Unusually Long‐Wavelength Emissions of Cyclopropanes: New Insight into C−C Bond Homolysis

Abstract

Bond homolysis (BHo) is a fundamental concept in chemical-bonding phenomena. To date, research studies on the BHo concept have provided crucial information for understanding the nature of chemical bonding and reactions. Two potential-energy minima, a σ-bonding isomer and a singlet-diradical isomer, have been known to exist in carbon-carbon BHo. Herein, a third isomer, that is, a puckered singlet diradical exhibiting unstructured long-wavelength fluorescence beyond 460 nm, was first observed in the excited states of 1,4-diarylbicyclo[2.1.0]pentane derivatives. The careful selection of appropriate substituents in the bicyclic structures enabled direct spectral detection. State-of-the-art ab initio quantum chemical calculations quantitatively reproduced the experimental observations. This new finding provides new insight into carbon-carbon bond-breaking and -forming processes.

New Insights into Bond Homolysis Process and Discovery of Novel Bonding System (C–π–C) by Generating Long-lived Singlet Diradicals

“…The puckered-type diradical 33 is the third minimal-energy structure found in the groundstate bond homolysis process and is an important intermediate that controls the stereoselectivity of radical coupling reactions� To determine whether the puckered-type structure exists in the bond dissociation process in an electronically excited state, we measured the emission spectrum of trans-32 (Figure 12). 47 The fluorescence spectrum (250 nm excitation) of a solution of trans-32 (1�5 × 10 -5 M) in 3-methylpentane (3-MP) was measured at liquid nitrogen temperature (77 K). Interestingly, fluorescence corresponding to a phenyl-group-derived vibrational structure was observed around 280-360 nm (lifetime 8.7 ns, Figure 12a) and broad fluorescence with no vibrational structure was noticed at 460-660 nm (lifetime 7.9 ns, Figure 12b).…”

Rates of Ring Opening of Radical Cation Intermediates Govern Differences in Thermoluminescence between 1‐ and 2‐Naphthyl‐Substituted Methylenecyclopropanes

“…[16,17] It is interesting to point out that the general mechanistic route responsible for γ- [10,11] or X-ray [12] induced TL is similar to that operating in organic light-emitting diodes (OLEDs), where emission takes place from excited species generated by CR between radical cations and radical anions. Organic radical LEDs (ORLEDs), [10,[18][19][20] which have several unique advantages over OLEDs, use luminescent organic radicals [21][22][23][24][25][26][27] as light emitting species. Consequently, we believed that the naphthyl substituted methylenecyclopropanes (MCPs) 1 b and 1 c are promising candidates for use in ORLEDs.…”