Highly Luminescent Aceno[6]helicenones by Intramolecular Radical Cyclization.

Helicenes and helicenoid structures are promising candidates for future applications exploiting circularly polarized light. Ideal candidates should possess near-quantitative photoluminescence quantum yield, a high luminescence dissymmetry factor and an adjustable HOMO-LUMO gap. However, carbo[n]helicenes are poorly luminescent compounds and they absorb light mainly in the ultraviolet region.

Carbonyl mediated fluorescence in aceno[]helicenones and fluoreno[]helicenes.

Helicenes are very attractive chiral non-planar polycyclic aromatic hydrocarbons possessing strong chiroptical properties. However, most of the helicenes absorb light mainly in the ultraviolet region, with only a small segment in the blue part of the visible spectrum. Furthermore, carbo[]helicenes exhibit only weak luminescence that limits their utilization.

Strongly Polarized π-Extended 1,4-Dihydropyrrolo[3,2-]pyrroles Fused with Tetrazolo[1,5-]quinolines.

A straightforward route to 1,4-dihydropyrrolo[3,2-]pyrroles comprised of two electron-withdrawing quinoline or tetrazolo[1,5-]quinoline scaffolds has been developed. The versatile multicomponent reaction affording 1,4-dihydropyrrolo[3,2-]pyrroles combined with intramolecular direct arylation enables assembly of these products in just three steps from anilines with overall yields exceeding 30%. The planarized, ladder-type heteroacenes possess up to 14 conjugated rings.

2,3-Dichloroanthracene crystal, a new rigid matrix for single molecule optical investigations.

Absorption and emission spectra of single crystals of 2,3-dichloroathracene (23DCA) and 23DCA dispersed in n-nonane matrix were studied at 5 K. Singlet and triplet excitonic bands in the crystal were estimated to be at about 415 nm and at wavelengths shorter than 700 nm, respectively. Thus, from the spectroscopic point of view, these crystals satisfy all criteria for a transparent and rigid matrix for low temperature optical studies of single molecules of dibenzoterrylene, which have their purely electronic S→S transition at around 785 nm.

Probing the in-plane dipole moment vector between ground and excited state of single molecules by the Stark effect.

Single molecules, embedded inside a well-defined insertion site of a single-crystalline host matrix, are sensitive probes of electric field via the induced Stark shift on their lifetime-limited electronic transition. Though the response of molecules to electric field has been shown to be relatively homogeneous, crystal symmetry allows for several, spectroscopically-indistinguishable, orientations of the net permanent dipole moment between the ground and excited state - the dipole vector - and this is problematic for measuring field orientation and magnitude. In this work, we measure for each terrylene molecule, embedded inside a new host matrix, the dipole vector independently by an electric field that we can rotate in the plane of the crystal.

Ultrafast excited state dynamics of pyridine N-oxide derivative in solution; femtosecond fluorescence up-conversion and theoretical calculations.

In this study we have investigated 2-ethylamino-4-nitro-6-methyl pyridine N-oxide (2E6M) molecule that belongs to important group of Proton Coupled Electron Transfer (PCET) compounds where both the charge transfer (CT) and proton transfer processes in excited states may proceed. In this case, this is possible due to the donors and acceptors of electrons and protons in this system, as well as due to the presence of intramolecular {N-H… O [2,566(3) Å}, hydrogen bond.Using stationary and time-resolved spectroscopy, as well as quantum chemical calculations on the DFT and TD DFT B3LYP/6-31G (d,p) level of theory, a partial CT nature of the S → S transition in both tautomeric forms (N and T) has been revealed.

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

Eleven conjoined coumarins possessing a chromeno[3,4-]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[]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.

High-resolution vibronic spectroscopy of a single molecule embedded in a crystal.

Vibrational levels of the electronic ground states in dye molecules have not been previously explored at a high resolution in solid matrices. We present new spectroscopic measurements on single polycyclic aromatic molecules of dibenzoterrylene embedded in an organic crystal made of para-dichlorobenzene. To do this, we use narrow-band continuous-wave lasers and combine spectroscopy methods based on fluorescence excitation and stimulated emission depletion to assess individual vibrational linewidths in the electronic ground state at a resolution of ∼30 MHz dictated by the linewidth of the electronic excited state.

Quadrupolar Dyes Based on Highly Polarized Coumarins.

The fluorescence and other photophysical parameters of highly polarized, quadrupolar bis-coumarins possessing an electron-rich pyrrolo[3,2-]pyrrole bridging unit are highly dependent on the linking position between both chromophores. Delocalization of the LUMO on the entire π-system results in intense emission and strong two-photon absorption.

Spectra and nature of the electronic states of [1]Benzothieno[3,2-b][1]benzothiophene (BTBT): Single crystal and the aggregates.

Absorption, fluorescence, and phosphorescence spectra of single crystals of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and BTBT dispersed in frozen n-nonane, n-hexadecane, and dichloromethane matrices were studied at 5 K. Observation of a new absorption band and related changes in the fluorescence to phosphorescence intensity ratio, when the concentration of BTBT in the matrix increased above 10M, indicated the presence of BTBT aggregates. Quantum-chemistry calculations performed for the simplest aggregate, isolated dimer, showed that its structure is similar to the "herringbone" element in the BTBT crystal unit cell and the lowest electronic excited singlet state of the dimer has the intermolecular charge-transfer character.

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.

Laser-Induced Frequency Tuning of Fourier-Limited Single-Molecule Emitters.

