PPPs substituted

PPP, substituted or unsubstituted, is not planar the band gap is relatively high and emission is shifted towards a higher energy with respect to PPV. So, while the emission of PPV is mostly centered in the yellow-green region, PPP emissions can reach the blue region of the spectrum. 

Semiempirical methods, of whieh there are quite a few, differ in the proportion of caleulations from first prineiples and the relianee on empirieal substitutions. Different methods of parameterization also lead to different semiempirieal methods. Huekel and extended Huekel ealeulations are among the simplest of the semiempirieal methods. In the next two seetions, we shall treat a semiempirieal method, the self eonsistent field method, developed by Paiiser and Parr (1953) and by Pople (1953), whieh usually goes under the name of the PPP method. 

The charge diagram of A-4-thiazoline-2-one is summarized in Table VII-9. This diagram and the one obtained by a HMO treatment (105) are consistent with the easy acetylation occurring in position 5 of the ring. However, PPP calculations indicate that this electrophilic substitution could also have occurred in position 4, which is not observed. 

As in the case of the free bases, the substitution of a nuclear hydrogen atom by a methyl group induces a bathochromic shift that decreases in the order of the position substituted 4->5->2- Ferre et al. (187) have proposed a theoretical model based on the PPP (tt) method using the fractional core charge approximation that reproduces quite correctly this Order of decreasing perturbation. 

Theoretical studies of the relative stabilities of tautomers 14a and 14b were carried out mostly at the semiempirical level. AMI and PM3 calculations [98JST(T)249] of the relative stabilities carried out for a series of 4(5)-substituted imidazoles 14 (R = H, R = H, CH3, OH, F, NO2, Ph) are mostly in accord with the conclusion based on the Charton s equation. From the comparison of the electronic spectra of 4(5)-phenylimidazole 14 (R2 = Ph, R = R3 = H) and 2,4(5)-diphenylimidazole 14 (R = R = Ph, R = H) in ethanol with those calculated by using ir-electron PPP method for each of the tautomeric forms, it follows that calculations for type 14a tautomers match the experimentally observed spectra better (86ZC378). The AMI calculations [92JCS(P1)2779] of enthalpies of formation of 4(5)-aminoimidazole 14 (R = NH2, R = R = H) and 4(5)-nitroimidazole 14 (R = NO2, R = R = H) point to tautomers 14a and 14b respectively as being energetically preferred in the gas phase. Both predictions are in disagreement with expectations based on Charton s equation and the data related to basicity measurements (Table III). These inconsistencies may be... 

Tile basic chromophors of indigo and of some tetraazafulvalenes are very similar and therefore the frontier orbitals are comparable (90JPC949). PPP calculations on the 1,3,5,7-tetraazafulvalene 90 (R = Me) showed a good agreement with its UV/VIS spectrum. Tlie substitution of both methyl-sulfanyl groups in 90 by diethylamino/piperidino moieties led to a hypso-chromic shift of about 45 nm. 

In addition to alkyl-substituted derivatives, soluble PPPs 6 are also known today containing alkoxy groups as well as ionic side groups (carboxy and sulfonic acid functions) [18]. Schliiter et al. recently described the generation of soluble PPPs decorated with densely packed stcrically demanding dendrons on the formation of cylindrically shaped dendrimers, so-called cylinder dendrimers ] 19]. 

Recently, Tour et al. [47] described attempts to use the Bergman cyclizat produce PPP derivatives starting from substituted endiynes, e.g. poi-. 1,4-phenylene) 22 starting from l-phenyl-hex-3-en-l,5-diyne or the structurally related poly(2-phenyl-1,4-naphthalene) 23 starting from l-phenylethynyl-2-ethynyl-benzene. 

Photophysics of Methyl-Substituted PPP-Type Ladder Polymers... 

Figure 9-1. Materials overview a few sclcclcd conjugated polymers and Ihcir properties have been compiled and ihe following abbreviations arc used DO-PPP...Poly(2-decyloxy-l,4-phcnylcnc), EHO-PPP...Poly(2-(2 -elhylliexyloxy)-l,4-phcnylenc), CN-PPP... Poly(2-(6 -cyano-6 -incthyl-licplyloxy)-l,4-phcnylene), m-LPPP... methyl-substituted ladder-type Poly( 1,4-phenylcne), and PLQY=phololuinincs-ecncc quanluni yield.

The investigation by Mustroph et al. (1981) was made using SCF-CI with the PPP approximation to calculate the influence of substituents on the longest wavelength band. This study is interesting because the authors come to the conclusion that 591 nm is the theoretical limit that can be obtained with substituted benzenediazo-nium ions. As far as we are aware, that limit had not been reached by the early 1990s. 

PPP calculations reproduce the nitro substituent effect and heterocyclic effect on the /,max. For example, the bathochromic shift by substitution of a nitro group is calculated (ca.20nm). It is in good agreement with the experimental value determined (A,max = 598 nm) in toluene. PPP calculation exactly predicts the bathochromic shift by benzo-annelation of the indoline and benzopyran residues (Table 2). In the neutral quinoid form, the calculated charge densities for the ground and first excited states by PPP... 

The band gap in substituted PPPs can be tuned to some extent by diluting the substituted phenylene rings with unsubstituted phenylenes. A bathochromic shift in absorption was observed for copolymer 473 (Eg = 2.95 eV) compared to the all-substituted homopolymer 472k (Eg = 3.1 eV) [581]. Sandwiching this polymer between ITO and Ca electrodes afforded a... 

---