Tuning the Singlet−Triplet Energy Gap in a Non-Kekulé Series by Designed Structural Variation. The Singlet States of N-Substituted-3,4-dimethylenepyrrole Biradicals

Abstract

Semiempirical quantum chemical calculations (AM1/CI and PM3/CI) confirm the qualitative perturbational prediction that electron-withdrawing groups on the ring nitrogen of a 3,4-dimethylenepyrrole should diminish the energy separation of the singlet and triplet states to near zero. Syntheses of a series of precursors of such biradicals have been developed. Study of the chemistry and spectroscopy of the biradicals has revealed persistent singlet states for the cases where the substituent is methyl, isobutyryl, and pivaloyl. In the cases of N-arenesulfonyl-3,4dimethylenepyrroles, both a singlet and a triplet form can be observed as persistent species. In this paper, the properties of the singlets in this series are described. Although energy transfer from the excited triplet state of the sensitizer xanthone to the diazene precursor of N-p-toluenesulfonyl-3,4-dimethylenepyrrole is observed by nanosecond timeresolved spectroscopy, the chemical behavior of the biradical intermediate is the same as that observed in the direct photolysis or thermolysis of the diazene. The reactive form of the biradical under these conditions appears to be the singlet. Experimental Design. Tuning the Multiplet Gap in N-Substituted-3,4-dimethylenepyrrole Biradicals.Prediction of the spacings of the lowest energy multiplets of π-conjugated non-Kekulé molecules is a stringent test of theory. Although in most cases these separations are difficult to measure, the effect of structure on the qualitative trends of their direction and magnitude should be estimable by theoretical calculation. We are particularly attracted to the use of computationally designed systematic structural variations as means of tuning the spacings and hence providing experimental tests of such estimates. The necessary elements of this approach are a parent system with near-degenerate multiplet states, a variable structural feature that can be used to perturb their separation, and a theoretical means for selecting potentially fruitful structural alterations. Since the singlet-triplet gap in such species can be made quite small, the tuning process ultimately might offer the power of choice over the multiplicity of the ground state.The disjoint hydrocarbon biradical tetramethyleneethane (TME, 1) provides a useful starting structure for such a process. Although the size of the energy separation between the lowest triplet and singlet states of TME is not yet known from experiment, theory 2 predicts a very small gap (E T -E S ∼ -1 to +2 kcal/mol). A heteroatom bridge connecting two termini of the TME molecule, as in previous work 3 from this laboratory on 3,4-dimethylenefuran and 3,4-dimethylenethiophene, introduces a variable source of electronic perturbation.The present paper describes the preparation and characterization of the singlet states of several such biradicals, and an accompanying paper 11b reports results of the search for the corresponding triplet states.Our present computational approach was implicit in theoretical considerations by Du, Hrovat, and Borden, 4 whic...

The effect of substituents on electronic states’ ordering in meta-xylylene diradicals: Qualitative insights from quantitative studies

“…[6][7][8][9][10][11][12][13][14] For example, for a 1,4-didehydrobenzene diradical, Clark and Davidson 6 found that substituents, which strongly interact with the benzyne system, do not interact with the space, and are only weakly electronegative, should yield the smallest singlet-triplet energy differences and might lead to the triplet ground state. Berson and co-workers 8,9 reported that electron-withdrawing substituents modulate the singlet-triplet gaps of the singlet tetramethyleneethane-type diradical. Ito et al 10 reported examples of -conjugated trianthrene systems showing the reverse spin-state preferences depending on the substitution patterns.…”

Thermal Rearrangements of Heteroatom‐Bridged Diallenes

“…The strong perturbation of the heteroatom X lowers the activation barrier (Figure 2), increasing the HOMO and LUMO energy separation and brings about the singlet nature of the products. [25] Vice versa, when the donor ability of X diminishes, the HOMO and LUMO of the products approach degeneracy allowing singlet-triplet (S-T) mixing ( Table 1) indicating the loss of cyclic delocalization due to protonation of X. The Gibbs reaction energies for protonated 12b, 12d and 12f are less exergonic than those of the non-protonated 12a, 12c and 12e (Figure 1), resulting from the inability to form an aromatic π-electron sextet.…”

“…Upon irradiation of some precursor diazenes (Scheme 3), Berson et al [23,24] generated the hetero-biradical 12 (X = O, S and NH) for ∆E ST "tuning" studies of these molecules. [22,25,26] Considering the heteroatom as a bridge connecting the two termini of the biradical tetra- Scheme 2. methyleneethane (TME), they found that it is possible to adjust ∆E ST by means of the variable character of the heteroatom lone pair p z -orbital acting on the non-bonding frontier π-orbitals (NBMOs) of TME. [25] Thus, the nature of the X heteroatom offers a tuning mechanism to control the total molecular electronic spin and ∆E ST .…”

“…[22,25,26] Considering the heteroatom as a bridge connecting the two termini of the biradical tetra- Scheme 2. methyleneethane (TME), they found that it is possible to adjust ∆E ST by means of the variable character of the heteroatom lone pair p z -orbital acting on the non-bonding frontier π-orbitals (NBMOs) of TME. [25] Thus, the nature of the X heteroatom offers a tuning mechanism to control the total molecular electronic spin and ∆E ST . Scheme 3. Apart from the 2,6-cyclizations of 8, heteroatom-bridged diallenes have not been examined with respect to other cyclization modes, i.e., through TS 25 , TS 16 and TS 17 (Scheme 4).…”

“…[25] Thus, the nature of the X heteroatom offers a tuning mechanism to control the total molecular electronic spin and ∆E ST . Scheme 3. Apart from the 2,6-cyclizations of 8, heteroatom-bridged diallenes have not been examined with respect to other cyclization modes, i.e., through TS 25 , TS 16 and TS 17 (Scheme 4). On the basis of our previous computational studies, [11,12] the 1,6-cyclization should be viable and competitive with the 2,6-reaction path.…”