The lower valence states of linear polyenes: Locations and identifications

Abstract

The experimental transition energy to the valence A, state of butadiene is used to validate a subset of the theoretical calculations that have been carried out on this molecule. The validated calculated A, transition energies of these calculations are then compared with the experimental B, transition energies to determine the relative state ordering in isolated, unsubstituted butadiene, hexatriene, and octatetraene. The A, origins are concluded to lie below the B , origins for all polyenes except butadiene. Experimental confirmation of this conclusion is presented. The extreme breadth of the A, transition, 1 .O eV, is noted. Some possible implications of this breadth are discussed.In this note, all currently available compatable experimental and theoretical information concerning the locations of the lower valence excited states of isolated, unsubstituted, short polyenes are compared, and an assignment is deduced that is consistent with this information and with the accepted major features of condensed phase and substituted polyene spectroscopy [ 11. Several predictions based on this analysis are presented.The experimental transition energies of two valence transitions each are now known for the all trans isomers of butadiene [2,3], hexatriene [4,5], and octatetraene [6,7] in the vapor phase. These are presented as solid symbols in Figure 1. The locations of the lower set of experimental transitions have long been known. They arise from allowed transitions to the B, state in each molecule [ 111. The B, transition energies indicated on the figure are origins. For use in our later analysis, this set is extrapolated to the decapentaene. The locations of the upper set of transitions have been determined by electron impact spectroscopy [ 3,5,7]. From experimental considerations, the upper set of transitions is assigned as arising from forbidden valence transitions. It is assumed, but not proven, that these are transitions to A, states. The transition energies indicated are vertical transitions.The first question that must be answered is whether or not the upper A, state observed in each molecule is the same. The answer is "no" for three reasons. The first is based on a comparison of the singlet A, and B, energy levels. The A,-B, energy gaps observed here in the triene and tetraene, 2.0 and 1.5 eV, respectively, are reasonable extrapolations of those observed in longer polyenes [ 121. The two transitions observed in each of these molecules thus correspond to the traditional B, and "cis" bands. The A,-B, gap observed in the diene, 1.6 eV, is significantly smaller than is 0 1986 John Wiley & Sons, Inc.

Embedding theory for excited states with inclusion of self-consistent environment screening

“…The excitations of isolated short organic molecules have attracted considerable attention because of the non-trivial character of their low-lying excited states [28][29][30]. For example, contrary to naive expectations the lowest excited state of the octatetraene molecule is not a one-electron (HOMO-LUMO) excitation, but has a HOMO 2 -LUMO 2 double-excitation character [29,30].…”

Two-Photon Photoemission Spectroscopy Study of 1,3-Butadiene on Cu(111): Electronic Structures and Excitation Mechanism

“…The electronic structures of isolated C 4 H 8 have been investigated using both experimental and theoretical methods. Although there is good agreement between experiment and ab initio theory for the two lowest triplet electronic states and a number of the low-lying Rydberg states, the assignment of the lowest excited singlet state is still a matter of debate. − The states of the cations , and anions − of butadiene have also been studied, assuming the approximate validity of Koopman's theorem (KT), which associates the experimentally determined ionization potentials (IP) and electron affinities (EA) with the negative energies of the filled and unfilled orbitals, respectively. The energies (with respect to the vacuum level) of the occupied states of gas-phase butadiene9.03 eV (π 2 , 1 b g ), 11.46 eV (π 1 , 1 a u ), and 12.23 eV (σ 7 , 7 a g )have been measured using PES …”