Realization of nitroaromatic chromophores with intense two-photon brightness.

Strong fluorescence is a general feature of dipyrrolonaphthyridinediones bearing two nitrophenyl substituents. Methyl groups simultaneously being weakly electron-donating and inducing steric hindrance appear to be a key structural parameter that allows for significant emission enhancement, whereas Et2N groups cause fluorescence quenching. The magnitude of two-photon absorption increases if 4-nitrophenyl substituents are present while the contribution of Et2N groups is detrimental.

Tuning the aromatic backbone twist in dipyrrolonaphthyridinediones.

This communication describes the photophysical behavior of three analogs of cyclophane bearing the dipyrrolonaphthyridinedione (DPND) core. In these molecules, intersystem crossing (ISC) can be successfully induced by distinct changes in the deviation from planarity within the DPND core, allowing at the same time the emission maximum to shift from the green to red region of the visible spectrum without any synthetic modifications of the chromophore structure. This finding may build the foundation for a new paradigm for inducing ISC-type transitions within other centrosymmetric and planar cross-conjugated chromophores.

Direct transformation of coumarins into orange-red emitting rhodols.

The lactone carbonyl group of coumarin derivatives has been shown to participate in intramolecular Knoevenagel condensations, enabling the unprecedented direct transformation of coumarins into rhodols. The resulting rhodols, possessing two ester groups, have very intense orange-red fluorescence.

Potent strategy towards strongly emissive nitroaromatics through a weakly electron-deficient core.

Nitroaromatics seldom fluoresce. The importance of electron-deficient (n-type) conjugates, however, has inspired a number of strategies for suppressing the emission-quenching effects of the strongly electron-withdrawing nitro group. Here, we demonstrate how such strategies yield fluorescent nitroaryl derivatives of dipyrrolonaphthyridinedione (DPND). Nitro groups near the DPND core quench its fluorescence. Conversely, nitro groups placed farther from the core allow some of the highest fluorescence quantum yields ever recorded for nitroaromatics. This strategy of preventing the known processes that compete with photoemission, however, leads to the emergence of unprecedented alternative mechanisms for fluorescence quenching, involving transitions to dark nπ* singlet states and aborted photochemistry. Forming nπ* triplet states from ππ* singlets is a classical pathway for fluorescence quenching. In nitro-DPNDs, however, these ππ* and nπ* excited states are both singlets, and they are common for nitroaryl conjugates. Understanding the excited-state dynamics of such nitroaromatics is crucial for designing strongly fluorescent electron-deficient conjugates.

Red emissive sulfone-rhodols as mitochondrial imaging agents.

The controlled hydrolysis of sulfone-rhodamines affords a series of core-modified red-emitting rhodols, the fluorescence of which is sensitive to solvent polarity with pronounced bathochromic shifts recorded in both DMSO and CH3CN combined with an up to 8-fold increase in the fluorescence quantum yield.

How an Eight-Membered Ring Alters the Rhodamine Chromophore.

Readily available phenylene-1,3-diamines can be converted into unprecedented analogues of rhodamine and malachite green possessing a central eight-membered ring in three steps. The overall process couples a cyanine chromophore with a urea bridge giving rise to new dyes possessing distinct spectral characteristics: absorption of orange light combined with a weak emission of red light both in solution and in the crystalline state. Their photophysics is governed by the twist of lateral phenyl rings and intramolecular and intermolecular CT transitions.

Deciphering the unusual fluorescence in weakly coupled bis-nitro-pyrrolo[3,2-b]pyrroles.

Electron-deficient π-conjugated functional dyes lie at the heart of organic optoelectronics. Adding nitro groups to aromatic compounds usually quenches their fluorescence via inter-system crossing (ISC) or internal conversion (IC). While strong electronic coupling of the nitro groups with the dyes ensures the benefits from these electron-withdrawing substituents, it also leads to fluorescence quenching. Here, we demonstrate how such electronic coupling affects the photophysics of acceptor-donor-acceptor fluorescent dyes, with nitrophenyl acceptors and a pyrrolo[3,2-b]pyrrole donor. The position of the nitro groups and the donor-acceptor distance strongly affect the fluorescence properties of the bis-nitrotetraphenylpyrrolopyrroles. Concurrently, increasing solvent polarity quenches the emission that recovers upon solidifying the media. Intramolecular charge transfer (CT) and molecular dynamics, therefore, govern the fluorescence of these nitro-aromatics. While balanced donor-acceptor coupling ensures fast radiative deactivation and slow ISC essential for large fluorescence quantum yields, vibronic borrowing accounts for medium dependent IC via back CT. These mechanistic paradigms set important design principles for molecular photonics and electronics.

