Photocatalytic Functionalization of Dehydroalanine-Derived Peptides in Batch and Flow

Unnatural amino acids, and their synthesis by the late-stage functionalization (LSF) of peptides, play a crucial role in areas such as drug design and discovery. Historically, the LSF of biomolecules has predominantly utilized traditional synthetic methodologies that exploit nucleophilic residues, such as cysteine, lysine or tyrosine. Herein, we present a photocatalytic hydroarylation process targeting the electrophilic residue dehydroalanine (Dha). This residue possesses an α,β-unsaturated moiety and can be combined with various arylthianthrenium salts, both in batch and flow reactors. Notably, the flow setup proved instrumental for efficient scale-up, paving the way for the synthesis of unnatural amino acids and peptides in substantial quantities. Our photocatalytic approach, being inherently mild, permits the diversification of peptides even when they contain sensitive functional groups. The readily available arylthianthrenium salts facilitate the seamless integration of Dha-containing peptides with a wide range of arenes, drug blueprints, and natural products, culminating in the creation of unconventional phenylalanine derivatives. The synergistic effect of the high functional group tolerance and the modular characteristic of the aryl electrophile enables efficient peptide conjugation and ligation in both batch and flow conditions.

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

A straightforward route to 1,4-dihydropyrrolo[3,2-b]pyrroles comprised of two electron-withdrawing quinoline or tetrazolo[1,5-a]quinoline scaffolds has been developed. The versatile multicomponent reaction affording 1,4-dihydropyrrolo[3,2-b]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. These nominally quadrupolar materials exhibit efficient fluorescence with wavelengths spanning most of the visible spectrum from green-yellow for the dyes possessing biaryl bridges and orange-red for the fully fused systems. In many cases, the fluorescence quantum yields are large, the solvatofluorochromic effects are strong, and the fluorescence is maintained even in crystalline state. Analysis of the electronic structure of these molecular architectures using quantum chemical methods suggests that the character and position of the flanking heterocycle determine the shape of HOMO and LUMO and their extension to N-aryl substituents, influencing the values of molar absorption coefficient. An experimental study of the two-photon absorption (2PA) properties has revealed that it occurs in the 700-800 nm range with apparent deviation from the Laporte parity selection rule, which may be attributed to Hertzberg-Teller contribution to vibronically allowed 2PA transition.

Quadrupolar, Highly Polarized Dyes: Emission Dependence on Viscosity and Selective Mitochondria Staining

Quadrupolar A-D-A-type 1,4-dihydropyrrolo[3,2-b]pyrroles (DHPPs) bearing pyridinium and quinolinium substituents emit in the 500-600 nm region. The enhancement of electronic communication between the electron-rich heterocyclic core and electron-deficient peripheral substituents turned out to be crucial for achieving emission enhancement in viscous media. DHPP bearing two 4-pyridinium substituents has optical brightness 34,000 in glycerol and only 700 in MeOH, as evidenced by measurements of the emission intensity and fluorescence lifetimes in a series of polar solvents. Such behavior makes it an excellent candidate for viscosity probes in fluorescence microscopy, as demonstrated by the fluorescence imaging of H9C2 cardiomyocytes.

Dipyrrolonaphthyridinedione - (still) a mysterious cross-conjugated chromophore

Dipyrrolonaphthyridinediones (DPNDs) entered the chemical world in 2016. This cross-conjugated donor-acceptor skeleton can be prepared in two steps from commercially available reagents in overall yield ≈15-20% (5 mmol scale). DPNDs can be easily and regioselectively halogenated which opens an avenue to numerous derivatives as well as to π-expansion. Although certain synthetic limitations exist, the current derivatization possibilities provided impetus for numerous explorations that use DPNDs. Structural modifications enable bathochromic shift of the emission to deep-red region and reaching the optical brightness 30 000 M-1 cm-1. Intense absorption and strong emission of greenish-yellow light attracted the interest which eventually led to the discovery of their strong two-photon absorption, singlet fission in the crystalline phase and triplet sensitization. Dipyrrolonaphthyridinedione-based twistacenes broadened our knowledge on the influence of twisting angle on the fate of the molecule in the excited state. Collectively, these findings highlight the compatibility of DPNDs with various applications within organic optoelectronics.

