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

HSA over BSA: Selective detection of Human Serum Albumin via a naphtho [2,1-b] furan-based system

Human serum albumin (HSA) is an important biomarker that can be used for the early diagnosis of many diseases. In this work, a TICT probe bearing fused naphtho-furan scaffold (NPNF) was developed and employed in the selective turn-on sensing of HSA. The probe's selectivity towards HSA was observed using steady-state fluorescence experiments, with limit of quantitation in micromolar levels. NPNF's capability to exclusively detect HSA over BSA was further studied/rationalized using anisotropy and time-resolved studies. Molecular docking was used to shed light on the location of NPNF in the subdomain IB of HSA. The practical application of the probe was also demonstrated by the detection of HSA in urine and the HSA-assisted detection of cerium.

Solvent-polarity dependence of ultrafast excited-state dynamics of trans-4-nitrostilbene

Ultrafast excited-state dynamics of the simplest nitrostilbenes, namely trans-4-nitrostilbene (t-NSB), was studied in solvents of various polarities with ultrafast broadband time-resolved fluorescence and transient absorption spectroscopies, and by quantum-chemical computations. The results revealed that the initially excited S1(ππ*) state deactivation dynamics is strongly influenced by the solvent polarity. Specifically, the t-NSB S1-state lifetime decreases by three orders of magnitude from ∼60 ps in high-polarity solvents to ∼60 fs in nonpolar solvents. The strong solvent-polarity dependence arises from the differences in dipole moments among the S1 and relevant states, including the major intersystem crossing (ISC) receiver triplet states, and therefore, the solvent polarity can modulate their relative energies and ISC rates. In nonpolar solvents, the sub-100 fs lifetime is due to a combination of efficient ISC and internal conversion. In medium-polarity solvents, the S1-state population decays via a competing ISC relaxation mechanism in a biphasic manner, and the ISC rates are found to obey the inverse energy gap law of the strong coupling case. In high-polarity solvents, the S1 state is stabilized to a much lower energy such that ISC becomes energetically infeasible, and the S1 state decays via barrier crossing along the torsion angle of the central ethylenic bond to the nonfluorescent perpendicular configuration. Regardless of the initial S1-state deactivation pathways in various solvents, the excited-state population is ultimately trapped in the metastable T1-state perpendicular configuration, at which a slower ISC occurs to bring the system to the ground state and bifurcate into either trans or cis form of NSB.

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.

Application of aza-BODIPY as a Nitroaromatic Sensor

Nitroaromatic explosive detection with high sensitivity and selectivity is requisite for civilian and military safety and the ecosystem. In this study, aza boron dipyrromethene (aza-BODIPY) dye was selected as a fluorescent-based chemosensor against nitroaromatic compounds (NACs) including 2,4,6-trinitrophenol (picric acid, TNP), 2,4,6-trinitrotoluene (TNT), and 2,4-dinitrotoluene (DNT). This dye molecule exhibits sharp fluorescent behavior with high quantum yields beyond the near-infrared region (NIR) and is considered as a potential candidate for the detection of NACs. O'Shea's approach was used to synthesize tetraphenyl-conjugated aza-BODIPY molecules. Quenching of fluorescence emission of aza-BODIPY at 668 nm after the exposure to NACs was investigated under acetonitrile-water and acetonitrile-ethanol solvent conditions. The quenching responses and its mechanism were examined by considering the Stern-Volmer relationship Stern-Volmer constants (K_sv) for TNP (in water), TNP (in ethanol), TNT, and DNT, which are predicted to be 1420, 1215, 1364, and 968 M^-1, respectively, all of which are sufficiently above the limit of detection (LOD) values. Thus, the present study opens up the possibility of the usage of aza-BODIPY molecules as a low-cost, light-weight sensor for the detection of NAC explosives.

Robust singlet fission process in strong absorption π-expanded diketopyrrolopyrroles

Singlet fission (SF) has drawn tremendous attention as a multiexciton generation process that could mitigate the thermal loss and boost the efficiency of solar energy conversion. Although a SF-based solar cell with an EQE above 100% has already been fabricated successfully, the practical efficiency of the corresponding devices is plagued by the limited scope of SF materials. Therefore, it is of great importance to design and develop new SF-capable compounds aiming at practical device application. In the current contribution, via a π-expanded strategy, we presented a new series of robust SF chromophores based on polycyclic DPP derivatives, Ex-DPPs. Compared to conventional DPP molecules, Ex-DPPs feature strong absorption with a fivefold extinction coefficient, good molecular rigidity to effectively restrain non-radiative deactivation, and an expanded π-skeleton which endow them with well-suited intermolecular packing geometries for achieving efficient SF process. These results not only provide a new type of high-efficiency SF chromophore but also address some basic guidelines for the design of potential SF materials targeting practical light harvesting applications.

Controlling the fluorescence quantum yields of benzothiazole-difluoroborates by optimal substitution

Precise tuning of the fluorescence quantum yield, vital for countless applications of fluorophores, remains exceptionally challenging due to numerous factors affecting energy dissipation phenomena often leading to its counterintuitive behavior. In contrast to the absorption and emission wavelength which can be precisely shifted to the desired range by simple structural changes, no general strategy exists for controllable modification of the fluorescence quantum yield. The rigidification of the molecular skeleton is known to usually enhance the emission and can be practically realized via the limiting molecular vibrations by aggregation. However, the subtle balance between the abundant possible radiative and non-radiative decay pathways makes the final picture exceptionally sophisticated. In the present study, a series of nine fluorophores obtained by peripheral substitution with two relatively mild electron donating and electron withdrawing groups are reported. The obtained fluorescence quantum yields range from dark to ultra-bright and the extreme values are obtained for the isomeric molecules. These severe changes in emission efficiency have been shown to arise from the complex relationship between the Franck-Condon excited state and conical intersection position. The experimental findings are rationalized by the advanced quantum chemical calculations delivering good correlation between the measured emission parameters and theoretical radiative and internal conversion rate constants. Therefore, the described substituent exchange provides a method to rigorously adjust the properties of molecular probes structurally similar to thioflavin T.