Rhodols - synthesis, photophysical properties and applications as fluorescent probes.

The formal replacement of one dialkylamino group in rhodamines with a hydroxyl group transforms them into rhodols. This apparently minor difference is not as small as one may think; rhodamines belong to the cyanine family whereas rhodols belong to merocyanines. Discovered in the late 19th century, rhodols have only very recently begun to gain momentum in the field of advanced fluorescence imaging. This is in part due to the increased understanding of their photophysical properties, and new methods of synthesis. Rationalization of how the nature and arrangement of polar substituents around the core affect the photophysical properties of rhodols is now possible. The emergence of so-called π-expanded and heteroatom-modified rhodols has also allowed their fluorescence to be bathochromically shifted into regions applicable for biological imaging. This review serves to outline applicable synthetic strategies for the synthesis of rhodols, and to highlight important structure-property relationships. In the first part of this Review, various synthetic methods leading to rhodols are presented, followed by structural considerations and an overview of photophysical properties. The second part of this review is entirely devoted to the applications of rhodols as fluorescent reporters in biological imaging.

Unforeseen 1,2-Aryl Shift in Tetraarylpyrrolo[3,2- b]pyrroles Triggered by Oxidative Aromatic Coupling.

Tetraarylpyrrolo[3,2- b]pyrroles (TAPPs) possessing [1,1'-biphenyl]-2-yl substituents attached to the pyrrolic nitrogen atoms undergo selective double dehydrogenative cyclization accompanied by twofold 1,2-aryl migration under oxidative aromatic coupling conditions. The structure of the product of the rearrangement has been unambiguously confirmed by X-ray crystallography, and the reaction pathway is supported by density functional theory (DFT) calculations. Six-membered ring formation (requiring rearrangement of aryl substituents around the core) is energetically preferred over seven-membered ring closure, and a 1,2-aryl shift occurs via arenium cation intermediate.

How a Small Structural Difference Can Turn Optical Properties of π-Extended Coumarins Upside Down: The Role of Non-Innocent Saturated Rings.

The fluorescence properties of two new families of heterocycles possessing either a seven- or five-membered ring attached at the core molecule are entirely different in solution and in the solid state. Crystallization has the effect of inhibiting non-radiative excited-state deactivation pathways, operative in solution for the seven-membered ring compounds, thus leading to significant fluorescence efficiency in the solid state, with quantum yields ranging from 0.10 to 0.36. Conversely, the five-membered ring derivatives, which display notable emission properties in solution, are almost non-emissive in the crystalline state, characterized by a long-range π-stacked arrangement. When embedded in polymeric films, both series show fluorescence features similar to the solution case, with remarkable fluorescence quantum yields ranging from 0.09 to 0.41. According to quantum chemical calculations, 3H-chromeno[3,4-c]pyridine-4,5-diones show the specific mechanism of fluorescence quenching. The derivatives bearing the seven-membered ring undergo, in solution, a significant structural deformation in the excited state, resulting in a large decrease of the energy gap between S1 and S0 and hence to a substantial contribution of the internal conversion in the relaxation process. The fluorescence quenching of the five-membered ring derivatives is in turn related to the intermolecular interaction between adjacent molecules prevailing to a greater extent in the crystal lattice.

Dipolar Dyes with a Pyrrolo[2,3-b]quinoxaline Skeleton Containing a Cyano Group and a Bridged Tertiary Amino Group: Synthesis, Solvatofluorochromism, and Bioimaging.

Two strongly polarized dipolar chromophores possessing a cyclic tertiary amino group at one terminus of the molecule and a CN group at the opposite terminus were designed and synthesized. Their rigid skeleton contains the rarely studied pyrrolo[2,3-b]quinoxaline ring system. The photophysical properties of these regioisomeric dyes were different owing to differing π conjugation between the CN group and the electron-donor moiety. These dipolar molecules showed very intense emission, strong solvatofluorochromism, and sufficient two-photon brightness for bioimaging. One of these regioisomeric dyes, namely, 11-carbonitrile-2,3,4,5,6,7-hexahydro-1H-3a,8,13,13b-tetraazabenzo[b]cyclohepta[1,2,3-jk]fluorene, was successfully utilized in two-photon imaging of mouse organ tissues and showed distinct tissue morphology with high resolution.

