Control and Switching of Aromaticity in Various All-Aza-Expanded Porphyrins: Spectroscopic and Theoretical Analyses

Near-Infrared S2 Fluorescence from Deprotonated Möbius Aromatic [32]Heptaphyrin

This study revealed S₂ fluorescence from deprotonated meso-pentafluorophenyl-substituted Möbius aromatic [32]heptaphyrin(1.1.1.1.1.1.1) that was formed upon treatment of neutral antiaromatic [32]heptephyrin with tetrabutylammonium fluoride. Higher excited-state dynamics and emission were studied by fs-transient absorption spectroscopy and a broad-band fluorescence upconversion technique. This is the first S₂ fluorescence from chromophores with twisted Möbius topology, and the observation of S₂ fluorescence in the near-infrared region has been unprecedented. The higher excited-state dynamics of neutral and deprotonated [32]heptaphyrins were compared by ultrafast transient absorption spectroscopy to understand the S₂ fluorescence origin. In the antiaromatic [32]heptaphyrin, a fast time component of 65 fs was assigned as an internal conversion process from the S_B state to the S_Q state, which occurs prior to relaxation to the optically dark, lowest electronic state (S_D). Therefore, the S_Q state of the antiaromatic [32]heptaphyrin acts as a trap state intervening radiative transitions from the S_B state to the S₀ state. In deprotonated [32]heptaphyrin, the internal conversion from the S_B state to the S_Q state proceeds with a slower time constant of 150 fs for owing to its rigid structure, helping the observation of its S₂ fluorescence.

Spectroscopic Diagnosis of Excited-State Aromaticity: Capturing Electronic Structures and Conformations upon Aromaticity Reversal

Aromaticity, the special energetic stability derived from cyclic [4 n + 2]π-conjugated electronic structures, has been the topic of intense interest in chemistry because it plays a critical role in rationalizing molecular stability, reactivity, and physical/chemical properties. Recently, the pioneering work by Colin Baird on aromaticity reversal, postulating that aromatic (antiaromatic) character in the ground state reverses to antiaromatic (aromatic) character in the lowest excited triplet state, has attracted much scientific attention. The completely reversed aromaticity in the excited state provides direct insight into understanding the photophysical/chemical properties of photoactive materials. In turn, the application of aromatic molecules to photoactive materials has led to numerous studies revealing this aromaticity reversal. However, most studies of excited-state aromaticity have been based on the theoretical point of view. The experimental evaluation of aromaticity in the excited state is still challenging and strenuous because the assessment of (anti)aromaticity with conventional magnetic, energetic, and geometric indices is difficult in the excited state, which practically restricts the extension and application of the concept of excited-state aromaticity. Time-resolved optical spectroscopies can provide a new and alternative avenue to evaluate excited-state aromaticity experimentally while observing changes in the molecular features in the excited states. Time-resolved optical spectroscopies take advantage of ultrafast laser pulses to achieve high time resolution, making them suitable for monitoring ultrafast changes in the excited states of molecular systems. This can provide valuable information for understanding the aromaticity reversal. This Account presents recent breakthroughs in the experimental assessment of excited-state aromaticity and the verification of aromaticity reversal with time-resolved optical spectroscopic measurements. To scrutinize this intriguing and challenging scientific issue, expanded porphyrins have been utilized as the ideal testing platform for investigating aromaticity because they show distinct aromatic and antiaromatic characters with aromaticity-specific spectroscopic features. Expanded porphyrins exhibit perfect aromatic and antiaromatic congener pairs having the same molecular framework but different numbers of π electrons, which facilitates the study of the pure effect of aromaticity by comparative analyses. On the basis of the characteristics of expanded porphyrins, time-resolved electronic and vibrational absorption spectroscopies capture the changes in electronic structure and molecular conformations driven by the change in aromaticity and provide clear evidence for aromaticity reversal in the excited states. The approaches described in this Account pave the way for the development of new and alternative experimental indices for the evaluation of excited-state aromaticity, which will enable overarching and fundamental comprehension of the role of (anti)aromaticity in the stability, dynamics, and reactivity in the excited states with possible implications for practical applications.

