photophysical properties of the conjugated bis-coumarins are controlled both by the dialkylamino substituents and by the π-system. Analogous but non-conjugated bis-coumarins emit blue light both in solution and in crystalline state. Unusually fast oxidation process in the crystalline state is responsible for the presence of two bands in their solid-state emission. Two-center, charge-transfer transition from an orbital delocalized on the entire molecule to the central benzene ring is responsible for photophysical properties.

Conformation of the Ester Group Governs the Photophysics of Highly Polarized Benzo[g]coumarins

Photosensitizers that display "unusual" emission from upper electronically excited states offer possibilities for initiating higher-energy processes than what the governing Kasha's rule postulates. Achieving conditions for dual fluorescence from multiple states of the same species requires molecular design and conditions that favorably tune the excited-state dynamics. Herein, we switch the position of the electron-donating NMe₂ group around the core of benzo[g]coumarins (BgCoum) and tune the electronic coupling and the charge-transfer character of the fluorescent excited states. For solvents with intermediate polarity, three of the four regioisomers exhibit fluorescence from two different excited states with bands that are well separated in the visible and the near-infrared spectral regions. Computational analysis, employing ab initio methods, reveals that the orientation of an ester on the pyrone ring produces two conformers responsible for the observed dual fluorescence. Studies with solid solvating media, which restricts the conformational degrees of freedom, concur with the computational findings. These results demonstrate how "seemingly inconsequential" auxiliary substituents, such as the esters on the pyrone coumarin rings, can have profound effects leading to "anti-Kasha" photophysical behavior important for molecular photonics, materials engineering, and solar-energy science.

Synthesis and Excited-State Properties of Donor-Acceptor Azahelical Coumarins

A set of three donor-acceptor azahelical coumarins (DA-AHCs), namely, H-AHC, Me-AHC, and Ph-AHC, were rationally designed and synthesized, and their excited-state properties were comprehensively investigated. All three DA-AHCs are shown to display very high fluorosolvatochromic shifts as a result of significant intramolecular charge transfer in their excited states. The para-quinoidal forms of the latter apparently contribute predominantly to large dipole moments in their excited states. By virtue of the fact that these helical systems structurally incorporate a highly fluorescent coumarin dye, they exhibit high quantum yields in both solution and solid states. Indeed, their emission behaviors in the crystalline media are shown to be remarkably correlated with their respective crystal packings. Incisive analyses demonstrate (i) strengthening of hydrogen bonding in the excited state promotes quenching (H-AHC), (ii) efficient crystal packing promotes high emission (Me-AHC) by precluding deactivations via vibrational motions, and (iii) loose crystal packing contributes to excited-state deactivation to account for low quantum yields of emission (Ph-AHC).

Influence of Triptycene Annulation on the Photochromism of Diphenylnaphthopyrans: Entropic Control of Thermal Reversion

Regioisomeric naphthopyrans annulated with triptycene, i. e., Prox-NP and Dist-NP, display divergent photochromic behaviors. While steady-state photolysis of Dist-NP led to a very labile colored intermediate that is not observable at room temperature, Prox-NP yielded a remarkably stable species characterized by X-ray crystallography as the TT isomer of o-quinonoid intermediate (Prox-NPQ) with t_1/2 ca. 0.18 years at 298 K. The kinetic analysis of thermal reversion reveals that the bleaching of Prox-NPQ is entropically controlled; the steric effect due to the rigid triptycene scaffold renders Prox-NP a highly constrained system such that the photogenerated colored o-quinonoid form is more entropically relaxed. This constitutes the first instance of an entropically-controlled thermal reversion for the celebrated class of photochromic naphthopyrans. Based on the response of Prox-NP and its colored intermediate Prox-NPQ to different stimuli, namely, light, heat, and acid, the molecular system can be likened to a logic gate with the 'INHIBIT' function.