Gold-Catalyzed 1,2-Aryl Shift and Double Alkyne Benzannulation

The tandem intramolecular hydroarylation of alkynes accompanied by a 1,2-aryl shift is described. Harnessing the unique electron-rich character of 1,4-dihydropyrrolo[3,2-b]pyrrole scaffold, we demonstrate that the hydroarylation of alkynes proceeds at the already occupied positions 2 and 5 leading to a 1,2-aryl shift. Remarkably, the reaction proceeds only in the presence of cationic gold catalyst, and it leads to heretofore unknown π-expanded, centrosymmetric pyrrolo[3,2-b]pyrroles. The utility is verified in the preparation of 13 products that bear six conjugated rings. The observed compatibility with various functional groups allows for increased tunability with regard to the photophysical properties as well as providing sites for further functionalization. Computational studies of the reaction mechanism revealed that the formation of the six-membered rings accompanied with a 1,2-aryl shift is both kinetically and thermodynamically favourable over plausible formation of products containing 7-membered rings. Steady-state UV/Visible spectroscopy reveals that upon photoexcitation, the prepared S-shaped N-doped nanographenes undergo mostly radiative relaxation leading to large fluorescence quantum yields. Their optical properties are rationalized through time-dependent density functional theory calculations. We anticipate that this chemistry will empower the creation of new materials with various functionalities.

The magic of biaryl linkers: the electronic coupling through them defines the propensity for excited-state symmetry breaking in quadrupolar acceptor-donor-acceptor fluorophores

Charge transfer (CT) is key for molecular photonics, governing the optical properties of chromophores comprising electron-rich and electron-deficient components. In photoexcited dyes with an acceptor-donor-acceptor or donor-acceptor-donor architecture, CT breaks their quadrupolar symmetry and yields dipolar structures manifesting pronounced solvatochromism. Herein, we explore the effects of electronic coupling through biaryl linkers on the excited-state symmetry breaking of such hybrid dyes composed of an electron-rich core, i.e., 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP), and pyrene substituents that can act as electron acceptors. Experimental and theoretical studies reveal that strengthening the donor-acceptor electronic coupling decreases the CT rates and the propensity for symmetry breaking. We ascribe this unexpected result to effects of electronic coupling on the CT thermodynamics, which in its turn affects the CT kinetics. In cases of intermediate electronic coupling, the pyrene-DHPP conjugates produce fluorescence spectra, spreading over the whole visible range, that in addition to the broad CT emission, show bands from the radiative deactivation of the locally excited states of the donor and the acceptors. Because the radiative deactivation of the low-lying CT states is distinctly slow, fluorescence from upper locally excited states emerge leading to the observed anti-Kasha behaviour. As a result, these dyes exhibit white fluorescence. In addition to demonstrating the multifaceted nature of the effects of electronic coupling on CT dynamics, these chromophores can act as broad-band light sources with practical importance for imaging and photonics.

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.

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.

Excited-state symmetry breaking in quadrupolar pull-push-pull molecules: dicyanovinyl vs. cyanophenyl acceptors

A significant number of quadrupolar dyes behave as their dipolar analogues when photoexcited in polar environments. This is due to the occurrence of excited-state symmetry breaking (ES-SB), upon which the electronic excitation, initially distributed over the whole molecule, localises preferentially on one side. Here, we investigate the ES-SB properties of two A-D-A dyes, consisting of a pyrrolo-pyrrole donor (D) and either cyanophenyl or dicyanovinyl acceptors (A). For this, we use time-resolved vibrational spectroscopy, comparing IR absorption and femtosecond stimulated Raman spectroscopies. Although dicyanovinyl is a stronger electron-withdrawing group, ES-SB is not observed with the dicyanovinyl-based dye even in highly polar media, whereas it already takes place in weakly polar solvents with dyes containing cyanophenyl accepting groups. This difference is attributed to the large electronic coupling between the D-A branches in the former dye, whose loss upon symmetry breaking cannot be counterbalanced by a gain in solvation energy. Comparison with analogues of the cyanophenyl-based dye containing different spacers reveals that interbranch coupling does not so much depend on the distance between the D-A subunits than on the nature of the spacer. We show that transient Raman spectra probe different modes of these centrosymmetric molecules but are consistent with the transient IR data. However, lifetime broadening of the Raman bands, probably due to the resonance enhancement, may limit the application of this technique for monitoring ES-SB.

