Abstract:An adiabatic photoreaction is a chemical process that occurs entirely on a single excited electronic energy surface. As a rule, most photoreactions of organic molecules start on an excited electronic surface but "jump" to a lower surface somewhere along the reaction coordinate. There are, however, exceptions to this general rule. For example, photoreactions involving small structural changes and minor alterations in covalent bonding (e. g., proton transfers and complex formation) are commonly found to occur ad… Show more
“…This finding is in line with Norrish type I photochemistry, where the reactive radical intermediates are often not observed. 43 These results would imply that the reaction proceeds according to an adiabatic mechanism, 44 where the products are capable of undergoing a reversible reaction to the starting material upon heating. Taking all of the above information into account, the distinguishing factor between the photoreactivity in 1/2 and 3 must therefore be the relative stability of the biradical IM-T 1 state.…”
N,C-chelate
organoborates represent an emerging class of photoresponsive
materials due to their photochromic switching at a boron center. Despite
the promising applicability of such systems, little is known about
the excited-state processes that lead to their unique photoreactivity,
which is detrimental to the design of next-generation smart materials
based on boron. As part of our ongoing effort to understand and improve
the utility of these organoboron compounds, we report some of the
first experimental evidence to support an excited-state mechanism
for N,C-chelate organoborates. Femtosecond transient absorption spectroscopy
combined with steady-state UV/vis and fluorescence measurements gives
direct insight into their underlying photochemical processes, such
as the formation of a common triplet charge-transfer state which either
relaxes radiatively or undergoes the desired photoisomerization through
a biradical intermediate. With this information, a complete mechanistic
picture of the excited-state reactivity of N,C-chelate organoborates
has been established, which is anticipated to lead to new smart materials
with improved performance.
“…This finding is in line with Norrish type I photochemistry, where the reactive radical intermediates are often not observed. 43 These results would imply that the reaction proceeds according to an adiabatic mechanism, 44 where the products are capable of undergoing a reversible reaction to the starting material upon heating. Taking all of the above information into account, the distinguishing factor between the photoreactivity in 1/2 and 3 must therefore be the relative stability of the biradical IM-T 1 state.…”
N,C-chelate
organoborates represent an emerging class of photoresponsive
materials due to their photochromic switching at a boron center. Despite
the promising applicability of such systems, little is known about
the excited-state processes that lead to their unique photoreactivity,
which is detrimental to the design of next-generation smart materials
based on boron. As part of our ongoing effort to understand and improve
the utility of these organoboron compounds, we report some of the
first experimental evidence to support an excited-state mechanism
for N,C-chelate organoborates. Femtosecond transient absorption spectroscopy
combined with steady-state UV/vis and fluorescence measurements gives
direct insight into their underlying photochemical processes, such
as the formation of a common triplet charge-transfer state which either
relaxes radiatively or undergoes the desired photoisomerization through
a biradical intermediate. With this information, a complete mechanistic
picture of the excited-state reactivity of N,C-chelate organoborates
has been established, which is anticipated to lead to new smart materials
with improved performance.
“…According to the semiempirical calculations, the sum of non-hydrogen atom displacements in proceeding from 2 to 1 (0.506 Å/atom) is slightly smaller than that to 19 (0.551 Å/atom). , Due to the more extended conjugation in 1 than in 2 , moreover, the excited triplet state energy of the product 1 is expected to be significantly lower than that of the reactant 2 . In such an energetic situation, photochemical transformation may well proceed adiabatically, especially in triplet excited states, directly delivering products in their electronically excited states. , The adiabatic conversion of 2 * into 1 * might be significantly less susceptible to the steric constraint imposed by the rigid media than the stepwise isomerization of 2 * to 16 and 19 , and might preferentially, practically irreversibly proceed in the organic glasses at low temperature. 2 …”
Irradiation of 6,7-benzobicyclo[3.2.0]hepta-3,6-dien-2-one (2) in a rigid glass at -196°C leads to the formation of 3,4-benzotropone (1), which exhibits a characteristic UV-vis absorption extending to 535 nm and dimerizes to give syn-and anti-[π8 + π10] dimers 7 and 8 upon thawing the glass. The rate constant of the thermal dimerization of 1 is 12 ( 3 M -1 s -1 at -78°C. Photochemically, 1 is stable under matrix isolation, but undergoes [π10 + π10] dimerization in solution. Compound 1 in a CFCl 3 -CF 2 BrCF 2 Br-CHCl 3 glass at -155°C shows an IR band at 1506 cm -1 which is assigned to a vibration primarily involving CdO stretching mode by comparing the spectrum with that of 18 O-labeled 1. Under the same conditions tropone exhibits a CdO vibrational band at a significantly higher frequency (1553 cm -1 ), suggesting that the 3,4-benzo annulation makes the tropone ring more strongly polarized. The large contribution of a polar resonance form 1b is reflected in the high affinity of 1 for protonation: 1 is nearly half-protonated in a 0.1 M solution of CHCl 2 CO 2 H in EPA (a 5:5:2 mixture of ether, isopentane, and ethanol). Furthermore, the electronic absorption spectrum undergoes a characteristic blue shift when the medium is changed from hydrocarbons to alcohols, suggesting that 1 is more polarized in the ground state than in the excited state. The generation of 1 from 2 is thermally effected also and, upon heating a mixture of 2 and olefin such as maleic anhydride or ethyl vinyl ether in benzene at 220°C, adducts of the latter to 1 are obtained. The photochemical behavior of 2 is dependent on the state of medium in which 2 is irradiated; the photolysis of 2 in rigid glass results almost exclusively in the valence isomerization providing 1, while in fluid solution predominantly in the di-π-methane rearrangement affording 3,4-benzotricyclo[3.2.0.0 2,7 ]hept-3-en-6-one (19). Plausible causes of the medium-dependent photochemical behavior of 2 are discussed.
“…550 nm)” were observed together with structured fluorescence from the phenyl ring at approximately 270 nm in the electronic excitation of 1,2‐diphenylcyclopropane derivatives in a glassy matrix at 77 K (Scheme a) . The authors proposed that the unusually long‐wavelength emissions were derived from the diradical‐like species generated after adiabatic C−C bond cleavage . If the proposed diradicals are isolable under certain conditions, conclusive evidence could be experimentally obtained to assign the emissive species the long‐wavelength emissions.…”
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.
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