Aromaticity as a Guiding Concept for Spectroscopic Features and Nonlinear Optical Properties of Porphyrinoids

With their versatile molecular topology and aromaticity, porphyrinoid systems combine remarkable chemistry with interesting photophysical properties and nonlinear optical properties. Hence, the field of application of porphyrinoids is very broad ranging from near-infrared dyes to opto-electronic materials. From previous experimental studies, aromaticity emerges as an important concept in determining the photophysical properties and two-photon absorption cross sections of porphyrinoids. Despite a considerable number of studies on porphyrinoids, few investigate the relationship between aromaticity, UV/vis absorption spectra and nonlinear properties. To assess such structure-property relationships, we performed a computational study focusing on a series of Hückel porphyrinoids to: (i) assess their (anti)aromatic character; (ii) determine the fingerprints of aromaticity on the UV/vis spectra; (iii) evaluate the role of aromaticity on the NLO properties. Using an extensive set of aromaticity descriptors based on energetic, magnetic, structural, reactivity and electronic criteria, the aromaticity of [4n+2] π-electron porphyrinoids was evidenced as was the antiaromaticity for [4n] π-electron systems. In agreement with previous studies, the absorption spectra of aromatic systems display more intense B and Q bands in comparison to their antiaromatic homologues. The nature of these absorption bands was analyzed in detail in terms of polarization, intensity, splitting and composition. Finally, quantities such as the average polarizability and its anisotropy were found to be larger in aromatic systems, whereas first and second hyperpolarizability are influenced by the interplay between aromaticity, planarity and molecular symmetry. To conclude, aromaticity dictates the photophysical properties in porphyrinoids, whereas it is not the only factor determining the magnitude of NLO properties.

Triplet state homoaromaticity: concept, computational validation and experimental relevance

Conjugation through space can give rise to aromaticity in the lowest excited triplet state, with impact for photochemistry.

Cyclic conjugation that occurs through-space and leads to aromatic properties is called homoaromaticity. Here we formulate the homoaromaticity concept for the triplet excited state (T₁) based on Baird's 4n rule and validate it through extensive quantum-chemical calculations on a range of different species (neutral, cationic and anionic). By comparison to well-known ground state homoaromatic molecules we reveal that five of the investigated compounds show strong T₁ homoaromaticity, four show weak homoaromaticity and two are non-aromatic. Two of the compounds have previously been identified as excited state intermediates in photochemical reactions and our calculations indicate that they are also homoaromatic in the first singlet excited state. Homoaromaticity should therefore have broad implications in photochemistry. We further demonstrate this by computational design of a photomechanical “lever” that is powered by relief of homoantiaromatic destabilization in the first singlet excited state.

New electron delocalization tools to describe the aromaticity in porphyrinoids

There are several possible pathways in the macrocycle of large porphyrinoids and, among aromaticity indices, only AV_minis capable of recognizing the most aromatic one.

meso-Aryl [20]π Homoporphyrin: The Simplest Expanded Porphyrin with the Smallest Möbius Topology

An unstable conjugated homoporphyrin was successfully stabilized by introducing meso-aryl substitutents. It was evident from the moderate diatropic ring current found by NMR analysis that the newly formed 20π conjugated free base and its protonated form exhibited Möbius aromatic character. Furthermore, complexation as a ligand with an Rh^I ion afforded a unique binding mode and retained the Möbius aromaticity. Overall, these compounds are the smallest Möbius aromatic molecules, as confirmed by spectral and crystal-structure analysis and supported by theoretical studies.

Cyclopropyl Group: An Excited-State Aromaticity Indicator?

