New Data on Biradicals

Morozova, I. D.; Dyatkina, M. E.

doi:10.1070/rc1968v037n05abeh001649

Cited by 20 publications

(5 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Chichibabin’s, Müller’s, and related hydrocarbons have usually a diradical or diradicaloid GS and a small Δ E ST . 6−8 These species are getting special attention owing to the tunable character of their GS that can be singlet [open- (OSS) or closed-shell (CS)] or triplet (T), depending on the length of the π-system and the exocyclic substituents. 9−11 In general, the longer the hydrocarbon, the greater the diradical character.…”

Section: Introductionmentioning

confidence: 99%

“…In addition, they determined that this molecule has a singlet ground state (GS) with a small singlet–triplet energy gap (Δ E ST ) and a large amount of diradical character, i.e., it is a Kekulé diradicaloid. Chichibabin’s, Müller’s, and related hydrocarbons have usually a diradical or diradicaloid GS and a small Δ E ST . − These species are getting special attention owing to the tunable character of their GS that can be singlet [open- (OSS) or closed-shell (CS)] or triplet (T), depending on the length of the π-system and the exocyclic substituents. − In general, the longer the hydrocarbon, the greater the diradical character . Indeed, many investigations support this relationship and also add the fact that the aromaticity is increased in larger CPH systems. , …”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Effect of Exocyclic Substituents and π-System Length on the Electronic Structure of Chichibabin Diradical(oid)s

et al. 2019

View full text Add to dashboard Cite

The ground state (GS) of Chichibabin’s polycyclic hydrocarbons (CPHs) can be singlet [open- or closed-shell (OSS or CS)] or triplet (T), depending on the elongation of the π-system and the exocyclic substituents. CPHs with either a small singlet–triplet energy gap (Δ E ST ) or even a triplet GS have potential applications in optoelectronics. To analyze the effect of the size and exocyclic substituents on the nature of the GS of CPHs, we have selected a number of them with different substituents in the exocyclic carbon atoms and different ring chain lengths. The OPBE/cc-pVTZ level of theory was used for the optimization of the systems. The aromaticity of the resulting electronic structures was evaluated with HOMA, NICS, FLU, PDI, I ring , and MCI aromaticity indices. Our results show that the shortest π-systems (one or two rings) have a singlet GS. However, systems with three to five rings favor OSS GSs. Electron-withdrawing groups (EWGs) and aromatic substituents in the exocyclic carbons tend to stabilize the OSS and T states, whereas electron-donating groups slightly destabilize them. For CS, OSS, and T states, aromaticity measures indicate a gain of aromaticity of the 6-membered rings of the CPHs with the increase in their size and when CPHs incorporate EWGs or aromatic substituents. In general, the CPHs analyzed present small singlet–triplet energy gaps, and in particular, the ones containing EWGs or aromatic substituents present the smallest singlet–triplet energy gaps.

show abstract

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Effect of Exocyclic Substituents and π-System Length on the Electronic Structure of Chichibabin Diradical(oid)s

et al. 2019

View full text Add to dashboard Cite

show abstract

“…These works showed that careful design of the molecular topology 9 can lead to an electronic structure with two unpaired electrons weakly interacting in two nearly-degenerate nonbonding molecular orbitals 10,11 -the relative stabilization of the resulting spin-triplet EPR measurements show that in Tschitschibabin's [14][15][16][17][18] diradical, the energy separation between the ground spin-singlet and the excited spin-triplet states is thermally accessible (∆𝐸 𝑆−𝑇 ~ 2.5 𝐾𝑐𝑎𝑙/𝑚𝑜𝑙) and MO calculations suggest the gap is 5.5 Kcalmol -1 in the Thiele hydrocarbon. 19,20 . Following these early works, this approach for designing stable organic radicals has been successfully applied to the general class of even alternant polyaromatic hydrocarbons (PAHs) 21 achieving almost degenerate spin states (∆𝐸 𝑆−𝑇 ~0), and more recently to analogues with unpaired electrons hosted on carbene- 22 and nitrogen- 23 centers, where ∆𝐸 𝑆−𝑇 can be modulated by varying the substituentsall these systems are symmetric, and to the best of our knowledge this approach has not been tested in asymmetric molecules.…”

