Article Singlet/Triplet State Anti/Aromaticity of CyclopentadienylCation: Sensitivity to Substituent Effect Milovan Stojanovi´c 1, Jovana Aleksi´c 1 and Marija Baranac-Stojanovi´c 2,* 1 Institute of Chemistry, Technology and Metallurgy, Center for Chemistry, University of Belgrade, Njegoševa 12, P.O. Box 173, 11000 Belgrade, Serbia; [email protected] (M.S.); [email protected] (J.A.) 2 Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, P.O. Box 158, 11000 Belgrade, Serbia * Correspondence: [email protected]; Tel.: +381-11-3336741 Abstract: It is well known that singlet state aromaticity is quite insensitive to substituent effects, in the case of monosubstitution. In this work, we use density functional theory (DFT) calculations to examine the sensitivity of triplet state aromaticity to substituent effects. For this purpose, we chose the singlet state antiaromatic cyclopentadienyl cation, antiaromaticity of which reverses to triplet state aromaticity, conforming to Baird’s rule. The extent of (anti)aromaticity was evaluated by using structural (HOMA), magnetic (NICS), energetic (ISE), and electronic (EDDBp) criteria. We find that the extent of triplet state aromaticity of monosubstituted cyclopentadienyl cations is weaker than the singlet state aromaticity of benzene and is, thus, slightly more sensitive to substituent effects. As an addition to the existing literature data, we also discuss substituent effects on singlet state antiaromaticity of cyclopentadienyl cation. Citation: Stojanovi´c,M.; Aleksi´c,J.; Baranac-Stojanovi´c,M. Keywords: antiaromaticity; aromaticity; singlet state; triplet state; cyclopentadienyl cation; substituent Singlet/Triplet State effect Anti/Aromaticity of CyclopentadienylCation: Sensitivity to Substituent Effect. Chemistry 2021, 3, 765–782. https://doi.org/10.3390/ 1. Introduction chemistry3030055 Aromaticity and antiaromaticity are important concepts in science since they explain the special physical and chemical properties of cyclically delocalized systems. Com- Academic Editors: Andrea Peluso pounds showing aromaticity delocalize (4n + 2), usually π–electrons, as stated by Hückel’s and Guglielmo Monaco rule [1–3]. They show enhanced thermodynamic stability, develop diamagnetic ring cur- rents when exposed to an external magnetic field, and tend to equalize bond lengths and Received: 25 June 2021 retain their cyclic delocalization during a chemical reaction [4]. In contrast, compounds Accepted: 19 July 2021 that delocalize 4n π–electrons have been named antiaromatic by Breslow et al. since they Published: 21 July 2021 are unstable and very reactive [5,6]. In addition, they develop paramagnetic ring currents when placed in a magnetic field and are characterized by alternating single and double Publisher’s Note: MDPI stays neutral bonds [4]. Thus, the extent and type of electron delocalization [7], molecular geometry [8], with regard to jurisdictional claims in energy [9,10], and magnetic properties [11] are often taken into account when defining a published maps and institutional affil- system as aromatic or antiaromatic. iations. In the first excited triplet state, the electron count is reversed: (4n + 2) π–electron systems are antiaromatic, and 4n π–electron ones are aromatic. This was first pointed out by Baird on the basis of molecular orbital considerations [12] and was extended to the first singlet excited state by Karadakov on the basis of calculated magnetic properties Copyright: © 2021 by the authors. of benzene and cyclobutadiene [13]. Theoretical investigations of electron delocaliza- Licensee MDPI, Basel, Switzerland. tion [14–16], molecular structure [17,18], stabilization energies [17,19–21] and magnetic This article is an open access article properties [13,17,21–25] supported Baird’s rule, for which experimental lines of evidence distributed under the terms and also exist [26–28]. A few review articles summarize these lines of evidence for the rule conditions of the Creative Commons Attribution (CC BY) license (https:// and explain earlier reports on various photochemical reactions on the basis of concepts creativecommons.org/licenses/by/ related to excited-state aromaticity and antiaromaticity [29,30]. It is important to mention 4.0/). a recently developed concept of adaptive aromaticity by Zhu et al., referring to the small Chemistry 2021, 3, 765–782. https://doi.org/10.3390/chemistry3030055 https://www.mdpi.com/journal/chemistry Chemistry 2021, 3, FOR PEER REVIEW 2 developed concept of adaptive aromaticity by Zhu et al., referring to the small number of cyclically delocalized compounds, which show aromaticity in both singlet and triplet states [31–33]. Since most compounds are derivatives of their basic skeleton, understanding of sub- stituent effects