Novel Method for the Synthesis of Merocyanines: New Photophysical Possibilities for a Known Class of Fluorophores

A new, transformative method for the preparation of rhodols and other merocyanines from readily available tetrafluorohydroxybenzaldehyde and aminophenols has been developed. It is now possible to prepare merocyanines bearing three fluorine atoms and additional conjugated rings, and the whole one-pot process occurs under neutral, mild conditions. Three heretofore unknown merocyanine-based architectures were prepared using this strategy from aminonaphthols and 4-hydroxycoumarins. The ability to change the structure of original rhodol chromophore into π-expanded merocyanines translates to a comprehensive method for the modulation of photophysical properties, such as shifting the absorption and emission bands across almost the entire visible spectrum, reaching a huge Stokes shift i. e. 4800 cm-1 , brightness approximately 80.000 M-1  cm-1 , two-photon absorption cross-section above 150 GM and switching-on/off solvatofluorochromism. A detailed investigation allowed to rationalize the different spectroscopic behavior of rhodols and new merocyanines, addressing solvatochromism and two-photon absorption.

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.

Nonaromatic naphthocorroles

New naphthocorrole ligands, display both the cavity size of corroles and the dianionic character of porphyrins. Nonaromatic and yet flaunting deceptively porphyrin-like optical spectra, they are readily accessible via a simple protocol.

Conversion of Ketones into Blue-Emitting Electron-Deficient Benzofurans

A novel heavy metal-free and safe synthetic methodology enabling one-step conversion of ketones into corresponding 4,5,6,7-tetrafluorobenzofurans (F4 BFs) has been developed. The presented approach has numerous advantageous qualities, including utilization of readily available substrates, broad scope, scalability, and good reaction yields. Importantly, some of the benzofurans prepared by this method were heretofore inaccessible by any other known transformation. Importantly, furo[2,3-b]pyrazines and heretofore unexplored difuro[2,3-c:3',2'-e]pyridazine can be prepared using this strategy. Spectroscopic studies reveal that for simple systems, absorption and fluorescence maxima fall within the UV spectral range, while π-electron system expansion red-shifts both spectra. Moreover, the good fluorescence quantum yields observed in solution, up to 96 %, are also maintained in the solid state. Experimental results are supported by density functional theory (DFT) calculations. The presented methodology, combined with the spectroscopic characteristics, suggest the possibility of using F4 BFs in the optoelectronic industry (i. e., organic light emitting devices (OLED), organic field-effect transistors (OFET), organic photovoltaics (OPV)) as inexpensive and readily available emissive or semiconductor materials.

Saddle-shaped aza-nanographene with multiple odd-membered rings

A saddle-shaped aza-nanographene containing a central 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) has been prepared via a rationally designed four-step synthetic pathway encompassing intramolecular direct arylation, the Scholl reaction, and finally photo-induced radical cyclization. The target non-alternant, nitrogen-embedded polycyclic aromatic hydrocarbon (PAH) incorporates two abutting pentagons between four adjacent heptagons forming unique 7-7-5-5-7-7 topology. Such a combination of odd-membered-ring defects entails a negative Gaussian curvature within its surface with a significant distortion from planarity (saddle height ≈ 4.3 Å). Its absorption and fluorescence maxima are located in the orange-red region, with weak emission originating from the intramolecular charge-transfer character of a low-energy absorption band. Cyclic voltammetry measurements revealed that this stable under ambient conditions aza-nanographene underwent three fully reversible oxidation steps (two one-electron followed by one two-electron) with an exceptionally low first oxidation potential of E _ox1 = -0.38 V (vs. Fc/Fc⁺).

On-off-on Control of Molecular Inversion Symmetry via Multi-stage Protonation: Elucidating Vibronic Laporte Rule

The Laporte rule dictates that one- and two-photon absorption spectra of inversion-symmetric molecules should display alternatively forbidden electronic transitions; however, for organic fluorophores, drawing clear distinction between the symmetric- and non-inversion symmetric two-photon spectra is often obscured due to prevalent vibronic interactions. We take advantage of consecutive single- and double-protonation to break and then reconstitute inversion symmetry in a nominally symmetric diketopyrrolopyrrole, causing large changes in two-photon absorption. By performing detailed one- and two-photon titration experiments, with supporting quantum-chemical model calculations, we explain how certain low-frequency vibrational modes may lead to apparent deviations from the strict Laporte rule. As a result, the system may be indeed considered as an on-off-on inversion symmetry switch, opening new avenues for two-photon sensing applications.

