Photochemical synthesis of matrix-isolated pleiadene

Kolc, J; Michl, Josef

doi:10.1021/ja00716a075

Cited by 54 publications

(27 citation statements)

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“…These reactions include thermal dimerization of pleiadene or electrocyclic ring closure of its dimethyl derivative, both of which can be reversed photochemically, as demonstrated 23 by Michl and co-workers. What links these two examples with cethrene 12a (o- 1 ) and its analogue biphenalenylidene 15a (o- 3 ) is the low activation energy of their formally symmetry-forbidden thermal transformations, a feature that arises on account of the diradicaloid character.…”

Section: Discussionmentioning

confidence: 96%

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch

et al. 2018

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We describe the synthesis and properties of 13,14-dimethylcethrene, a prototype of a chiral diradicaloid photochemical switch that can be transformed reversibly via conrotatory electrocyclization to its more stable closed form by light (630 nm) or heat and back to its open form by light (365 nm). This system illustrates how the chemical reactivity of a diradicaloid molecule can be translated into a switching function, which alters substantially all electronic parameters, namely, the HOMO–LUMO and the singlet–triplet (ST) energy gaps, and the degree of helical twist. As a result, distinct changes in the optical and chiroptical properties of this system were observed, which allowed us to monitor the switching process by a variety of spectroscopic techniques, including NMR, UV–vis, and CD. In comparison to the previously reported parent molecule cethrene, this system benefits from two methyl substituents installed in the fjord region, which account for the stability of the closed form against oxidation and racemization. The methyl substituents increase the ST energy gap of 13,14-dimethylcethrene by ∼4 kcal mol–1 in comparison to cethrene. Our DFT calculations reveal that the larger ST gap is a result of electronic and geometric effects of the methyl substituents and show the potential of related systems to act as magnetic switches at room temperature.

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Section: Discussionmentioning

confidence: 96%

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch

et al. 2018

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“…21 Generation of 3 in a milder condition and photochemical synthesis in a low-temperature matrix was performed by Michl et al in 1970. 24 Irradiation of a cyclobutene precursor in a 3-methylpentane glass afforded a stable yellow-green 3, which disappeared in 1015 s at the glass-melting temperature (110 K). The product was the dimer 4 and the mechanism of the rapid reaction from two 3 to 4 is quite interesting because it is formally a thermally forbidden [4 + 4] configuration, which is closely related to singlet biradical character, and the admixing of the ground state configuration with the doubly excited configuration would make the transition state lower in energy.…”

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confidence: 99%

Recent Progress in Quinoidal Singlet Biradical Molecules

2014

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“…In certain cases of 2s + 2s reactions large barriers in the S state were actually detected experimentally in agreement with orbital and state correlation diagrams. 31 The van der Lugt-Oosterhoff mechanism has been criticized as involving a purely hypothetical and unobserved doubly excited state,32 but such states have more recently been observed both in polyenes3' and in a a-electron biradicaloid molecule. 34 Results of another c a l c~l a t i o n~~ failed to produce a minimum in the D state at energies below the S state, perhaps because insufficient extent of C I was used; the D above S ordering was also indicated in the original qualitative diagrams of Abrahamson and Longuet-Higgins.…”

Section: Extrapolation To 2s + 2s and Related Pericyclic Processesmentioning

confidence: 99%

Excited potential energy hypersurfaces for hydrogen (H4) at trapezoidal geometries. Relation to photochemical 2s + 2s processes

Gerhartz¹,

Poshusta²,

Michl³

1976

J. Am. Chem. Soc.

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Ab initio VB calculations, complete within minimum basis set (STO-4G), were performed on the Born-Oppenheimer potential surfaces of the three lowest singlet electronic states of the H4 molecule for the three-dimensional subspace of all trapezoids. Perspective drawings of equipotential surfaces aid the visualization of the results. The hypersurface of the ground state is in excellent agreement with previous results and suggests that the transition state of the isotopic exchange reaction Hz + Dz e 2HD lies outside the trapezoidal subspace. An exothermic reaction of singlet excited H2 (BIZ,+) with ground state Hz (XIZ;,+) without activation energy and adiabatic formation of an H4* excimer is predicted. The bonding in the excimer is due to exciton resonance and charge transfer. The state is purely "zwitterionic". However, at all energetically favorable excimer geometries, a purely dissociative state originating in overall singlet coupling of two parallel triplet Hz molecules is almost degenerate with the excimer state, and this may limit the lifetime of the excimer The dissociative doubly excited state is approximately half zwitterionic and half covalent and can also be viewed as originating in two crossed ground state Hz molecules.Paths of steepest descent on this surface lead to 4H, H2 + 2H, or 2H2 (diagonal bonding). The case of HZ + H2 can be viewed as the simplest possible model for organic 2s + 2s processes. Our results support the picture outlined by van der Lugt and Oosterhoff for photochemical pericyclic processes (return to So through a "pericyclic" minimum in a doubly excited state) and suggest answers to questions concerning photocycloadditions and other pericyclic processes: (i) the relation of the pericyclic minimum to the excimer minimum; (ii) the origin of activation energy; (iii) the physical factors affecting the ordering of excited states at the geometry of the pericyclic minimum; (iv) occurrence of diagonal bonding; (v) occurrence of excited product formation; (vi) relation of the photochemical process to triplet-triplet annihilation; (vii) its relation to radical ion recombination; and (viii) interrelation of the "supermolecule" correlation diagrams to interaction and correlation diagrams based on MO's and states of partners in the photocycloaddition.Born-Oppenheimer hypersurfaces for electronically excited singlet states in the six-dimensional nuclear configuration space of H4 are of interest for several reasons. First, their availability might form a basis for eventual better understanding of processes such as collisional energy transfer, one of the simplest conceivable bimolecular photochemical reactions (H2 + H2* -products), and one of the simplest molecular ion recombination reactions (H3+ + H--products).Although the last named reaction is one between two ground state species, it starts on an electronically excited surfaces of the total H4 system and therefore has potential for production of H**, particularly since excess energy can be carried away by Hz so that no stabilization by a third b...

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Photochemical synthesis of matrix-isolated pleiadene

Cited by 54 publications

References 0 publications

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch

Recent Progress in Quinoidal Singlet Biradical Molecules

Excited potential energy hypersurfaces for hydrogen (H4) at trapezoidal geometries. Relation to photochemical 2s + 2s processes

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