on molecular properties has attracted great attention from scientists. In unsaturated molecules, substituents mostly interact with the π–electronic system, increas- ing delocalization in conjugated butadiene moiety but decreasing it in an aromatic system [34]. In a competition between aromaticity and substituent effect, the former wins in the Chemistry 2021, 3 case of monosubstitution [35,36]. However, the extent of aromaticity766 can be decreased upon polysubstitution [37]. On the other hand, antiaromaticity, as a destabilizing effect, is often attenuated by substituent [38,39], particularly in electron-deficient and cationic systems,number of cyclicallysuch as delocalized borole [40] compounds, and cyclopentadien which show aromaticityyl cation in both [41,42], singlet respectively, and where it is triplet states [31–33]. significantlySince most compoundsreduced by are electron-donating derivatives of their basic groups. skeleton, Additionally, understanding of the recent computa- tionalsubstituent work effects proved on molecular that propertiesaromatic has benzene attracted greatis almost attention insensitive from scientists. to substituent effect, whereasIn unsaturated electron molecules, delocalization substituents mostly in its interact nonaroma with theticπ dication–electronic is system, greatly in- affected by substit- creasing delocalization in conjugated butadiene moiety but decreasing it in an aromatic uentsystem [43]. [34]. In a competition between aromaticity and substituent effect, the former wins in the caseHow of monosubstitution similar are singlet- [35,36]. However, and triplet-state the extent of aromaticityaromaticity can beand decreased antiaromaticity? Does the formerupon polysubstitution resist the substituent [37]. On the other effect, hand, and antiaromaticity, is the latter as influenced a destabilizing by effect, it to the same extent as is often attenuated by substituent [38,39], particularly in electron-deficient and cationic thesystems, singlet-state such as borole properties? [40] and cyclopentadienyl A few works cation were [41 aimed,42], respectively, to answer where these it is questions. Two sub- stituents,significantly reducedF and byCN, electron-donating were considered groups. Additionally,when examining the recent the computational aromaticity of cyclooctatet- raenework proved in its that triplet aromatic state. benzene The authors is almost insensitivefound a decrease to substituent in effect,aromaticity whereas upon increasing the electron delocalization in its nonaromatic dication is greatly affected by substituent [43]. numberHow similarof substituents are singlet- and[39]. triplet-state The triplet- aromaticitystate dicationic and antiaromaticity? benzene, Does formally the 4n π–electronic system,former resist was the found substituent to be effect, more and delocalized is the latter influenced than its singlet by it to the state same and extent showed less sensitivity toas thesubstituent singlet-state effect properties? [43]. A Recently, few works wereit was aimed shown to answer computationally these questions. Twothat a single substituent substituents, F and CN, were considered when examining the aromaticity of cyclooctate- cantraene have in its tripleta large state. impact The authors on the found triplet-state a decrease in aromaticity(anti)aromaticity upon increasing of benzene. the Depending on thenumber substituent’s of substituents electronic [39]. The triplet-state properties, dicationic the benzene,triplet formallystate of 4n benzeneπ–electronic can be tuned between highlysystem, was antiaromatic found to be more and delocalized strongly than aromatic its singlet [44]. state and showed less sensitivity to substituent effect [43]. Recently, it was shown computationally that a single substituent can haveIt is a largethe impactaim of on this the triplet-statework to provide (anti)aromaticity more ofinformation benzene. Depending about on the the sensitivity of triplet- statesubstituent’s aromaticity electronic on properties, substituent the triplet effect, state ofwhich benzene we can find be tuned important between highly because of the tight rela- tionantiaromatic between and stronglyaromaticity aromatic and [44 ].molecular properties. The results may be useful for future It is the aim of this work to provide more information about the sensitivity of triplet- studiesstate aromaticity on the onpossible substituent tuning effect, of which triplet we state find importantaromaticity because and, of thus, the tight physical and chemical propertiesrelation between of aromaticityorganic compounds. and molecular properties. We chose The resultsthe antiaromatic may be useful for cyclopentadienyl future cation as astudies model on thesystem possible