César Augusto Guarín Durán

A series of seven pyrrolo[3,4‐ b ]pyridin‐5‐ones were synthesized via a one‐pot process involving an Ugi‐Zhu reaction followed by a cascade sequence ( N ‐acylation/ aza Diels–Alder cycloaddition/decarboxylation/dehydration) incorporating different substituents no aromatic and aromatic such as furan, tetrahydrofuran, and pyridine. The pyrrolo[3,4‐ b ]pyridin‐5‐ones were optically characterized through their photoluminescence spectra, quantum yields, and fluorescence lifetimes. By combining spectroscopic measurements with DFT/TDDFT calculations, photophysical behavior was elucidated, specifically addressing solvent effects on transition magnitudes and the electronic nature of absorption and emission states. Structure–property relationship analyses revealed that steric and electronic substituent effects govern the interplay between charge‐transfer (CT) and locally excited (LE) states. Finally, the most promising candidates were successfully evaluated as fluorogenic microviscosity and polarity sensors, demonstrating their potential for advanced sensing applications.

A new study is presented to elucidate the photodynamics of model carbonyl-substituted polyaromatics targeting the relevance of carbonyl-group orientation and torsional re-equilibration on intersystem crossing (ISC). Our experiments focused on 9-acetylanthracene (9AA) using femtosecond resolved spectroscopy. In the ground state of this molecule, steric interactions force the carbonyl substituent into a near-perpendicular orientation relative to the aromatic system. The time-resolved signals from 9AA show that ISC takes place after spectral shifts that reflect the evolution of the carbonyl group to a slanted geometry as it adjusts to a dihedral angle of around 40° with respect to the aromatic plane. Depending on the solvent, in 9AA manifold crossing takes place on the 3 to 25 ps time-scale. On the other hand, for 2-acetylanthracene (2AA) which is coplanar in both S₀ and S₁, the emission lifetimes can reach several nanoseconds. Analysis of these systems at the highest available theoretical levels reveals further insights into the excited-state dynamics. For 9AA and in contrast with previous publications, it is established that for all relevant geometries, the first excited singlet retains a ππ* character and decays through ISC with no involvement of other singlet states. The manifold crossing involves the interaction with the triplet manifold through states which's transition orbitals are partially localized at the acetyl substituent. Specifically, the slanted geometry of the carbonyl group in 9AA and the potential energy surface around the equilibrium S₁ geometry implies significant spin-orbit interactions and accelerated manifold-crossings. The present results highlight the relevance of substituent reorientation and their slanted geometries which appear to be a dominant feature in carbonyl and nitrated aromatic systems which show rapid ISC dynamics. In the article, we include details on the differences in the mechanisms operating in these two kinds of systems which show the fastest ISC rates among organic chromophores.

In recent years, nanomaterials have become a source of solutions to effectively address various environmental, biological, pharmaceutical and food problems, often in novel ways, with high sensitivity and at low-cost. These materials have attracted a great deal of interest in the scientific community because they improve their properties, or unexpected effects arise compared to bulk materials. This article explores the concept of nanomaterials, their classification, some fabrication methods and the various applications of nanomaterials in miniaturized devices, whose purpose is to obtain systems that are easier to handle and transport due to their small size. The text does not pretend to be exhaustive but rather focuses on explaining to the reader the ins and outs of this fascinating world of nanomaterials and its newly emerging area of research known as nanotechnology.

The effect on the two‐photon absorption of heptamethine cyanines (HMCy) upon polymethinic chain substitution is explained via measurements with a femtosecond laser and time‐dependent/density functional theory calculations. An analysis of the optical properties of several HMCy is presented, emphasizing the similarities and differences between IR780 and HITC (used as a case study of HMCy), mainly by analyzing meso‐substitution and investigating the effect of intermediate structures between both molecules. The two‐photon absorption cross‐section of HITC diluted in methanol was measured, yielding a peak of 1838 GM at 830 nm. Spectroscopic and electronic structure computational data show that the change of polymethine chain substituents significantly affects excited state S ₄ of the molecules, modulating their transition moment and relative energy which, together with changes in the molecular geometry, govern non‐linear absorption.

The nonlinear optical (NLO) properties of meta-nitroaniline (m-NA) are evaluated via Hückel–Agrawal’s approximation in a solvent environment. In this context, both the ¹B and the intramolecular charge transfer (ICT) electronic transitions are considered. The benzene ring currents on the clockwise or counterclockwise direction and the corresponding Brillouin zone from 0 to π are also considered. Besides, the Bloch equations were applied to a single cell n = 1 defined on the benzene ring. We have considered that the light beam was directed along the ring benzene bonds of m-NA; this topological hypothesis changed the crystal structure to a linear chain and the calculated optical properties were found near the experimental ones. In addition, the Fermi’s golden rule was applied to the crystal state and then the calculated refraction index of m-NA had an error of less than 3% of the experimental one. On the other hand, the molar absorptivity ε of m-NA in acetonitrile for the ¹B and intramolecular ICT transitions was experimentally determined to be 11 981 and 1192 L mol^–1 cm^–1, respectively. With this methodology, we found that the change of the charge in the NO₂ group has also a strong influence on the linear and NLO properties. In addition, the dipole transition moments, which are originated from the carbon between the carbons joined to NO₂ and NH₂, are mainly involved in the NLO properties. Thus, the first hyperpolarizability β_z was 1.69 × 10^–30 esu at λ_Laser = 1064 nm, 27% of the experimental value. We attribute this difference to the evaluation of the excited dipole moment. If we attribute a separation of charge of 0.1 e in the excited state, the new dipole moment allows for the simulation of the experimental value. Besides, the calculated value of χ⁽³⁾ for m-NA in a solution of acetonitrile is 2.9 × 10^–13 esu at λ_Laser = 1064 nm, 158% of the experimental value. The discrepancy between these values is attributed to the influence of the electronic correlation effects, that is, because of resonance structures of the aromatic ring and the zwitterionic pair of nitro and aniline. Besides, we have also evaluated the second hyperpolarizability γ, the second-order susceptibility χ⁽²⁾ of m-NA and their values have similar differences to the experimental values. This type of approach is important because it reduces computing time and gives insight into the molecular causes responsible for linear and NLO properties in this type of functional groups, which can be used as building blocks in more complex polymer systems.