Laser flash photolysis study of 2,6-dimethyl-3,5-diphenyl-4-pyrone and related chromones. Evidence for triplet state structural relaxation from quenching behaviors
The 308- or 337.1-nm laser excitation of 2,6-dimethyl-3,5-diphenyl-4-pyrone (1), 2-phenylchromone (2), and 2,3-diphenylchromone (3) results in the formation of short-lived triplets (tau/sub 0//sup T/ = 1-5 ..mu..s, lambda/sub max//sup T/ = 330-365 nm) in high yields (0.5-1.0 in benzene). From phosphorescence spectra at 77 K, the spectroscopic triplet energies of these aromatic enones are estimated at 69, 62, and 67 kcal mol/sup -1/, respectively. However, in fluid solutions at room temperature, the triplets are poorly quenched by 1-methylnaphthalene (E/sub T/ = 59.6 kcal mol/sup -1/), dienes (E/sub T/ approx. = 59 kcal mol/sup -1/), and ferrocene (E/sub T/ approx. = 40 kcal mol/sup -1/). In spite of the apparent exothermicity, the energy-transfer processes are inefficient; this suggests geometric distortion in these triplets, very probably in the form of partial twisting about the ene double bonds, causing a lowering of vertical energy (at relaxed geometry) by approx.30 kcal mol/sup -1/. The quenching behavior of the triplet of 2, however, is normal, the rate constants for quenching by the above-mentioned quenchers being in the limit of diffusion control. A major reason for the excited-state torsional distortion in 1 and 3 appears to be the steric interaction between adjacent substituents (phenyl and methyl) on the carbon atoms of the ene double bond.
- Research Organization:
- Univ. of Notre Dame, IN
- OSTI ID:
- 7169830
- Journal Information:
- J. Phys. Chem.; (United States), Vol. 90:22
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
PYRONES
PHOSPHORESCENCE
PHOTOLYSIS
RADIATIONLESS DECAY
STRUCTURAL CHEMICAL ANALYSIS
CHEMICAL REACTION KINETICS
ENERGY TRANSFER
EXCIMER LASERS
EXPERIMENTAL DATA
LOW TEMPERATURE
MEDIUM TEMPERATURE
VISIBLE SPECTRA
CHEMICAL REACTIONS
DATA
DE-EXCITATION
DECOMPOSITION
ENERGY-LEVEL TRANSITIONS
GAS LASERS
HETEROCYCLIC COMPOUNDS
INFORMATION
KINETICS
LASERS
LUMINESCENCE
NUMERICAL DATA
ORGANIC COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PYRANS
REACTION KINETICS
SPECTRA
400500* - Photochemistry
400201 - Chemical & Physicochemical Properties