Pyrrolo[3,2-b]pyrroles-From Unprecedented Solvatofluorochromism to Two-Photon Absorption.

A combined experimental and theoretical study of the two-photon absorption (2PA) properties of a series of quadrupolar molecules possessing a highly electron-rich heterocyclic core, pyrrolo[3,2-b]pyrrole, is presented. In agreement with quantum-chemical calculations, large 2PA cross-section values, σ2PA ≈10(2) -10(3)  GM (1 GM=10(50)  cm(4) s photon(-1) ), are observed at wavelengths of 650-700 nm, which correspond to the two-photon allowed but one-photon forbidden transitions. The calculations also predict that increased planarity of this molecule through removal of two N-substituents leads to further increase in the σ2PA values. Surprisingly, the most quadrupolar pyrrolo[3,2-b]pyrrole derivative, containing two 4-nitrophenyl substituents at positions 2 and 5, demonstrates a very strong solvatofluorochromic effect, with a fluorescence quantum yield as high as 0.96 in cyclohexane, whereas the fluorescence vanishes in DMSO.

Subtle structural changes in octupolar merocyanine dyes influence the photosensitized production of singlet oxygen.

The photophysical properties of two indoline-based octupolar merocyanine dyes and of the corresponding quinoline-based dyes were examined. This seemingly subtle structural change in the chromophore of these molecules has an appreciable effect on the yields with which these respective compounds sensitize the production of singlet molecular oxygen, O2(a(1)Δg). The indoline-based dyes are reasonably efficient O2(a(1)Δg) sensitizers (Ï•Δ âˆ¼ 0.35), whereas the quinoline-based dyes are poor O2(a(1)Δg) sensitizers (Ï•Δ âˆ¼ 0.005). A series of experiments, including Laser-Induced Optoacoustic Calorimetric (LIOAC) measurements, reveal that this difference principally reflects the fact that the excited singlet state of the quinoline-based dyes rapidly and efficiently decays via nonradiative channels to regenerate the ground state molecule. It is likely that a charge-transfer state mediates this efficient coupling between the excited and ground states. Such subtle, structure-dependent effects are important in elucidating and ultimately understanding phenomena that influence the efficiency of photosensitized O2(a(1)Δg) production. In turn, the knowledge gained facilitates the rational design and preparation of O2(a(1)Δg) sensitizers with explicitly controlled properties.

V-shaped bis-coumarins: synthesis and optical properties.

A highly efficient procedure for the synthesis of bis-coumarins fused at the pyranone ring has been developed. The electron-rich phenols reacted with esters of coumarin-3-carboxylic acids, leading to substituted chromeno[3,4-c]chromene-6,7-diones. The reaction is catalyzed by both Lewis acids and 4-dimethylaminopyridine. The most probable mechanistic pathway involves Lewis acid catalyzed or DMAP catalyzed transesterification, followed by intramolecular conjugate addition of α,ß-unsaturated esters to phenols and subsequent oxidation of the initially formed intermediate. The reaction is compatible with various functionalities such as NO2, Br, and OMe. Not only benzene derivatives but also dihydroxynaphthalenes are reactive in this reaction, and the structure of the product can be controlled by adjusting the reaction conditions. Furthermore, a double addition is possible, leading to a horseshoe-shaped system comprised of seven conjugated rings. Compounds with four structurally unique skeletons have been obtained and have been shown to strongly absorb in the violet, blue, and/or green regions of the visible spectrum. Most of them display strong greenish yellow fluorescence, which can be modulated by both structural changes and the character of the solvents. Again, introduction of an electron-donating group in the chromeno[3,4-c]chromene-6,7-diones caused a significant red shift in both the absorption and emission maxima, and the effect became especially noteworthy in the case of amino substituents.

Experimental and theoretical investigation of the first-order hyperpolarizability of octupolar merocyanine dyes.

