Phosphorescence excitation spectrum of the T1(n,π*)←S transition of 4H-pyran-4-one

Abstract

The phosphorescence excitation (PE) spectrum of 4H-pyran-4-one (4PN) vapor at 40-50 degrees C was recorded near 366 nm. The most intense vibronic feature in this region of the spectrum is the T(1)(n,pi*)<--S(0) origin band. The value of nu(0) for the 0(0)(0) transition was determined to be 27 291.5 cm(-1) by comparing the observed spectrum to a simulation in the T(1)<--S(0) origin-band region. Attached to the origin band in the PE spectrum are several Deltav=0 sequence bands involving low-frequency ring modes. From the positions of these bands, together with the known ground-state combination differences, fundamental frequencies for nu(18') (ring bending), nu(13') (ring twisting), and nu(10') (in-plane ring deformation) in the T(1)(n,pi*) excited state were determined to be 126, 269, and 288 cm(-1), respectively. These values represent drops of 15%, 32%, and 43%, compared to the respective fundamental frequencies in the S(0) state. The changes in these ring frequencies indicate that the effects of T(1)(n,pi*)<--S(0) excitation extend beyond the nominal carbonyl chromophore and involve the conjugated ring atoms as well. The delocalization may be more extensive for T(1)(n,pi*) than for S(1)(n,pi*) excitation.

Triplet and Singlet (n,π*) Excited States of 4H-Pyran-4-one Characterized by Cavity Ringdown Spectroscopy and Quantum-Chemical Calculations

“…Table 1 lists band positions in the CRD spectrum and T 1 (n,π*) ← S 0 vibronic assignments. The assigned bands include those previously observed in the PE spectrum 2 as well as bands newly detected in the present study. Table 1 also incorporates corrections to certain assignments we made previously.…”

“…The broad hump farther to the blue contains overlapping ΔN = +2 (S-form) branches. 2,20 The S 1 ← S 0 features in the CRD spectrum (Figure 2) are much broader than the T 1 ← S 0 vibronic bands. The greater widths in the S 1 ← S 0 region are likely due to the presence of closely spaced, unresolved satellite bands associated with the ring-bending mode, ν 18 .…”

“…Microwave spectroscopy shows that the S 0 ground state of 4PN has a C 2v equilibrium geometry, in which all atoms are co-planar. 1 Electronic spectra of the T 1 (n,π*) ← S 0 2 and S 1 (n,π*) ← S 0 3 band systems indicate that the S 1 (n,π*) and T 1 (n,π*) electronic excited states are also planar but with diminished structural rigidity due to the π* ← n excitation.…”

“…In 2008, we reported the T 1 (n,π*) ← S 0 phosphorescence excitation (PE) spectrum of 4PN vapor near 367 nm, measured at 40 °C. 2 We assigned the T 1 (n,π*) ← S 0 origin band along with about a dozen vibronic hot bands (v″ > 0) involving low-frequency ring vibrational modes. From the vibronic analysis, we concluded that the n,π* chromophore for T 1 excitation, nominally associated with the carbonyl moiety, is extensively delocalized to include the ring atoms.…”

“…In 2008, we reported the T 1 (n,π*) ← S 0 phosphorescence excitation (PE) spectrum of 4PN vapor near 367 nm, measured at 40 °C . We assigned the T 1 (n,π*) ← S 0 origin band along with about a dozen vibronic hot bands ( v ″ > 0) involving low-frequency ring vibrational modes.…”