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

• Published in: The Journal of chemical physics 2008-03, Vol.128 (10)
• Main Authors: Hoffelt, Laura M., Springer, Mitchell G., Drucker, Stephen
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.2834922

The phosphorescence excitation (PE) spectrum of 4H-pyran-4-one (4PN) vapor at 40–50°C was recorded near 366nm. The most intense vibronic feature in this region of the spectrum is the T1(n,π*)←S0 origin band. The value of ν0 for the 000 transition was determined to be 27291.5cm−1 by comparing the observed spectrum to a simulation in the T1←S0 origin-band region. Attached to the origin band in the PE spectrum are several Δv=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 ν18′ (ring bending), ν13′ (ring twisting), and ν10′ (in-plane ring deformation) in the T1(n,π*) excited state were determined to be 126, 269, and 288cm−1, respectively. These values represent drops of 15%, 32%, and 43%, compared to the respective fundamental frequencies in the S0 state. The changes in these ring frequencies indicate that the effects of T1(n,π*)←S0 excitation extend beyond the nominal carbonyl chromophore and involve the conjugated ring atoms as well. The delocalization may be more extensive for T1(n,π*) than for S1(n,π*) excitation.