The cyclopropyl (cPr) group, which is a well-known probe for detecting radical character at atoms to which it is connected, is tested as an indicator for aromaticity in the first ππ* triplet and singlet excited states (T₁ and S₁ ). Baird's rule says that the π-electron counts for aromaticity and antiaromaticity in the T₁ and S₁ states are opposite to Hückel's rule in the ground state (S₀ ). Our hypothesis is that the cPr group, as a result of Baird's rule, will remain closed when attached to an excited-state aromatic ring, enabling it to be used as an indicator to distinguish excited-state aromatic rings from excited-state antiaromatic and nonaromatic rings. Quantum chemical calculations and photoreactivity experiments support our hypothesis; calculated aromaticity indices reveal that openings of cPr substituents on [4n]annulenes ruin the excited-state aromaticity in energetically unfavorable processes. Yet, polycyclic compounds influenced by excited-state aromaticity (e.g., biphenylene), as well as 4nπ-electron heterocycles with two or more heteroatoms represent limitations.

Enhancing the reactivity of nickel(ii) in hydrogen evolution reactions (HERs) by β-hydrogenation of porphyrinoid ligands

Fine-tuning of the porphyrin β-periphery is important for naturally occurring metal tetrapyrroles to exert diverse biological roles. Here we describe how this approach is also applied to design molecular catalysts, as exemplified by Ni(ii) porphyrinoids catalyzing the hydrogen evolution reaction (HER). We found that β-hydrogenation of porphyrin remarkably enhances the electrocatalytic HER reactivity (turnover frequencies of 6287 vs. 265 s-1 for Ni(ii) chlorin (Ni-2) and porphyrin (Ni-1), and of 1737 vs. 342 s-1 for Ni(ii) hydroporpholactone (Ni-4) and porpholactone (Ni-3), respectively) using trifluoroacetic acid (TFA) as the proton source. DFT calculations suggested that after two-electron reduction, β-hydrogenation renders more electron density located on the Ni center and thus prefers to generate a highly reactive nickel hydride intermediate. To demonstrate this, decamethylcobaltocene Co(Cp*)2 was used as a chemical reductant. [Ni-2]2- reacts ca. 30 times faster than [Ni-1]2- with TFA, which is in line with the electrocatalysis and computational results. Thus, such subtle structural changes inducing the distinctive reactivity of Ni(ii) not only test the fundamental understanding of natural Ni tetrapyrroles but also provide a valuable clue to design metal porphyrinoid catalysts.

S2 Fluorescence from [26]Hexaphyrin Dianion

S₂ fluorescence from meso-hexakis(pentafluorophenyl)-substituted [26]hexaphyrin dianion was observed as the first example of expanded porphyrins despite its large molecular size and small HOMO-LUMO gap. The population kinetics among S₂, S₁, and S₀ states have been studied by using femtosecond time-resolved absorption and fluorescence spectroscopies. Broad-band fluorescence upconversion spectroscopy allowed for simultaneous observation of S₂ fluorescence decay in the visible region and S₁ fluorescence rise in the NIR region, both with a time constant of 0.22 ps. The transient absorption spectroscopy revealed the presence of a direct decay path from the S₂ state to the S₀ state. The observation of S₂ fluorescence from highly conjugated molecular systems is quite rare, and S₂ fluorescence beyond 700 nm is also quite rare.

Ethynylene-linked Figure-Eight Octaphyrin(1.2.1.1.1.2.1.1): Synthesis and Characterization of Its Two Oxidation States

An octaphyrin(1.2.1.1.1.2.1.1) containing two conjugated ethynylene bridges has been synthesized and characterized. The macrocycle reveals complex conformational dynamics dependent on its protonation and oxidation state. The [40]annulenoid macrocycle and its [38]annulenoid oxidized form display residual macrocyclic ring currents. In spite of its low apparent aromaticity the new octaphyrin is a potent chromophore with a vis-NIR absorption profile strongly influenced by the redox and acid-base chemistry.

Optical and magnetic properties of antiaromatic porphyrinoids

Magnetic and spectroscopic properties of a number of formally antiaromatic carbaporphyrins, carbathiaporphyrins and isophlorins with 4n π electrons have been investigated at density functional theory and ab initio levels of theory.

A new architecture for high spin organics based on Baird's rule of 4n electron triplet aromatics

This work reports triplet aromatic high spin ground states of cyclopentadienyl cation polyads as alternative to high spin polyradicals or polycarbenes.