Unraveling the Photophysical Characteristics, Aromaticity, and Stability of π-Extended Acene-Quinodimethyl Thioamides†.

Poly-aromatic systems that contain quinodimethyl (QDM) units are appealing for several photonic and spintronic applications owing to the unique electronic structure, aromaticity, and spin state(s) of the QDM ring. Herein, we report the synthesis and characterization of novel QDM-based chromophores 1-3, which exhibit unique photo-excited behavior and aromaticity. Extending the aromatic core with a biphenyl/phenanthryl- and a pyrrolo-fragment led to reducing the optoelectronic bandgap and modulating the photophysics QDM 1-3. Yet, QDM 2 and 3 suffer from "aromaticity imbalance" and become relatively unstable compared to the parent compound QDM 1. Further assessment of local aromaticity using computational tools revealed that the pseudo-quinoidal ring B is the main driving force allowing to easily populate the excited triplet state of these chromophores. The present study provides complementary guidelines for designing novel non-classical poly-aromatic systems.

Photophysical Insights of Halogenated Dipyrrolonaphthyridine-Diones as Potential Photodynamic Therapy Agents†.

We report the synthesis and photophysical characterization of novel halogenated dipyrrolonaphthyridine-diones (X2 -DPNDs, X = Cl, Br, and I), as candidates for photodynamic therapy (PDT) application. Apart from the heavy atom-induced spin-orbit coupling (SOC) dynamics in the investigated X2 -DPNDs, it was found that the position of the halogen atom (relative to the nitrogen of the pyrrole ring) also influenced the triplet excited state behavior. Interestingly, the faster/efficiency sensitization of 3 O2 to 1 O2 using X2 -DPND correlates with the rate of triplet population, kISC >1.6 × 108 s-1 for I2 -DPND vs kISC >2.9 × 109 s-1 for Cl2 -DPND and Br2 -DPND (where τISC  = 343 ± 3 ps for I2 -DPND and τISC  = 5-6 ns for Cl2 -DPND and Br2 -DPND are the lowest time constants/values for ISC). Furthermore, the heavy atom-induced SOC in Cl2 -DPND and Br2 -DPND did not lead to a reduction of the corresponding fluorescence (ca 75% vs 67% for the parent DPND). The attractive photophysical characteristics of Cl2 /Br2 -DPND put them on the landscape as not only promising PDT agents but also as fluorescence probes. The present study is a stepping stone in the development of novel organic photosystems for synergistic photomedicinal applications.

Estimation of Singlet Oxygen Quantum Yield Using Novel Green-Absorbing Baird-type Aromatic Photosensitizers.

We report two new organic green-absorbing singlet oxygen (1 O2 ) photosensitizers: Quinoidal naphthyl thioamide (QDM) and bis-iodol-dipyrrolonaphthyridine-dione (I2 -DPND), with triplet energies of 40.8 and 47.5 kcal mol-1 (at 77 K in a glassy matrix) , respectively. The UV-vis absorption and emission characteristics of QDM and I2 -DPND are similar to other commercially available organic 1 O2 photosensitizers such as Rose Bengal, which was used as standard/reference to estimate the 1 O2 quantum yield (Φ∆ ) of the chromophores under study. Using 9,10-diphenylanthracene (DPA) as an 1 O2 quencher, we estimated the Φ∆ ≈ 67-85% for QDM and Φ∆ ≈ 25-32% for I2 -DPND. The discrepancy in the Φ∆ values could be explained by the apparent photo-decomposition of the later dye. Nevertheless, the high Φ∆ value for QDM is unprecedented, as this chromophore exhibits relatively low structural complexity and could further be derivatized to create novel photodynamic agents.

Design, synthesis, and biological evaluation of novel imidazo[1,2-a]pyridinecarboxamides as potent anti-tuberculosis agents.