A sensitive zinc probe operating via enhancement of excited-state intramolecular charge transfer

Novel highly sensitive fluorescent probes for zinc cations based on the diketopyrrolopyrrole scaffold were designed and synthesized. Large bathochromic shifts (≈80 nm) of fluorescence are observed when the Zn²⁺-recognition unit (di-(2-picolyl)amine) is bridged with the fluorophore possessing an additional pyridine unit able to participate in the coordination process. This effect originates from the dipolar architecture and the increasing electron-withdrawing properties of the diketopyrrolopyrrole core upon addition of the cation. The new, greenish-yellow emitting probes, which operate via modulation of intramolecular charge transfer, are very sensitive to the presence of Zn²⁺. Introduction of a morpholine unit in the diketopyrrolopyrrole structure induces a selective six-fold increase of the emission intensity upon zinc coordination. Importantly, the presence of other divalent biologically relevant metal cations has negligible effects and typically even at a 100-fold higher concentration of Mg²⁺/Zn²⁺, the effect is comparable. Computational studies rationalize the strong bathochromic shift upon Zn²⁺-complexation. Decorating the probes with the triphenylphosphonium cation and morpholine unit enables selective localization in the mitochondria and the lysosome of cardiac H9C2 cells, respectively.

Green-Emitting 4,5-Diaminonaphthalimides in Activity-Based Probes for the Detection of Thrombin

The natures of electron-donating groups as well as the bridge between them determine the fate of substituted 1,8-naphthalimide molecules in the excited state. An activity-based probe constructed from a selective peptide sequence, a reactive warhead, and the brightest green-emitting fluorophore displays impressive performance for thrombin protease detection in a newly constructed series of 1,8-naphthalimides.

Multifunctional Heteropentalenes: From Synthesis to Optoelectronic Applications

In the broad family of heteropentalenes, the combination of two five-membered heterocyclic rings fused in the [3,2-b] mode has attracted the most significant attention. The relatively straightforward access to these structures, being a consequence of the advances in the last two decades, combined with their physicochemical properties which match the requirements associated with many applications has led to an explosion of applied research. In this Perspective, we will discuss the recent progress of heteropentalenes' usefulness as an active element of organic light-emitting diodes and organic field-effect transistors. Among the myriad of possible combinations for the different heteroatoms, thieno[3,2-b]thiophenes and 1,4-dihydropyrrolo[3,2-b]pyrroles are subject to the most intense studies. Together they comprise a potent optoelectronics tool resulting from the combination of appreciable photophysical properties, chemical reactivity, and straightforward synthesis.

All-Polymer Microcavities for the Fluorescence Radiative Rate Modification of a Diketopyrrolopyrrole Derivative

Controlling the radiative rate of emitters with macromolecular photonic structures promises flexible devices with enhanced performances that are easy to scale up. For instance, radiative rate enhancement empowers low-threshold lasers, while rate suppression affects recombination in photovoltaic and photochemical processes. However, claims of the Purcell effect with polymer structures are controversial, as the low dielectric contrast typical of suitable polymers is commonly not enough to provide the necessary confinement. Here we show all-polymer planar microcavities with photonic band gaps tuned to the photoluminescence of a diketopyrrolopyrrole derivative, which allows a change in the fluorescence lifetime. Radiative and nonradiative rates were disentangled systematically by measuring the external quantum efficiencies and comparing the planar microcavities with a series of references designed to exclude any extrinsic effects. For the first time, this analysis shows unambiguously the dye radiative emission rate variations obtained with macromolecular dielectric mirrors. When different waveguides, chemical environments, and effective refractive index effects in the structure were accounted for, the change in the radiative lifetime was assigned to the Purcell effect. This was possible through the exploitation of photonic structures made of polyvinylcarbazole as a high-index material and the perfluorinated Aquivion as a low-index one, which produced the largest dielectric contrast ever obtained in planar polymer cavities. This characteristic induces the high confinement of the radiation electric field within the cavity layer, causing a record intensity enhancement and steering the radiative rate. Current limits and requirements to achieve the full control of radiative rates with polymer planar microcavities are also addressed.

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.

Probing the flux of mitochondrial potassium using an azacrown-diketopyrrolopyrrole based highly sensitive probe

The diketopyrrolopyrrole bearing an aza-18-crown-6 as a binding unit as well as a PPh₃⁺ group is highly sensitive towards K⁺ and localizes selectively in mitochondria of cardiac H9C2 cells. Fast efflux/influx of mitochondrial K⁺ can be observed upon stimulation with nigericin.