Design, synthesis and applications of NIR-emissive scaffolds of diketopyrrolopyrrole-aza-BODIPY hybrids

Herein, we report a facile synthetic method towards diketopyrrolopyrrole-aza-BODIPY hybrids by a two-step strategy. The three resulting fluorophores (LAB-TH4 ∼ LAB-TH6) showed absorption maxima up to 637 nm with molar extinction coefficients (ε) up to 32 300 M^-1 cm^-1 and emission maxima at 661 nm with a fluorescence quantum yield of 25.2% in solution. Due to good biocompatibility and strong ROS generation ability, they are useful fluorophores for bioimaging and photodynamic therapy (PDT). In addition, as a useful scaffold, a novel NIR-emissive conjugated polymer can be constructed by further bromination and Suzuki coupling polymerization.

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.

1,3-Dipolar cycloaddition of azomethine ylides and imidoyl halides for synthesis of π-extended imidazolium salts

A new synthetic approach to π-extended imidazolium salts is developed based on 1,3-dipolar cycloaddition of polycyclic aromatic azomethine ylides with imidoyl chlorides in the presence of cesium fluoride as a key additive.

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.

Janus Pyrrolopyrrole Aza-dipyrrin: Hydrogen-Bonded Assemblies and Slow Magnetic Relaxation of the Cobalt(II) Complex in the Solid State

A novel pyrrolopyrrole azadipyrrin (Janus-PPAD) with Janus duality was synthesized by a Schiff base-forming reaction of diketopyrrolopyrrole. The orthogonal interactions of the hydrogen-bonding ketopyrrole and metal-coordinating azadipyrrin moieties in Janus-PPAD enabled the metal ions to be arranged at regular intervals: zinc(II) and cobalt(II) coordination provided metal-coordinated Janus-PPAD dimers, which can subsequently form hydrogen-bonded one-dimensional arrays both in solution and in the solid state. The supramolecular assembly of the zinc(II) complex in solution was investigated by ¹ H NMR spectroscopy based on the isodesmic model, in which a binding constant for the elongation of assemblies is constant. Owing to the tetrahedral coordination, in the solid state, the cobalt(II) complex exhibited a slow magnetic relaxation due to the negative D value of -27.1 cm^-1 with an effective relaxation energy barrier U_eff of 38.0 cm^-1 . The effect of magnetic dilution on the relaxation behavior is discussed. The relaxation mechanism at low temperature was analyzed by considering spin lattice interactions and quantum tunneling effects. The easy-axis magnetic anisotropy was confirmed, and the relevant wave functions were obtained by ab initio CASSCF calculations.

New Synthetic Approaches to N-Aryl and π-Expanded Diketopyrrolopyrroles as New Building Blocks for Organic Optoelectronic Materials

Diketopyrrolopyrrole (DPP) as a building block has been intensively investigated for organic semiconductors and light emitting materials. The synthesis of N-aryl DPPs remains challenging. Herein, we firstly report a new easily handled synthetic method toward N-aryl DPPs through H-DPP with diaryliodonium salt in the presence of CuI, which shows broad reaction scope. Sixteen N-aryl DPPs, including phenyl, furan and thiophene as flanking aromatic groups, were synthesized with yields up to 78 %. These N-aryl DPPs are fluorescent in both solutions and solid states, with quantum yields up to 96 % and 40 %, respectively. Moreover, we show that the reaction between H-DPP and diaryliodonium salt can lead to π-expanded DPPs by using Pd(OAc)₂ as catalyst. Nine π-expanded DPPs were obtained in 27-61 % yields. These π-expanded DPPs exhibit good semiconducting properties with hole mobility of 0.71 cm²  V^-1  s^-1 , demonstrating that they are useful building blocks for high performance organic semiconductors.

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.

Keep glowing and going: recent progress in diketopyrrolopyrrole synthesis towards organic optoelectronic materials

Recent progress in the syntheses of DPP derivatives is summarized as well as the structure–property relationships of the derivatives, including the syntheses of DPP cores, N-functionalization reactions, and π-extensions on and along the DPP cores.

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.

Fluorescent Chromophores Containing the Nitro Group: Relatively Unexplored Emissive Properties

Apart from numerous applications, for example in azo dye precursors, explosives, and industrial processes, the nitro group (-NO₂ ) appears on countless molecules in photochemical research owing to its unique characteristics such as a strong electron-withdrawing ability and facile conversion to the reduced substituent. Although it is well known as a fluorescence quencher, fluorescent chromophores that contain the nitro group have also emerged, with 3-nitrophenothiazine being recently reported to have 100 % emission quantum yield in nonpolar solvents. The diverse characters of nitro-containing chromophores motivated us to systematically review those chromophores with nitro substituents, their associated photophysical properties, and applications. In this Review, we succinctly elaborate the advance of the fluorescent nitro chromophores in fields of intramolecular charge transfer, fluorescent probes and nonlinear properties. Special attention is paid to the rationalization of the associated emission spectroscopy, so that the readers can gain insights into the structure-photophysics relationship and hence gain insights for the strategic design of nitro chromophores.

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.