The π‐Orbital Energies of the Acenes

Abstract

The 'observed' n-orbital energies cv,j = -Iv, j derived from the vertical ionization potentials obtained by a photoelectron spectroscopic investigation of the acenes benzene A ( l ) , naphthalene A(2), anthracene A(3), naphthacene A(4) and pentacene A(5) have been compared with n-orbital energies calculated by three different approximations : (a) the standard Huckel HMO model; (b) a first-order perturbation treatment, based on (a), that takes into account bond length changes which follow the ionization process; (c) a SCF n-electron model of the type proposcd by Pople and by Pariser & Purr. I n agreement with previous experience i t is found that model (b) yields the most satisfactory parametrization of the experimental data. l) 2) Part 37 of 'Applications of Photoelectron Spectroscopy'. Part 36: [l]Permanent Address :

Electronic Structure and Photophysical Properties of Isoindole and its Benzo[f]‐ and Dibenzo[e,g]‐Derivatives

“…parameters we prefered to use the more lucid Hiickel model which yields accurate predictions of IP's. for a variety of conjugated hydrocarbons if allowance is made for the bond lengths changes upon ionization by a perturbative treatment [31] were then performed with hN = 1 and 0.5, respectively, where the results of the latter (Fig. 7c) were in better agreement with experiment.…”

“…Invoking Koopmans' theorem, we choose as basis energies Aj of the decoupled hydrocarbon moieties those observed IP's. of the corresponding acenes which are associated with ionization from a, and b3g orbitals [31] [39]. The basis energy AN of -NMe-may be located near -9.6 eV in line with previous experience [37].…”

“…Choosing the pattern traced by the PE. spectra of the acenes [31] as a guideline to interpret the spectra of the series N-methyl-pyrrole [32], 1 and 2, the correlation shown in Fig. 7a suggests itself on purely empirical grounds.…”

“…The value of the valence state IP. of carbon -which need not be fixed for the calculation of electronic spectra -was chosen as 9.8 eV, the value determined by Heilbronner et al [31] from a linear regression between PPP. eigenvalues and experimental IP's.…”

Exciton Electro-Absorption at Non-Zero Wave Vector

“…In fact, although vibronic distortion in an individual molecule is larger for excitation than for ionization (1.57 vs. 1 [13][14][15]), but in a CT state two molecules are distorted at the same time, instead of one. Therefore we have adopted for all states the uniform value of b = 1.57, obtained from the absorption spectrum of the isolated molecule [14,15].…”