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

Abstract

The 4H-pyran-4-one (4PN) molecule serves as a model for investigating structural changes following π* ← n electronic excitation. We have recorded the cavity ringdown (CRD) absorption spectrum of 4PN vapor at room temperature, over the wavelength region from 350 to 370 nm. This spectral region includes the T1(n,π*) ← S0 band system as well as the low-energy portion of the S1(n,π*) ← S0 system. Aided by predictions from ab initio (equation-of-motion excitation energies with dynamical correlation incorporated at the level of coupled cluster singles doubles, EOM-EE-CCSD) and density functional theory (time-dependent density functional theory with PBE0 functional, TDPBE0) calculations, we have made vibronic assignments for about 30 features in the CRD spectrum, mostly T1(n,π*) ← S0 transitions. We have used these results to correct certain vibronic assignments appearing in the previous literature for both T1(n,π*) ← S0 and S1(n,π*) ← S0 band systems. We conclude that the lowest-energy carbonyl wagging fundamentals (ν27, in-plane and ν17, out-of-plane) undergo significant frequency drops (28 and 50%, respectively) upon T1(n,π*) ← S0 excitation and similar drops (29 and 39%, respectively) for S1(n,π*) ← S0 excitation. We find that vibrational modes involving the conjugated ring atoms undergo relatively small frequency changes upon π* ← n excitation, for both T1 and S1 states. We have used the present spectroscopic results and vibronic assignments to test the accuracy of computed excited-state frequencies for 4PN. This benchmarking process shows that the economical time-dependent density functional theory method is impressively accurate for certain (but not all) vibrational modes. The highly correlated EOM-EE-CCSD ab initio method is capable of making accurate frequency predictions, but the results, unexpectedly, depend sensitively on basis set family. This anomaly is traceable to a computed conical intersection between the T1(n,π*) and T2(π,π*) surfaces near the T1(n,π*) potential minimum. Relatively small errors in the location of the conical intersection lead to enhanced mixing of the two electronic states and incorrect T1(n,π*) vibrational frequencies when certain triple-ζ quality basis sets are used.

Jet-Cooled Phosphorescence Excitation Spectrum of the T1(n,π*) ← S0 Transition of 4H-Pyran-4-one

“…Under the present jet-cooled conditions, the maximum of the 0 0 0 band is at 27 293.2 cm −1 . This value is slightly different from the location of the origin-band maximum in the roomtemperature CRD spectrum, 4 27 290.2 cm −1 . In the latter case, the maximum occurs within the Q-form branch, whereas in the jet-cooled spectrum, the maximum is in the R-form branch.…”

“…18 The gold standards are too expensive for treating molecules having more than a few heavy atoms (such as 4PN), but EOM-EE-CCSD is feasible for excited-state geometry optimization and harmonic frequency calculations of monocylic molecules. 4,19 For even larger molecules, the very inexpensive TDDFT method is in widespread use. For example, accurate TDDFT calculations of vibronically resolved spectra are available for molecules such as conjugated dyes containing several fused rings.…”

“…Though 4PN is near the upper size limit for EOM-EE-CCSD frequency calculations (with triple-ζ quality basis sets), it is possible to carry out these calculations within a few days by using modern parallelizable algorithms. 22 In a prior investigation of 4PN, 4 we measured the CRD absorption spectrum of the T 1 (n,π*) ← S 0 transition at room temperature. Most of the CRD assignments involved lowfrequency, nontotally symmetric ring modes.…”

Cavity Ring-Down Spectroscopy: Recent Technological Advancements, Techniques, and Applications

“…Drucker and co-workers investigated the triplet and singlet ( n ,π*) excited states of 4H-pyran-4-one (4PN) using the CRDS technique within the spectral range of 350–370 nm at room temperature. They assigned 30 experimental vibronic transitions of 4PN in combination with the predictions from ab initio and density functional theory.…”

An overview of recent advances in the catalytic synthesis of substituted pyrans

“…Furthermore, they are important building blocks in organic synthesis. [24][25][26] Apart from these, they have varied importance in fields of dyes, [27,28] pH sensitizers, [29] semiconductors, [30,31] and materials. [32,33] Some biologically active compounds containing pyran scaffolds are depicted in Figure 2.…”