From 2,5-Diformyl-1,4-dihydropyrrolo[3,2-b]pyrroles to Quadrupolar, Centrosymmetric Two-Photon-Absorbing A-D-A Dyes

An original approach has been developed for the insertion of formyl substituents at positions 2 and 5 of 1,4-dihydropyrrolo[3,2-b]pyrroles by conversion of thiazol-2-yl substituents. The synthetic utility of these formyl groups was investigated, and a series of centrosymmetric A−π–D−π–A frameworks were constructed. The two-photon absorption of the quadrupolar pyrrolo[3,2-b]pyrrole possessing two dicyanovinylidene flanking groups is attributed to an S₀ → (S₁) → S₄ transition which has a large TPA cross-section (1300 GM) for a molecule of this size.

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.

The Kröhnke synthesis of benzo[a]indolizines revisited: towards small, red light emitters

Benzo[a]indolizines with an ordered arrangement of various electron-withdrawing substituents (NO2, CF3, CN, CO2R and COPh) were prepared directly from pyridinium salts and chloronitroarenes, allowing for refined control of the photophysical...

Going beyond the borders: pyrrolo[3,2-b]pyrroles with deep red emission

A two-step route to strongly absorbing and efficiently orange to deep red fluorescent, doubly B/N-doped, ladder-type pyrrolo[3,2-b]pyrroles has been developed. We synthesize and study a series of derivatives of these four-coordinate boron-containing, nominally quadrupolar materials, which mostly exhibit one-photon absorption in the 500-600 nm range with the peak molar extinction coefficients reaching 150 000, and emission in the 520-670 nm range with the fluorescence quantum yields reaching 0.90. Within the family of these ultrastable dyes even small structural changes lead to significant variations of the photophysical properties, in some cases attributed to reversal of energy ordering of alternate-parity excited electronic states. Effective preservation of ground-state inversion symmetry was evidenced by very weak two-photon absorption (2PA) at excitation wavelengths corresponding to the lowest-energy, strongly one-photon allowed purely electronic transition. π-Expanded derivatives and those possessing electron-donating groups showed the most red-shifted absorption- and emission spectra, while displaying remarkably high peak 2PA cross-section (σ _2PA) values reaching ∼2400 GM at around 760 nm, corresponding to a two-photon allowed higher-energy excited state. At the same time, derivatives lacking π-expansion were found to have a relatively weak 2PA peak centered at ca. 800-900 nm with the maximum σ _2PA ∼50-250 GM. Our findings are augmented by theoretical calculations performed using TD-DFT method, which reproduce the main experimental trends, including the 2PA, in a nearly quantitative manner. Electrochemical studies revealed that the HOMO of the new dyes is located at ca. -5.35 eV making them relatively electron rich in spite of the presence of two B^--N⁺ dative bonds. These dyes undergo a fully reversible first oxidation, located on the diphenylpyrrolo[3,2-b]pyrrole core, directly to the di(radical cation) stage.

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.

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-b]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.

Langmuir-Blodgett Films of Diketopyrrolopyrroles with Tunable Amphiphilicity

In this work, we present the formation of H- and J-aggregates of amphiphilic centrosymmetric diketopyrrolopyrroles containing aliphatic or aromatic amino groups. The inherent amphiphilicity of these dyes predestines their assembly at interfaces to form ordered supramolecular structures. In this work, we employed the Langmuir-Blodgett (LB) technique to generate, manipulate, and deposit such supramolecular structures. The aforementioned amines provide an additional means to control the formation of the supramolecular assemblies. In the resulting LB films, both H- and J-aggregates of the dyes can be realized, leading to very broad absorption spectra. In contrast to many reports on H- and J-aggregates, the interactions between the symmetric diketopyrrolopyrroles are controlled via interface assembly and π-stacking and not by dipolar interactions. We show that in the case of the aliphatic, but not for the aromatic amine functionalization, the usage of an acidic subphase enables the transition from H- to J-aggregate-dominated LB films via an increase in the surface pressure during deposition.

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.

The Synthesis and Photophysical Properties of Weakly Coupled Diketopyrrolopyrroles

Three centrosymmetric diketopyrrolopyrroles possessing either two 2-(2′-methoxyphenyl)benzothiazole or two 2-(2′-methoxyphenyl)benzoxazolo-thiophene scaffolds were synthesized in a straightforward manner, and their photophysical properties were investigated. Their emission was significantly bathochromically shifted as compared with that of simple DPPs reaching 650 nm. Judging from theoretical calculations performed with time-dependent density functional theory, in all three cases the excited state was localized on the DPP core and there was no significant CT character. Consequently, emission was almost independent of solvents’ polarity. DPPs possessing 2,5-thiophene units vicinal to DPP core play a role in electronic transitions, resulting in bathochromically shifted absorption and emission. Interestingly, as judged from transient absorption dynamics, intersystem crossing was responsible for the deactivation of the excited states of DPPs possessing para linkers but not in the case of dye bearing meta linker.

