Transient changes in aromaticity and their effect on excited-state proton transfer reactions

Arpa, Enrique M.; Durbeej, Bo

doi:10.1039/d2cp00494a

Cited by 11 publications

(19 citation statements)

References 42 publications

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“…Interestingly, in the S 2 state the benzene ring was found to be aromatic, and as a result, the ESIPT process in this state is unfavorable (Figure 22). [186] At this point it should be stressed that it has been concluded that by considering only the (anti‐)aromatic character of excited‐state reactants and products in ESPT reactions one may neglect important (anti‐)aromaticity effects along the reaction paths which are decisive [188] . This is especially critical when there is an interaction between two electronically excited states.…”

Section: Relief Of Esaamentioning

confidence: 99%

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Photochemistry Driven by Excited‐State Aromaticity Gain or Antiaromaticity Relief

Yan

Slanina

Bergman

et al. 2023

Chemistry A European J

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Gain of aromaticity or relief of antiaromaticity along a reaction path are important factors to consider in mechanism studies. Analysis of such changes along potential energy surfaces has historically focused on reactions in the electronic ground state (S 0 ), but can also be used for excited states. In the lowest ππ* states, the electron counts for aromaticity and antiaromaticity follow Baird's rule where 4n π-electrons indicate aromaticity and 4n + 2 π-electrons antiaromaticity. Yet, there are also cases where Hückel's rule plays a role in the excited state. The electron count reversals of Baird's rule compared to Hückel's rule explain many altered physicochemical properties upon excitation of (hetero)annulene derivatives. Here we illustrate how the gain of excited-state aromaticity (ESA) and relief of excited-state antiaromaticity (ESAA) have an impact on photoreactivity and photostability. Emphasis is placed on recent findings supported by the results of quantum chemical calculations, and photoreactions in a wide variety of areas are covered.

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Section: Relief Of Esaamentioning

confidence: 99%

“…The connection between ESIPT processes and ESAA relief was explored computationally in salicylic acid and 2‐(2’‐hydroxyphenyl)benzoxazole (HBO) derivatives [185–188] . Lampkin et al.…”

Section: Relief Of Esaamentioning

confidence: 99%

Photochemistry Driven by Excited‐State Aromaticity Gain or Antiaromaticity Relief

Yan

Slanina

Bergman

et al. 2023

Chemistry A European J

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“…[39,40] It is thus plausible that excited-state (anti)aromaticity modulates the height of the activation barriers of the retro-aldol reaction in 3 BPT. In fact, NICS-scans [41,42] (nucleus-independent chemical shift) on the pyrazine subunit of the pterin chromophore register a decrease of antiaromaticity along the 3 BPT! 3 I2 retroaldol reaction coordinate (Section 5.2 of the Supporting Information).…”

Section: Chemistry-a European Journalmentioning

confidence: 99%

Unveiling Photodegradation and Photosensitization Mechanisms of Unconjugated Pterins

Arpa

Corral

2023

Chemistry A European J

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Unconjugated pterins are ubiquitous molecules that participate in countless enzymatic processes and are potentially involved in the photosensitization of singlet oxygen, amino acids, and nucleotides. Following electronic excitation with UV‐A light, some of these pterins degrade, producing hydrogen peroxide as the main side product. This process, which is known to take place in vivo, contributes to oxidative stress and melanocyte destruction in vitiligo. In this work, we present for the first time mechanistic insight into the formation of transient triplet species that simultaneously trigger Type I and Type II photosensitizing processes and the initiation of degradation processes. Our calculations reveal that photodegradation of 6‐biopterin, which accumulates in the skin of vitiligo patients, leads to 6‐formylpterin through a retro‐aldol reaction, and subsequently to 6‐carboxypterin through a water‐mediated aldehyde oxidation. Additionally, we show that the changes in the photosensitizing potential of these systems with pH come from the modulation of their excited‐state redox potentials.

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“…However, already in 1972, following earlier work by Dewar 25 and Zimmerman, 26 the concepts of excitedstate aromaticity (ESA) and antiaromaticity (ESAA) [27][28][29][30][31] were given a solid theoretical foundation through the discovery by Baird 32 that annulenes with 4n and 4n + 2 p-electrons are respectively aromatic and 5 (20) antiaromatic in their lowest triplet pp* excited state (T1), thus implying a reversal of (anti)aromatic character in this state relative to the ground state as asserted by Hückel's rule. With time, and particularly since the past decade or so, 33,34 ESA and ESAA have established themselves as very useful concepts for explaining and predicting photophysical properties [35][36][37][38] and photochemical reactivities [39][40][41][42][43][44][45][46][47][48][49][50][51][52] of both triplet and singlet excited states of organic molecules.…”

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confidence: 99%

“…In this light, it is of particular interest to establish how the HOMA index performs for excited states, as this index is probably the easiest to calculate, requiring only a quantum-chemical geometry optimization. Indeed, for this reason, HOMA has often been employed to highlight photochemical consequences of ESA and ESAA, [40][41][42]44,45,[48][49][50] despite that it is yet to be parameterized for excited states.…”

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confidence: 99%

HOMER: A reparameterization of the harmonic oscillator model of aromaticity (HOMA) for excited states

Arpa

Durbeej

2023

Preprint

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Excited-state aromaticity (ESA) and antiaromaticity (ESAA) are by now well-established concepts for explaining photophysical properties and photochemical reactivities of cyclic, conjugated molecules. However, their application is less straightforward than the corresponding process by which the thermal chemistry of such systems is rationalized in terms of ground-state aromaticity (GSA) and antiaromaticity (GSAA). Recognizing that the harmonic oscillator model of aromaticity (HOMA) provides an easy way to measure aromaticity on geometric grounds, it is therefore notable that this model is yet to parameterized for excited states. Against this background, we here present a new parameterization of HOMA − termed HOMER − for the T1 state of both carbocyclic and heterocyclic compounds based on high-level quantum-chemical calculations. Considering CC, CN, NN and CO bonds and testing the parametrization using calculated magnetic data as reference, we find that the description of ESA and ESAA by HOMER is superior to that afforded by the original HOMA scheme, and that it reaches the same overall quality as HOMA does for GSA and GSAA. Furthermore, we demonstrate that the derived HOMER parameters can be used for predictive modeling of ESA and ESAA at very different levels of theory. Altogether, the results highlight the potential of HOMER to facilitate future studies of ESA and ESAA.

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Transient changes in aromaticity and their effect on excited-state proton transfer reactions

Abstract: Quantum chemical modelling shows that energy barriers for excited-state proton transfer reactions involving 2-pyridone are governed by the transient onset of antiaromaticity upon interaction between the photoexcited state and a second excited state.

Cited by 11 publications

References 42 publications

Photochemistry Driven by Excited‐State Aromaticity Gain or Antiaromaticity Relief

Photochemistry Driven by Excited‐State Aromaticity Gain or Antiaromaticity Relief

Unveiling Photodegradation and Photosensitization Mechanisms of Unconjugated Pterins

HOMER: A reparameterization of the harmonic oscillator model of aromaticity (HOMA) for excited states

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