Phosphorescence excitation spectrum of the T-1(n,pi(*)) - S-0 transition of 4H-pyran-4-one

摘要

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 Delta v=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. (c) 2008 American Institute of Physics.