Bowl-Shaped Pentagon- and Heptagon-Embedded Nanographene Containing a Central Pyrrolo[3,2-b]pyrrole Core

A bowl-shaped nitrogen-doped nanographene composed of a pyrrolo[3,2-b]pyrrole core substituted with six arene rings circularly bonded with one another has been prepared via a concise synthetic strategy encompassing the multicomponent tetraarylpyrrolopyrrole (TAPP) synthesis, the Scholl reaction, and intramolecular direct arylation. This synthesis represents the first case of programmed sequential intramolecular direct arylation reactions utilizing the different reactivity of C-Br and C-Cl bonds. The target compound contains two central pentagons confined between two adjacent heptagons-the inverse Stone-Thrower-Wales topology. The presence of both five- and seven-membered rings in the final structure is responsible for interesting properties such as a perpendicularly aligned dipole moment, absorption and fluorescence in the orange-red region, weak emission originating from the charge-transfer character of a low-energy absorption band, and a high lying HOMO. In the solid state slipped convex-to-convex π-π stacking dominates.

Development of a universal conductive platform for anchoring photo- and electroactive proteins using organometallic terpyridine molecular wires

The construction of an efficient conductive interface between electrodes and electroactive proteins is a major challenge in the biosensor and bioelectrochemistry fields to achieve the desired nanodevice performance. Concomitantly, metallo-organic terpyridine wires have been extensively studied for their great ability to mediate electron transfer over a long-range distance. In this study, we report a novel stepwise bottom-up approach for assembling bioelectrodes based on a genetically modified model electroactive protein, cytochrome c553 (cyt c553) and an organometallic terpyridine (TPY) molecular wire self-assembled monolayer (SAM). Efficient anchoring of the TPY derivative (TPY-PO(OH)2) onto the ITO surface was achieved by optimising solvent composition. Uniform surface coverage with the electroactive protein was achieved by binding the cyt c553 molecules via the C-terminal His6-tag to the modified TPY macromolecules containing Earth abundant metallic redox centres. Photoelectrochemical characterisation demonstrates the crucial importance of the metal redox centre for the determination of the desired electron transfer properties between cyt and the ITO electrode. Even without the cyt protein, the ITO-TPY nanosystem reported here generates photocurrents whose densities are 2-fold higher that those reported earlier for ITO electrodes functionalised with the photoactive proteins such as photosystem I in the presence of an external mediator, and 30-fold higher than that of the pristine ITO. The universal chemical platform for anchoring and nanostructuring of (photo)electroactive proteins reported in this study provides a major advancement for the construction of efficient (bio)molecular systems requiring a high degree of precise supramolecular organisation as well as efficient charge transfer between (photo)redox-active molecular components and various types of electrode materials.

Acylation of electrophilic bicyclo[1.1.0]butanes via Co/Ni-catalyzed reductive cross-coupling

Cyclobutane scaffold is appreciated in medicinal chemistry for its strained, well-defined three-dimensional structure. Consequently, methods for the synthesis of cyclobutyl derivatives have become highly desired, particularly those offering access to compounds with new patterns of substituents. Herein, an acylation of electrophilic strained molecules at the bridgehead carbon with [Formula: see text]-acyl-glutarimides is reported. For this, the polarity-reversal strategy based on cobalt catalysis that enables the generation of cyclobutyl radicals in a strain release event was harnessed. These nucleophilic species, in the presence of a Ni-complex, couple with [Formula: see text]-acyl-glutarimides to give cyclobutyl ketones in decent yields.

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.

How does tautomerization affect the excited-state dynamics of an amino acid-derivatized corrole?

In the first two decades of the XXI century, corroles have emerged as an important class of porphyrinoids for photonics and biomedical photonics. In comparison with porphyrins, corroles have lower molecular symmetry and higher electron density, which leads to uniquely complementary properties. In macrocycles of free-base corroles, for example, three protons are distributed among four pyrrole nitrogens. It results in distinct tautomers that have different thermodynamic energies. Herein, we focus on the excited-state dynamics of a corrole modified with L-phenylalanine. The tautomerization in the singlet-excited state occurs in the timescales of about 10-100 picoseconds and exhibits substantial kinetic isotope effects. It, however, does not discernably affect nanosecond deactivation of the photoexcited corrole and its basic photophysics. Nevertheless, this excited-state tautomerization dynamics can strongly affect photoinduced processes with comparable or shorter timescales, considering the 100-meV energy differences between the tautomers in the excited state. The effects on the kinetics of charge transfer and energy transfer, initiated prior to reaching the equilibrium thermalization of the excited-state tautomer population, can be indeed substantial. Such considerations are crucially important in the design of systems for artificial photosynthesis and other forms of energy conversion and charge transduction.

