Electronic spectra aromatics

Nuclear magnetic resonance measurements have led to the conclu-sion that 2-pyridones have about 35% of the aromaticity of benzene and that the formally related l,2-dihydro-2-methylenepy-ridine is not aromatic. A substantial contribution by such resonance is indicated by the electronic spectrum of 2-quinolone, which is... 

The reduced symmetry of the chromophore, which still contains 187t-electrons and is therefore an aromatic system, influences the electronic spectrum which shows a bathochromic shift and a higher molar extinction coefficient of the long-wavelength absorption bands compared to the porphyrin, so that the photophysical properties of the chlorins resulting from this structural alteration render them naturally suitable as pigments for photosynthesis and also make them of interest in medical applications, e.g. photodynamic tumor therapy (PDT).2... 

In electron donor-acceptor (EDA) complexes, there is always a donor molecule and an acceptor. The donor may donate an unshared pair (an n donor) or a pair of electrons in a ti orbital of a double bond or aromatic system (a it donor). One test for the presence of an EDA complex is the electronic spectrum. These complexes generally exhibit a spectrum (called a charge-transfer spectrum) that is not the same as the sum of the spectra of the two individual molecules. Because the first excited state of the complex is relatively close in energy to the ground state, there is usually a... 

Solutions of the alkylammonium salts of Cl , Br , r in acetonitrile show no visible absorptions beyond 300 nm. The aromatic it-acceptor, tetracyanopy-razine (TCP) is characterized by strong absorptions in the 220-300 nm range and a shoulder at 350 nm. However, the electronic spectrum of a mixture of the bromide salt and TCP reveals a new absorption band at Xct = 400 nm... 

Thus, the historical development of the chemistry of metallocorrolates until 1980 includes complexes with Cu2+, Ni2+, Pd2+, Fe3+, Co3+, Rh+, Mo5+ and Cr5+. The palladium complex has been isolated as its pyridinium salt since the neutral species was too unstable to be isolated or spectroscopically characterized [19]. The nickel complex was non-aromatic, with one of the potentially tautomeric hydrogens displaced from nitrogen to carbon in such a way as to interrupt the chromophore. In contrast the electronic spectrum of the paramagnetic copper complex is similar to those of the fully conjugated lV(21)-methyl derivatives [11],... 

Mason and Smith (1969) found that for a series of mono- and bicyclic aromatic hydrocarbons the changes in the fluorescence spectrum with acidity reflected the ground state protonation reaction. The p Sj )-values calculated for benzene, toluene, naphthalene, azulene, and indolizine do not correspond to observable processes since the rate of protonation is too slow to compete with deactivation of the Sj state. Photochemical deuterium and tritium exchange experiments in 1 mole dm-3 perchloric acid indicate that the radiative deactivation rate of an electronically excited aromatic hydrocarbon is faster than the rate of protonation by a factor >10s. 

SbCla is a relatively weak Lewis acid, since in the presence of a mixture of mesity-lene and benzoyl chloride it gives a complex with mesitylene Antimony penta-chloride is stronger and gives a 1 1 complex with perylene whose electronic spectrum is similar to that of the perylene cation Complexation has been claimed between antimony pentafluoride and aromatic olefins such as 1,1-diphenylethylene and stilbene ... 

As would be expected for an aromatic macrocycle, pentaphyrins display exceptionally strong absorbance bands in the visible portion of the electronic spectrum. For instance, pentaphyrin 6.35 displays a sharp Soret-like absorbance band at 458 nm (CH2CI2, = 238 000 cm ), and two Q-like absorbances at 642 and 695 nm (e = 5990 and 3540M cm , respectively). They thus resemble spectroscopically both the sapphyrins and smaragdyrins, as well as the tetrapyrrolic porphyrins from which they are conceptually derived. Like these various aromatic cousins , the... 

---