Janus-Type AIE Fluorophores: Synthesis and Properties of π-Extended Coumarin-Bearing Triskelions

Janus-type triskelion-shaped fluorophores comprising coumarins bearing various electron-donating substituents (1aad, 1add, 1ccd, and 1cdd) were successfully synthesized via an intramolecular Ullmann coupling. Density functional theory (DFT) calculations indicated that all the compounds presented two different molecular surfaces, similar to Janus-type molecules. The absorption and fluorescence spectra of asymmetrical derivatives 1aad, 1add, 1ccd, and 1cdd exhibited a bathochromic shift due to their narrow highest occupied molecular orbital (HOMO) –lowest unoccupied molecular orbital (LUMO) gap. Natural transition orbital (NTO) analysis indicated that the excited state orbital overlaps differ among the C3 symmetrical and asymmetrical dyes. These triskelion-shaped fluorophores were found to form molecular nanoaggregates in THF/H2O mixtures and demonstrated aggregation-induced emission (AIE) enhancement characteristics as a result of restricting their molecular inversion. These results indicate that Janus-type AIE fluorophores are potentially applicable as solid-state fluorescent chiral materials, which can be optimized by controlling their molecular rearrangement in the solid state.

Polarized, V-Shaped, and Conjoined Biscoumarins: From Lack of Dipole Moment Alignment to High Brightness

Eleven conjoined coumarins possessing a chromeno[3,4-c]chromene-6,7-dione skeleton have been synthesized via the reaction of electron-rich phenols with esters of coumarin-3-carboxylic acids, catalyzed by either Lewis acids or 4-dimethylaminopyridine. Furthermore, Michael-type addition to angular benzo[f]coumarins is possible, leading to conjugated helical systems. Arrangement of the electron-donating amino groups at diverse positions on this heterocyclic skeleton makes it possible to obtain π-expanded coumarins with emission either sensitive to, or entirely independent of, solvent polarity with large Stokes shifts. Computational studies have provided a rationale for moderate solvatochromic effects unveiling the lack of collinearity of the dipole moments in the ground and excited states. Depending on the functional groups present, the obtained dyes are highly polarized with dipole moments of ∼14 D in the ground state and ∼20–25 D in the excited state. Strong emission in nonpolar solvents, in spite of the inclusion of a NO₂ group, is rationalized by the fact that the intramolecular charge transfer introduced into these molecules is strong enough to suppress intersystem crossing yet weak enough to prevent the formation of dark twisted intramolecular charge transfer states. Photochemical transformation of the dye possessing a chromeno[3,4-c]pyridine-4,5-dione scaffold led to the formation of a spirocyclic benzo[g]coumarin.

Coronene-embedded 'super' coumarins

A non-planar π-enlarged coronene-embedded coumarin achieved via pyranone annulation and its cyanated derivative possess bright orange to red emission in both solution and solid states, with scope for bioimaging applications. The multiple redox states, frontier molecular orbital energy levels and strong cofacial π-stacking ability point towards applications in organic optoelectronics.

Polarized Helical Coumarins: [1,5] Sigmatropic Rearrangement and Excited-State Intramolecular Proton Transfer

The tandem process of phenol addition to a cyclic α,β-unsaturated ester followed by intramolecular transesterification and [1,5] sigmatropic rearrangement affords a series of helical coumarins based upon a previously unknown 3-amino-7-hydroxybenzo[3,4]cyclohepta[1,2-c]chromen-6-one core. These novel polarized coumarins, possessing a β-ketoester moiety, have been employed to synthesize more rigid and helical coumarin–pyrazolones, which display green fluorescence. The enhanced emission of coumarin–pyrazolones in polar solvents depends on the nature of the S₁ state. The coumarin–pyrazolones are predicted to have two vertical states close in energy: a weakly absorbing S₁ (¹LE) followed by a bright S₂ state (¹CT). In polar solvents, the ¹CT can be stabilized below the ¹LE and may become the fluorescent state. Solvatochromism of the fluorescence spectra confirms this theoretical prediction. The presence of an N—H···O=C intramolecular hydrogen bond in these coumarin–pyrazolone hybrids facilitates excited-state intramolecular proton transfer (ESIPT). This process leads to a barrierless conical intersection with the ground electronic state and opens a radiationless deactivation channel effectively competing with fluorescence. Solvent stabilization of the CT state increases the barrier for ESIPT and decreases the efficiency of the nonradiative channel. This results in the observed correlation between solvatochromism and an increase of fluorescence intensity in polar solvents.