Control of Molecular Catalysts for Oxygen Reduction by Variation of pH and Functional Groups

In the search for replacement of the platinum-based catalysts for fuel cells, MN₄ molecular catalysts based on abundant transition metals play a crucial role in modeling and investigation of the influence of the environment near the active site in platinum-group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts. To understand how the ORR activity of molecular catalysts can be controlled by the active site structure through modification by the pH and substituent functional groups, the change of the ORR onset potential and the electron number in a broad pH range was examined for three different metallocorroles. Experiments revealed a switch between two different ORR mechanisms and a change from 2e^- to 4e^- pathway in the pH range of 3.5-6. This phenomenon was shown by density functional theory (DFT) calculations to be related to the protonation of the nitrogen atoms and carboxylic acid groups on the corroles indicated by the pK_a values of the protonation sites in the vicinity of the ORR active sites. Control of the electron-withdrawing nature of these groups characterized by the pK_a values could switch the ORR from the H⁺ to e^- rate-determining step mechanisms and from 2e^- to 4e^- ORR pathways and also controlled the durability of the corrole catalysts. The results suggest that protonation of the nitrogen atoms plays a vital role in both the ORR activity and durability for these materials and that pK_a of the N atoms at the active sites can be used as a descriptor for the design of high-performance, durable PGM-free catalysts.

Switch-On Diketopyrrolopyrrole-Based Chemosensors for Cations Possessing Lewis Acid Character

For the first time diketopyrrolopyrroles (DPPs) have been synthesized directly from nitriles possessing (aza)crown ethers leading to macrocycle-dye hybrids. Depending on the nature of the linkage between DPP and macrocyclic ring, various coordination effects are found. The strong interaction of the cations possessing Lewis acid character such as Li⁺ , Mg²⁺ and Zn²⁺ with 2-aminopyridin-4-yl-DPPs, leading to a bathochromic shift of both emission and absorption, as well as to strong enhancement of fluorescence was rationalized in terms of strong binding of these cations to the N=C-NR₂ functionality. The same effect has been observed for protonation. Depending on the size and the structure of the macrocyclic ring the complexation of cations by aza-crown ethers plays an important but secondary role. The interaction of Na⁺ and K⁺ with 2-aminopyridin-4-yl-DPPs leads to moderate enhancement of fluorescence due to the aza-crown ethers binding. The very weak fluorescence of DPP bearing 2-dialkylamino-pyridine-4-yl substituents is due to the closely lying T₂ state and the resulting intersystem crossing.

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.

Method for the Large-Scale Synthesis of Multifunctional 1,4-Dihydro-pyrrolo[3,2-b]pyrroles

A thorough investigation has enabled the optimization of the synthesis of 1,4-dihydro-pyrrolo[3,2-b]pyrroles. Although salts of such metals as vanadium, niobium, cerium, and manganese were found to facilitate the formation of 1,4-dihydro-pyrrolo[3,2-b]pyrroles from amines, aldehydes, and diacetyl, we confirmed that iron salts are the most efficient catalysts. The conditions identified (first step: toluene/AcOH = 1:1, 1 h, 50 °C; second step: toluene/AcOH = 1:1, Fe(ClO₄)₃·H₂O, 16 h, 50 °C) resulted in the formation of tetraarylpyrrolo[3,2-b]pyrroles in a 6–69% yield. For the first time, very electron-rich substituents (4-Me₂NC₆H₄, 3-(OH)C₆H₄, pyrrol-2-yl) originating from aldehydes and sterically hindered substituents (2-ClC₆H₄, 2-BrC₆H₄, 2-CNC₆H₄, 2-(CO₂Me)C₆H₄, 2-(TMS-C≡C)C₆H₄) present on anilines can be appended to the pyrrolo[3,2-b]pyrrole core. It is now also possible to prepare 1,4-dihydropyrrolo[3,2-b]pyrroles bearing an ordered arrangement of N-substituents and C-substituents ranging from coumarin, quinoline, phthalimide to truxene. These advances in scope enable independent regulations of many desired photophysical properties, including the Stokes shift value and emission color ranging from violet-blue through deep blue, green, yellow to red. Simultaneously, the optimized conditions have finally allowed the synthesis of these extremely promising heterocycles in amounts of more than 10 g per run without a concomitant decrease in yield or product contamination. Empowered with better functional group compatibility, novel derivatization strategies were developed.

