Phosphorescence Spectrum of Acetophenone; An Example of Pseudo-Jahn-Teller Distortion

Abstract

The phosphorescence spectra and lifetimes of acetophenone, aceto-d3-phenone and acetophenone-d5 were recorded in a variety of close packed and relaxed matrices. The variable vibronic lifetime changes, temperature dependence, and deuteration effects on the acetophenone phosphorescence from solvent to solvent are manifestations of perturbation between close lying 3ππ* and 3nπ* states. The acetophenone phosphorescence problem is marked by environmental perturbations superimposed on pseudo-Jahn-Teller interactions. The result is an environmental sensitive potential surface for the lowest triplet state marked by failure of the Condon approximation. Five cases are distinguished. (i) Only the dynamical pseudo-Jahn-Teller effect is operative as in acetophenone vapor. The emission here is interpreted to be from a distorted, i.e., largely torsionally twisted acetophenone about the COCH3 group. (ii) and (iii) There are environmental (static) interactions larger than dynamical interactions leading to strong characteristic out-of-plane vibrations and medium lifetime, e.g., COCH3 group modes in methylbenzoate matrix, where 3nπ* is lowest, and ring out-of-plane modes in acetophenone-d5 matrix, where 3ππ* is lowest. (iv) There is a dynamical Jahn-Teller interaction stronger than the environmental interaction. In some instances the resulting triple minimum potential curve results in dual phosphorescence emissions even at low temperatures (1.68°K) from the lowest 3nπ* state—one with a very short lifetime from a highly distorted region of the potential surface (the zero point region) and the other with a medium lifetime from an excited vibronic region, e.g., in pentane and other relaxed hydrocarbon matrices. The upper vibronic region is kept from rapidly cascading to the lower region of the triplet state by a potential barrier leading to temperature dependent phosphorescence. Breakdown of the Condon approximation for the lowest triplet state is observed, probably due to rapid change of the singlet—triplet transition moment with the distortion coordinate in the vicinity of near degeneracy of the zeroth order states. (v) Again the dynamical Jahn-Teller interaction is stronger than the environmental interaction except that the zeroth order 3ππ* state is now lowest, e.g., at 77°K in a benzene matrix the emission with its vibrational structure and lifetime characteristic of 3nπ is from the zeroth order region—whose electronic wavefunction is described largely by 3nπ. At 4.2°K, the vibrational structure and lifetime represent phosphorescence from an excited vibronic region of the lowest triplet state whose electronic wavefunction is largely 3ππ*. The rapid change in transition moment with vibrational coordinate also renders the Condon approximation inappropriate in this case.

Ultrafast electron diffraction: Excited state structures and chemistries of aromatic carbonyls

“…The conclusion is that the T 1 -T 2 energy gap is even less than that of benzaldehyde. 23,44 The energies and geometries of these lowlying electronic states have been studied theoretically, yielding similar results. An estimate of the T 1 -T 2 energy gap is only 565 cm −1 .…”

“…26 In the triplet manifold, the well-known T 1 ͑n * ͒ state has its origin at 25 791 cm −1 by sensitized phosphorescence 8 and 25 786 cm −1 by its direct emission. 44 Like benzaldehyde, the T 2 ͑ * ͒ state was not directly observed in the vapor. A congestion of spectral features noted 600 cm −1 above the T 1 origin has been attributed to possible mixing with the nearby T 2 .…”

“…30,31 Like for benzaldehyde, the prolific study of acetophenone is largely due to its intense phosphorescence facilitated by efficient intersystem crossing from S 1 into the triplet manifold. The photophysics of acetophenone have been studied in the gas phase, [6][7][8][9]20,26,[43][44][45][46][47] solution, 21,48 and in matrices. 22,23,44 Phosphorescence detected in the vapor phase is seen to decrease in intensity with increasing photon energy and exhibits a drop-off as excitation approaches S 2 .…”

“…The photophysics of acetophenone have been studied in the gas phase, [6][7][8][9]20,26,[43][44][45][46][47] solution, 21,48 and in matrices. 22,23,44 Phosphorescence detected in the vapor phase is seen to decrease in intensity with increasing photon energy and exhibits a drop-off as excitation approaches S 2 . The phosphorescence quantum yield at the S 1 origin is near unity, 7,45,47 while at the S 2 origin it is reduced to ϳ10 −4 .…”

Excited state molecular structures and reactions directly determined by ultrafast electron diffraction

“…The phosphorescence spectra of both molecules have been characterized in the vapor phase [2][3][4] and in matrices. 5,6 High yields of phosphorescence upon excitation into their S 1 ͑ 1 n * ͒ states correspond to a very efficient coupling with the triplet manifold through the non-emitting "dark" T 2 ͑ 3 * ͒. 7 The emission originates from the T 1 ͑ 3 n * ͒ state, although line congestion observed in the vapor phase just above the T 1 origin is suggestive of the T 2 ͑ 3 * ͒ state lying within ϳ1000 cm −1 , 4,7-10 a placement also proposed by theoretical inquiries.…”