Tuberculosis (TB) is a highly infectious disease that has been plaguing the human race for centuries. The emergence of multidrug-resistant strains of TB has been detrimental to the fight against tuberculosis with very few safe therapeutic options available. As part of an ongoing effort to identify potent anti-tuberculosis agents, we synthesized and screened a series of novel imidazo[1,2-a]pyridinecarboxamide derivatives for their anti-tuberculosis properties. These compounds were designed based on reported anti-tuberculosis properties of the indolecarboxamides (I2Cs) and imidazo[1,2-a]pyridinecarboxamides (IPAs). In this series, we identified compounds 15 and 16 with excellent anti-TB activity against H37Rv strain of tuberculosis (MIC = 0.10-0.19 µM); these compounds were further screened against selected clinical isolates of Mtb. Compounds 15 and 16 showed excellent activities against multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains of TB (MIC range: 0.05-1.5 µM) with excellent selectivity indices. In addition, preliminary ADME studies on compound 16 showed favorable pharmacokinetic properties.

New perspectives for triplet-triplet annihilation based photon upconversion using all-organic energy donor & acceptor chromophores.

It is recognized that metal organic complexes that serve as sensitizers can present various degrees of challenges viz. synthesis and stability for photonic applications such as triplet-triplet annihilation based photon upconversion (TTA-PUC). Presently, researchers, including our group, are turning their attention toward purely organic triplet sensitizers, which can be handled more easily for photon management science. In this review, we surveyed recently developed all-organic chromophoric systems that were devised and used for TTA-PUC research. Knowing that TTA-PUC research has mainly been focused on the design and synthesis of the triplet sensitizers, we detailed the underlying photophysics and thermodynamics that served as the starting point for the synthesis of the purely organic chromophores in question. Accordingly, this review details triplet sensitizers that operate on (i) spin-orbit coupling or heavy atom effect, (ii) Baird-type aromaticity and antiaromaticity, (iii) open-shell characteristics or doublet excited state and (iv) thermally activated delayed fluorescence.

A Naphtho-p-quinodimethane Exhibiting Baird's (Anti)Aromaticity, Broken Symmetry, and Attractive Photoluminescence.

We report a novel reductive desulfurization reaction involving π-acidic naphthalene diimides (NDI) 1 using thionating agents such as Lawesson's reagent. Along with the expected thionated NDI derivatives 2-6, new heterocyclic naphtho-p-quinodimethane compounds 7 depicting broken/reduced symmetry were successfully isolated and fully characterized. Empirical studies and theoretical modeling suggest that 7 was formed via a six-membered ring oxathiaphosphenine intermediate rather than the usual four-membered ring oxathiaphosphetane of 2-6. Aside from the reduced symmetry in 7 as confirmed by single-crystal XRD analysis, we established that the ground state UV-vis absorption of 7 is red-shifted in comparison to the parent NDI 1. This result was expected in the case of thionated polycyclic diimides. However, unusual low energy transitions originate from Baird 4nπ aromaticity of compounds 7 in lieu of the intrinsic Hückel (4n + 2)π aromaticity as encountered in NDI 1. Moreover, complementary theoretical modeling results also corroborate this change in aromaticity of 7. Consequently, photophysical investigations show that, compared to parent NDI 1, 7 can easily access and emit from its T1 state with a phosphorescence 3(7a)* lifetime of τP = 395 µs at 77 K indicative of the formation of the corresponding "aromatic triplet" species according to the Baird's rule of aromaticity.

Generation and Reactivity Studies of Diarylmethyl Radical Pairs in Crystalline Tetraarylacetones via Laser Flash Photolysis Using Nanocrystalline Suspensions.

The nanosecond electronic spectra and kinetics of the radical pairs from various crystalline tetraarylacetones were obtained using transmission laser flash photolysis methods by taking advantage of aqueous nanocrystalline suspensions in the presence of submicellar CTAB, which acts as a surface passivator. After showing that all tetraarylacetones react efficiently by a photodecarbonylation reaction in the crystalline state, we were able to detect the intermediate radical pairs within the ca. 8 ns laser pulse of our laser setup. We showed that the solid-state spectra of the radical pairs are very similar to those detected in solution, with λmax in the 330-360 nm range. Kinetics in the solid state was observed to be biexponential and impervious to the presence of oxygen or variations in laser power. A relatively short-lived component (0.3-1.7 µs) accounts for only 3-8% of the total decay with a longer-lived component having a time constant in the range of 40-90 µs depending on the nature of the substituents.