Access to Corrole-Appended Persubstituted Benzofurans by a Multicomponent Reaction: The Dual Role of p-Chloranil

A multicomponent reaction among dipyrranes, aryl-propargyl aldehydes, and p-chloranil leading to 10-(benzofuran-2-yl)corroles is described. p-Chloranil was identified as a crucial reagent playing a twofold role: an oxidant taking part in the formation of the corrole macrocycle and a component undergoing heteroannulation to the incipient 10-arylethynylcorrole. A series of corroles bearing persubstituted benzofuran-2-yl moieties have been synthesized, and their fundamental electronic properties have been studied via UV-vis absorption and fluorescence spectroscopies.

How To Make Nitroaromatic Compounds Glow: Next-Generation Large X-Shaped, Centrosymmetric Diketopyrrolopyrroles

Red‐emissive π‐expanded diketopyrrolopyrroles (DPPs) with fluorescence reaching λ=750 nm can be easily synthesized by a three‐step strategy involving the preparation of diketopyrrolopyrrole followed by N‐arylation and subsequent intramolecular palladium‐catalyzed direct arylation. Comprehensive spectroscopic assays combined with first‐principles calculations corroborated that both N‐arylated and fused DPPs reach a locally excited (S₁) state after excitation, followed by internal conversion to states with solvent and structural relaxation, before eventually undergoing intersystem crossing. Only the structurally relaxed state is fluorescent, with lifetimes in the range of several nanoseconds and tens of picoseconds in nonpolar and polar solvents, respectively. The lifetimes correlate with the fluorescence quantum yields, which range from 6 % to 88 % in nonpolar solvents and from 0.4 % and 3.2 % in polar solvents. A very inefficient (T₁) population is responsible for fluorescence quantum yields as high as 88 % for the fully fused DPP in polar solvents.

Change of Quadrupole Moment upon Excitation and Symmetry Breaking in Multibranched Donor-Acceptor Dyes

Upon photoexcitation, a majority of quadrupolar dyes, developed for large two-photon absorption, undergo excited-state symmetry breaking (ES-SB) and behave as dipolar molecules. We investigate how the change of quadrupole moment upon S₁ ←S₀ excitation, ΔQ, influences the propensity of a dye to undergo ES-SB using a series of molecules with a A-π-D-π-A motif where D is the exceptionally electron-rich pyrrolo[3,2-b]pyrrole and A are accepting groups. Tuning of ΔQ is achieved by appending a secondary acceptor group, A', on both sides of the D core and ES-SB is monitored using a combination of time-resolved IR and broadband fluorescence spectroscopy. The results reveal a clear correlation between ΔQ and the tendency to undergo ES-SB. When A is a stronger acceptor than A', ES-SB occurs already in non-dipolar but quadrupolar solvents. When A and A' are identical, ES-SB is only partial even in highly dipolar solvents. When A is a weaker acceptor than A', the orientation of ΔQ changes, ES-SB is observed in dipolar solvents only and involves major redistribution of the excitation over the D-π-A and D-A' branches of the dye.

Direct Observation of Different One- and Two-Photon Fluorescent States in a Pyrrolo[3,2-b]pyrrole Fluorophore

Two-photon fluorophores are frequently employed to obtain superior spatial resolution in optical microscopy applications. To guide the rational design of these molecules, a detailed understanding of their excited-state deactivation pathways after two-photon excitation is beneficial, especially to assess the often-assumed presumption that the one- and two-photon excited-state dynamics are similar after excitation. Here, we showcase the breakdown of this assumption for one- and two-photon excitation of a centrosymmetric pyrrolo[3,2-b]pyrrole chromophore by combining time-resolved fluorescence and broadband femtosecond transient absorption spectroscopy. Compared to one-photon excitation, where radiative decay dominates the photodynamics, two-photon excitation leads to dynamics arising from increased nonradiative decay pathways. These different photodynamics are manifest to different quantum yields, thus highlighting the types of time-resolved studies described here to be valuable guideposts in the design of two-photon fluorophores for imaging applications.