Q bands of porphyrins

The free-base form of sapphyrin also displays three relatively weak Q-type transitions in the 620-710 nm region of the visible spectrum. On protonation, these bands increase in intensity, and are generally blue-shifted to between 615 and 690 nm. Depending upon which acid is used, the number of Q-bands observed for protonated sapphyrins varies from two to four in number. These bands too are red-shifted relative to the Q-bands of porphyrins. In fact, because of this bathochromic shifting, the sapphyrins actually absorb light at the edge of the physiological window of transparency that occurs between ca. 100 and 900 nm. This makes sapphyrin... 

Porphyrins are well known for their ligation with various metal centers to form metalloporphyrins. Coordination of the metal ions typically has a dramatic effect on the It delocalization in the molecule. Inherited changes are very obvious in spectroscopic analysis and in their properties. Especially, the electronic spectra of metalloporphyrins are dependent on the identity of the metal ion, axial ligation, oxidation level, and spin state. Typically, metal coordination reduces the number of observed Q bands of porphyrin, indicating the acquisition of higher symmetry of the chromophore relative to the free base. X-ray structural determinations reveal that the bond length of porphyrin changes upon coordination with metal ions [39]. 

FIG. 16 Photocurrent spectra corresponding to the photo-oxidation of DCMFc by ZnYPPC" at the water-DCE interface under chopped illumination and lock-in detection. The main features of the spectra coincide with the onset of the Soret band and the Q-bands of the porphyrin ring. (From Ref. 73. Reproduced by permission of the Royal Society of Chemistry.)... 

Much like the porphyrin and TPP complexes, the observed MCD of the Q band of the TAP and Pc complexes is relatively straightforward to reproduce. TDDFT calculations find that this transition is dominated by the alu—>eg one-electron excitation (136,151-153) and the calculated Aj parameters are large and positive. 

In addition to the hypsochromic shift of the Soret and Q bands of diamagnetic Ni TPP, there are various optically invisible lower-energy electronic states in which an electron is promoted from the metal to the porphyrin ring or vice versa and those that correspond to d-d excitation within the metal orbitals. Calculations have shown that there is a multitude of CT states below the first optically allowed state and explain its lack of fluorescence (42). The radiationless depopulation of the Q state... 

As expected for a delocalized 26 7c-electron aromatic system, the rubyrins, when protonated, display an intense Soret-like absorbance band in the visible spectrum that is considerably red-shifted (by ca. 100 nm) relative to that of the porphyrins (Figure 7.1.1). For instance, the Soret band of the bis-HCl salt of dodecaalkylrubyrin 7.25 appears at 505 nm (e = 302000M cm in CH2CI2). The rubyrins also exhibit three long-wavelength Q-like transitions in their dicatio-nic forms. These latter bands also show a marked red shift when compared to those of the porphyrins. For instance, the Q-bands of 7.25 appear at 711, 791, and 850 nm (s = 11000, 15 500, and 38 000M cm , respectively). ... 

However, the fluorescence of the P-CD sandwiched porphyrin 81 (schematic representation reprinted with permission from reference 240) was not quenched by 1,4-benzoquinone [240]. Efficient quenching and biexponential decay of the emission were observed in presence of 1,4-naphthoquinone, of 9,10-anthraquinone-2-sulfonate and of a benzoquinone functionalized with adamantane. The fluorescence quenching was always accompanied by variations of the absorption Q band of the porphyrin, which indicated ground-state association between 81 and the quinones. These data indicate that association between 81 and the quinones is a prerequisite for electron transfer. 

In other words, the Q bands are forbidden and simply shouldn t exist. The fact that they do is because of molecular vibrations within the porphyrin macrocycle. These have the effect of marginally lifting the degeneracy of bj and b2 so that the difference in intensities represented by Qy and is no longer equal to zero, i.e. the Q bands become weakly allowed. Their weakness allows them to show vibrational fine structure. In the case of a porphyrin dication or metal complex, where the x and y directions for the components of the transition dipoles are equivalent, this gives rise to two Q bands. For porphyrin free bases, where the x and y directions are perpendicular to each other, each component has two associated Q bands, so the total number is four. 

The incorporation of phthalocyanines into a multichromophore supra-molecular system was attempted by attachment to imidazolyl zinc porphyrin 65 (Fig. 28). A combination of strong absorption of the Soret band of porphyrin and of the Q-band of phthalocyanine makes compound 66 an attractive dyestuff similar to chlorophylls and covering the whole visible re-... 

In contrast to the previous research, the length of the anchoring group remained the same. Thus, no influence of the spacer nature on the electron injection or collection efficiency was expected. Instead, light absorbance should be increased. The increase of absorbance efficiency was posmlated because of broadening the absorbance spectra in the series of LAC-1 to LAC-5, accompanied by the red shift of both the Soret and Q-bands of the porphyrins. In fact, PCE increased from LAC-1 to LAC-3 (Table 5). However, an increase of the spacer conjugation from LAC-3 to LAC-5 led to a rapid decrease of PCE almost to zero (Table 5). 

Meso substitution of porphyrines to give tetraazaporphyrines, so-called porphyrazines, modulates the electronic character of the macrocycle. While porphyrazines have received considerably less attention than porphyrines over many years, this has changed due to the development of efficient syntheses of soluble derivatives.1 6-1809 Also, various porphyrazines (and phtalocyanines) with peripheral groups for metal ion coordination have been prepared and used for the construction of multimetallic complexes.1806 Ni porphyrazines (695) typically show absorptions spectra with a strong Q band at around 615nm. 

---