On Excited States of Alkyl Bridged [14] Annulenes

Abstract

SummaryThe linear dichroic absorption spectrum of 1,3,6,8-trans-15,16-hexamethyldihydropyrene has been measured in stretched polyethylene at 77 K, and CNDO-CI calculations with inclusion of singly and doubly excited configurations have been carried out on a series of alkyl bridged [ 14lannulenes with pyrene-and anthraceneshaped perimeters. Transitions to e3g+ e4u type 'L and lB states are well described, and the results indicate that additional low-energy excited states originate from e3g+ e5g and e2u-' e4u type configurations interacting strongly with doubly excited configurations of the e3g, e3g+ e4u, e4u type. The second excited singlet state responsible for the weak transition observed between the 'L bands may be assigned to one of these additional states, but it is probably of complex nature, similar to the 'phantom' state in linear polyenes.Introduction. -The electronic structure of bridged annulenes has been the subject of several investigations, e.g. [ 1-81. The electronic absorption spectra of these compounds are particularly interesting in that they provide a direct application of Platt's classification of electronic states of cyclic polyenes [9], and they further offer the opportunity for studying the effects of distortions from planarity and inductive and hyperconjugative perturbations on the spectroscopic properties of conjugated z-electron systems.The electronic absorption spectrum and polarized fluorescence of trans-15,16-dimethyl-dihydropyrene 1 (Scheme 1) and some of its simple derivatives were investigated some time ago by Heilbronner et al. [2], and the spectra of 1,6: 8,13-cyclopropanediylidene [ 14lannulene (2), 1,6 : 8,13-ethanediylidene [ 14lannulene (3), 1,6: 8,13-propane-1,3-diylidene[l4]annulene (4), and 1,6: 8,13-butane-l,4-diylidene [14]annulene (5) (Scheme 1) were studied recently by Michl et al. [7] by means of several experimental techniques, including linear dichroism in stretched polymer sheets. Characteristic absorption bands in the series of alkyl bridged [ 14lannulenes 1-5 could in all cases be assigned to L,, La, B, and B, states derived from Platt's perimeter model [9]. However, several additional') transitions were observed. In par-

Bis(trifluoromethyl)sulfene: generation and cycloaddition reactions

“…Decoupling experiments were required to determine the 'H chemical shifts and coupling constants unambiguously. The relatively large 15 Hz coupling constant for the vinyl protons H6,H7 in the adducts is consistent with trans but not cis vinyl proton stereochemistry (3Jtrans = 12-18 Hz, 3Jcir [6][7][8][9][10][11][12]. This immediately rules out the [4 + 21 cycloadducts 22 and the [2 + 21 cycloadducts 23.…”

Conjugative, Exciton and Charge‐Resonance Interactions in D2d‐9,9′‐Spirobifluorene and D2d‐Tetrabenzotricyclo [5.5.0.02,8]dodeca‐3,5,9,11‐tetraene. Photoelectron and Polarized Absorption Spectra

“…[l 11 in several essential points. CNDO/S calculations were performed using a modified version [27] of the computer program published by Baumann [28]. lnput geometries were estimated from the results of MNDO [29] calculations with partial geometry optimization; D2d-symmetry was assumed for 1 and 2, Czv for 3.…”

Two-photon spectroscopy of dipole-forbidden transitions

“…To our opinion this is due to the fact that the importance of this question has not been recognized so clearly at that time. However, some more recent applications, where HEC are included in CNDO/S calculations [58,59] do not deal with this problem, too. It is therefore one of the main aspects of this paper to investigate to which extent the general results obtained from PPP theory can be transferred to NDO-type methods.…”