One-Pot Synthesis of 4-Carboalkoxy-Substituted Benzo[h]coumarins from α- and β-Naphthols and Their Excited-State Properties

One-pot synthesis has been developed for 4-carboethoxybenzo[h]coumarins starting from α-/β-naphthols. Accordingly, diverse 4-carboethoxybenzocoumarins can be synthesized in moderate-to-excellent (31-75%) isolated yields. The synthesis involves initial oxidation of naphthols to the intermediary 1,2-naphthoquinones with 2-iodoxybenzoic acid followed by a cascade of reactions, namely, Wittig olefination, Michael addition, β-elimination, and cyclization. Furthermore, we have comprehensively investigated the excited-state properties of differently substituted 4-carboalkoxybenzo[h]coumarins. It is shown that they exhibit low to high fluorescence quantum yields (1-36%) and excited-state lifetimes (ca. 1-7 ns) depending on the substitution pattern and solvent employed.

Photochemical Functionalization of Helicenes

Herein, a visible-light photochemical approach for practical helicene functionalization at very mild reaction conditions is described. The photochemical reactions allow for the regiospecific and innate late-stage functionalization of helicenes and are easily executed either through the activation of C(sp2 )-Br bonds in helicenes using K2 CO3 as inorganic base or direct C(sp2 )-H helicene bond functionalization under oxidative photoredox reaction conditions. Overall, using these transformations six different functional groups are introduced to the helicene scaffold through C-C and four different C-heteroatom bond-forming reactions.

The Interplay between Solvation and Stacking of Aromatic Rings Governs Bright and Dark Sites of Benzo[g]coumarins

Coumarins are classic, strongly polarized fluorophores with multiple applications, and significant efforts have been put into modifying their emission characteristics and elucidating their photophysics. Expecting that π-expansion of these donor-acceptor chromophores could modify their ground- and excited-state characteristics, the authors performed combined, detailed photophysical and computational studies of linearly π-expanded coumarins, that is, 8-dialkylamino-3-carboxyalkyl-benzo[g]coumarins. The investigation led to the conclusion that emission is only possible thanks to the stabilizing effect of the solvent and that breaking of the lactone ring leads to the conical intersection with the ground state and induces the radiationless decay of the electronic excitation. Aiming at the fine-tuning the excited state properties through the construction of covalently linked dye assemblies, the authors designed and synthesized a new bis(benzo[g]coumarin), built from two similar moieties that exhibit different degrees of polarization due to the electron donor at position 8: one possesses a dialkylamino, and the other a weaker amide donor. Comprehensive studies have shown that the observed weak fluorescence of the system is the result of the interplay between the solvation-induced separation of the benzo[g]coumarin moieties, which stabilizes the emitting locally excited singlet state and the π-stacking interactions, favoring their sandwiched orientation and leading to the non-emissive charge-transfer state.

Accessing [g]-Face π-Expanded Fluorescent Coumarins by Scholl Cyclization

[g]-Face π-expanded coumarins are synthesized by employing the Scholl cyclization method. These new arene-annulated dipolar coumarins display interesting absorption and fluorescent properties. The large Stokes shifts, tuneable fluorescent quantum yields, and high photostability reveal promise in bioimaging application.