The local interaction of charges and light in organic solids is the basis of distinct and fundamental effects. We here observe, at the single-molecule scale, how a focused laser beam can locally shift by hundreds of times their natural line width and, in a persistent way, the transition frequency of organic chromophores cooled at liquid helium temperature in different host matrices. Supported by quantum chemistry calculations, the results can be interpreted as effects of a photoionization cascade, leading to a stable electric field, which Stark-shifts the molecular electronic levels.

Luminescent properties of chameleon-like metal-organic framework between zinc(II) dichloride and two quinoline-N-oxide molecules.

Zinc compounds in the form of inorganic/organic hybrids containing both zinc halides and heterocyclic ligands show various interesting optical and physicochemical properties. Due to these properties, there is a potential for development of various innovative technologies and applications within the life sciences. In this study, experimental and theoretical results on the absorption and emission (steady state and time-resolved) properties of the hybrid ZnCl(QO) complex formed between ZnCl and quinoline N-oxide, have been reported.

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.

Matrix-induced Linear Stark Effect of Single Dibenzoterrylene Molecules in 2,3-Dibromonaphthalene Crystal.

Absorption and fluorescence from single molecules can be tuned by applying an external electric field - a phenomenon known as the Stark effect. A linear Stark effect is associated to a lack of centrosymmetry of the guest in the host matrix. Centrosymmetric guests can display a linear Stark effect in disordered matrices, but the response of individual guest molecules is often relatively weak and non-uniform, with a broad distribution of the Stark coefficients.

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.

Unprecedented rearrangement of diketopyrrolopyrroles leads to structurally unique chromophores.

Diketopyrrolopyrroles possessing thienyl, furyl and benzofuryl substituents undergo unprecedented skeletal rearrangement in the presence of trimethylsilyl bromide resulting in the formation of thieno[2,3-f]isoindole-5,8-diones and furo[2,3-f]isoindole-5,8-diones. These relatively small dyes possess favorable photophysical properties with the emission maxima within the range of 573-624 nm, large fluorescence quantum yields, moderate sensitivity of emission to solvent polarity and a HOMO-LUMO gap of ca. 1.

Anomalous doping of a molecular crystal monitored with confocal fluorescence microscopy: Terrylene in a p-terphenyl crystal.

Highly terrylene doped single crystals of p-terphenyl, obtained by co-sublimation of both components, showed bright spots in the confocal fluorescence images. Polarization of the fluorescence excitation spectra, blinking and bleaching, and saturation behavior allowed us to attribute them to single molecules of terrylene anomalously embedded between two neighbor layers of the host crystal, in the (a,b) plane. Such an orientation of terrylene molecules results in much more efficient absorption and collection of the fluorescence photons than in the case of previously investigated molecules embedded in the substitution sites.

Photophysical behavior of a potential drug candidate, trans-[2-(4-methoxystyryl)]quinoline-1-oxide tuned by environment effects.

Styryl and/or quinoline structural fragments, present in a large number of bioactive substances, inspired the design of various new drug candidates. In this paper, we describe the photophysical behavior of trans-[2-(4-methoxystyryl)]quinoline-1-oxide (trans-MSQNO) on the basis of X-ray analysis data, theoretical calculations as well as steady state and time-resolved spectroscopy experiments in various media. The molecule crystallizes in orthorhombic unit cell containing eight molecules of N-oxide, space group Pbca.

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.

Unexpected formation of π-expanded isoquinoline from anthracene possessing four electron-donating groups via the Duff reaction.

New synthetic methods leading towards π-expanded heterocycles are sought after mainly due to their promising opto-electronic properties. Subjecting 1,5,9,10-tetramethoxyanthracene to the modern Duff reaction conditions led to the formation of a compound possessing the 2-azabenzoanthrone (dibenzo[de,h]isoquinolin-7-on) skeleton instead of the expected dialdehyde. This non-typical course of reaction can be rationalized by the double electrophilic aromatic substitution at two neighboring electron-rich positions of anthracene followed by oxidation of the resulting intermediate to form a pyridine ring.

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.

Dipyrrolonaphthyridinediones--structurally unique cross-conjugated dyes.

Red-emissive dyes based on a previously unknown skeleton--dipyrrolo[1,2-b:1',2'-g][2,6]naphthyridine-5,11-dione--can be easily synthesized from simple and inexpensive reagents by one- or two-step routes. A careful selection of the substituents gives access to a variety of dipyrrolonaphthyridinedione derivatives with intense fluorescence in the range of 520-740 nm.

Solvatofluorochromic, non-centrosymmetric π-expanded diketopyrrolopyrrole.

A novel non-centrosymmetric π-expanded diketopyrrolopyrrole was designed and synthesized. Strategic placement of tert-butyl groups at the periphery of a diketopyrrolopyrrole allowed us to selectively fuse one moiety via tandem Friedel-Crafts-dehydration reactions, resulting in a non-centrosymmetric dye. The structure of the dye was confirmed by X-ray crystallography, revealing that it contains a nearly flat arrangement of four fused rings.

Double head-to-tail direct arylation as a viable strategy towards the synthesis of the aza-analog of dihydrocyclopenta[hi]aceanthrylene--an intriguing antiaromatic heterocycle.

The first case of double head-to-tail direct arylation of aromatic compounds and the unusual photophysical properties of the resulting 2,2a(1,5)b(1,7)-tetraazacyclopenta[hi]aceanthrylene are reported. This molecule, comprising of two imidazo[1,2-a]pyridine units, is antiaromatic due to the changes in the efficiency of π-electron ring current and it belongs to a class of seldom encountered compounds with a dark lowest electronically excited singlet state.