Section: Introductionmentioning

confidence: 99%

Biradical Formation by Deprotonation in Thiazole-Derivatives: The Hidden Nature of Dasatinib

Heras

Reta

Valero

et al. 2019

Preprint

View full text Add to dashboard Cite

The formation of stable organic biradicals by a deprotonation process is reported for a series of conjugated heterocycles that share a Ph-N(H)-2-thiazole structural motif. We characterise the paramagnetic electronic ground state by means of continuous-wave and pulse EPR. We propose a simple valence bond mechanism for a deprotonation-induced formation of paramagnetic organic molecules, based on the interplay between the electronegativity of heteroatomic groups and the recovery of aromaticity to stabilise the biradical species. The Ph-N(H)-2-thiazole motif is found in a variety of biologically active molecules, exemplified here with the anticancer drug Dasatinib, and our results suggest a radical-based mechanism for the protein kinase inhibition activity of the drug. The existence of this structure-property relationship for an elementary chemical motif suggests that biradical species may be more prevalent than previously thought and have an important role in bioorganic chemistry.

show abstract

“…We also thought that for compounds with the same π-conjugated path the torsional differences could tune the singlet–triplet gaps. While it has been shown that alkyl substitution in Tschitschibabin-type compounds can influence their diradical character, the fundamental cause of the effect (e.g., steric or electronic) has not been sufficiently explored. − Toward this end, two derivatives of the parent TMTQ compound were designed and synthesized with a hexyl chain oriented either toward or away from the central annulene to give TMTQ-In and TMTQ-Out, respectively (Scheme ). Torsional effects in TMTQ-In may enhance a diradical configuration of the compound, while TMTQ-Out offers a control to assess substituent effects in an orientation with less thiophene–M10A torsional clashing.…”

Section: Introductionmentioning

confidence: 99%

Torsional Bias as a Strategy To Tune Singlet–Triplet Gaps in Organic Diradicals

et al. 2018

View full text Add to dashboard Cite

Quinoidal compounds with proaromatic structures possess differing degrees of diradical character, where the open-shell diradical resonance form has restored aromaticity throughout the compound. Methods to tune the diradical character of these compounds have traditionally focused on altering the length and the molecular composition of the π-conjugated backbones. However, other molecular design strategies to tune the singlet−triplet gap of π-conjugated quinoidal molecules have not been extensively explored. We previously reported a strikingly small energy gap between the quinoidal and diradical states of a quinoidal small molecule containing methano[10]annulene (TMTQ) that was dictated in large part by the unusual aromaticity of the central annulene ring. Here, we report on two alkylated derivatives of TMTQ that present substantially different torsional biases to the planarity of the TMTQ π-system. Using a combination of electronic and vibrational spectroscopies, magnetic measurements, and quantum chemical calculations, we demonstrate here how a steric effect rather than π-electron compositional molecular engineering can dramatically narrow the singlet−triplet gap of a quinoidal compound to as small as −0.52 kcal/mol, determined experimentally. This study offers important insight for the continued development of open-shell diradical molecules that need not rely exclusively on the design of synthesis of new and complex πconjugated systems.

show abstract

New Data on Biradicals

Cited by 20 publications

References 0 publications

Effect of Exocyclic Substituents and π-System Length on the Electronic Structure of Chichibabin Diradical(oid)s

Effect of Exocyclic Substituents and π-System Length on the Electronic Structure of Chichibabin Diradical(oid)s

Biradical Formation by Deprotonation in Thiazole-Derivatives: The Hidden Nature of Dasatinib

Torsional Bias as a Strategy To Tune Singlet–Triplet Gaps in Organic Diradicals

Contact Info

Product

Resources

About