Coumarin-Annelated Regioisomeric Heptahelicenes: Influence of Helicity on Excited-State Properties and Chiroptical Properties

Two regioisomeric pairs of heptahelical mono- and biscoumarins that are differentiated by "inward" and "outward" disposition of the pyran-2-one moiety have been synthesized and investigated to understand the influence of helicity on excited-state and chiroptical properties. A slight variation in the helicities is found to manifest in contrasting excited-state properties of coumarin-annelated heptahelicenes; in addition to the intramolecular charge transfer, structural relaxation in the excited state is shown from theoretical calculations to cause decrease in the fluorescence quantum yield for a system with higher helicity. The optically pure enantiomers of heptahelical coumarins exhibit helicity-dependent chiroptical properties, namely, specific rotations, molar ellipticities, Cotton effects, and anisotropic dissymmetry factors. Theoretical calculations point to factors that are not readily explicable.

Building Molecular Complexity from Quinizarin: Conjoined Coumarins and Coronene Analogs

The double Knoevenagel condensation of 1,4-dibenzoyloxyanthraquinone with methyl esters of arylacetic acids affords a series of compounds based upon a previously unknown 1,8-dioxa-benzo[e]pyrene-2,7-dione heterocyclic core. The aryl groups incorporated in the 3- and 6-positions can be oxidatively coupled to the π-expanded backbone to produce a further new heterocyclic core: 1,10-dioxa-dibenzo[dj]coronene-2,9-dione. The intriguing optical properties of these π-expanded coumarin derivatives are discussed and rationalized through quantum chemical calculations. The broad absorption bands of 1,8-dioxa-benzo[e]pyrene-2,7-dione-based dyes are attributed to both HOMO-1→LUMO and HOMO→LUMO transitions, which have a similar energy. Weakly coupled electron-donating aryl substituents result in a moderate bathochromic shift of both the absorption and emission by 30-60 nm in toluene. The emissive properties of these compounds are in part determined by the oscillator strength of the main transition, lifetimes of the excited state, and by the energy match of the excited state with a triplet state of a similar energy. The 1,10-dioxa-dibenzo[dj]coronene-2,9-dione displays a much smaller Stokes shift, yet a markedly increased fluorescence quantum yield of 90 % owing to the increased rigidity compared with the 1,8-dioxa-benzo[e]pyrene-2,7-dione core.

The synthesis and photophysical properties of tris-coumarins

A structurally unique cyclic tris-coumarin possessing three identical coumarin units bridged by amide linkers as well as two linear analogs has been synthesized. There is a remarkable agreement between crystallographic data, 1H NMR and results of calculations for the cyclic tris-coumarin, showing in all cases a non-symmetric arrangement of identical coumarin moieties. Weak polarization of the coumarin subunits, resulting from the presence of only CONH- groups as electron-donors, results in a hypsochromic shift of both absorption and emission in this dye. We have proven that in non-cyclic, head-to-tail linked tris-coumarins, the photophysics is controlled not only by the substituents but also by the conformation of the molecule, which in turn depends on the nature of the linker's interactions. These can be controlled by the presence/absence of an amide-type hydrogen atom responsible for the formation of intramolecular hydrogen bonds. The presence of a hydrogen bond favors a stretched trans conformation of the dye, while in its absence, folding of the molecule occurs leading to a more compact conformation. Although, the increased number of covalently linked coumarin units does not drastically change the preferred conformation, the fluorescence quantum yields of tris-coumarins are significantly lower than for analogous bis-coumarins composed of the same units.

Anionic merocyanine dyes based on thiazol-2-hydrazides: reverse solvatochromism, preferential solvation and multiparametric approaches to spectral shifts

Reverse solvatochromism, preferential solvation and multiparametric approaches to spectral shifts of (4-nitro/cyanophenyl)-substituted thiazol-2-hydrazide colored merocyanine dyes are presented.

Effect of conformational flexibility on photophysics of bis-coumarins

The fluorescence of bis-coumarins linked via CONH and COO functionalities is strongly dependant on solvent polarity.