Hyper-Rayleigh scattering experiments and quantum chemical calculations are combined to investigate the second-order nonlinear optical responses of a series of three-arm merocyanine derivatives. They exhibit an octupolar hyperpolarizability response with lower amplitude than crystal violet due to a lower extent of the photoinduced charge transfer and reduced bond length alternation. Strong effects on the second-order optical response measured close to the two-photon absorption level are clearly evidenced; for example, the effective measured polarization ratio deviates below the ideal octupolar value of 3/2 even at very low excitation power. These effects are attributed to two-photon absorption resonance, which we believe modifies dynamically the population of the ground state versus that of the excited state.

Pentacyclic coumarin-based blue emitters - the case of bifunctional nucleophilic behavior of amidines.

An unexpected discovery of the novel cyclocondensation reaction of 1,8-diazabicyclo[5.4.0]undec-8-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) with alkyl coumarin-3-carboxylates is described. First, an unknown skeleton is generated through the concomitant formation of new nitrogen-carbon and carbon-carbon bonds followed by the oxidation of the intermediate product by the second equivalent of the starting coumarin. Pentacyclic DBN-derivatives exhibit strong fluorescence both in solutions (Φfl∼ 0.6-0.8) and in the solid state, while non-planar DBU-derivatives exhibit strong fluorescence in the solid state only.

Nonlinear optical chemosensor for sodium ion based on rhodol chromophore.

As part of a strategy to identify good fluorescent probes based on two-photon excited fluorescence (TPEF), the sensor for sodium cation has been designed bearing a rhodol chromophore linked with an aza-crown ether. An efficient synthetic route to rhodol derivatives possessing five-membered heterocycles at position 9 and their precursors that contain xanthylium salt has been developed. The synthesis involves condensation of xanthylium salts bearing vinamidinium moiety at position 9, with phenylhydrazine derivatives as the key step. To accomplish the synthesis of derivatives bearing 1-aza-15-crown-5 and 1,10-diaza-18-crown-6, the Buchwald-Hartwig reaction has been employed in the final stage. Electronic spectra of all prepared rhodols display strong absorption in the range of 450-550 nm with well-resolved vibronic bands, which maintains its fine structure in a wide range of solvents. The most intensive two-photon absorption (2PA) band in the rhodol spectrum (165 GM), located at shorter wavelengths, matches well with the short-wavelength absorption band in the linear electronic spectrum and is most probably related to the two-photon allowed electronic transition S0→S2. The influence of cation binding on one- and two-photon spectroscopic properties of rhodol linked with 1-aza-15-crown-5 via the phenylpyrazole bridge has been investigated. This probe exhibits high sensitivity and good selectivity for Na(+) in CH3CN. The mechanism involves the complexation of the Na(+) by 1-aza-15-crown-5 in the probe, which induces prominent fluorescence enhancement via quenching of electron-transfer. Interestingly, the complexation with Na(+) led to a significant increase of the 2PA band in the 750-800 nm region (corresponding to a two-photon allowed, one-photon forbidden transition) for rhodol bearing 1-aza-15-crown-5, which led to the overall enhancement of the TPEF signal (approximately an order of magnitude). Thus, a turn-on fluorescent probe for sodium ion, which does not respond to many other metal species, has been constructed.

Octupolar merocyanine dyes: a new class of nonlinear optical chromophores.

A set of new octupolar merocyanine chromophores was designed and synthesized. These compounds were prepared from the reaction of 1,3,5-triformyl-2,4,6-trihydroxybenzene with heterocyclic nucleophiles. Octupolar dyes were formed exclusively in their open-dye form. The one- and two-photon-absorption spectra of the dyes consist of two bands: The long-wavelength band in the two-photon absorption spectrum (a few hundreds GM above 1000 nm) matches well with the intense, long-wavelength-absorption band that is located in the visible region in the linear spectrum. Interestingly, an additional, much-more-intense TPA band in the NIR region is observed at higher energy, which corresponds to a weakly allowed one-photon electronic transition. Changing the peripheral heterocyclic moieties allows tuning of the optical properties to approach the cyanine limit (i.e., polymethine state), thus resulting in a red-shift of the low-energy one-photon-absorption band as well as to the rise of an intense two-photon-absorption band in the NIR region. To the best of our knowledge, this is the first synthesis and TPA characterization of octupolar merocyanine chromophores with typical low-bond-length alternation.