Franck notation

In terms of synthesis, the rosarins are most easily prepared via the condensation of three a-free bipyrroles with three equivalents of an aromatic aldehyde. For instance, rosarin 7.36 was prepared in > 70 /o yield via the condensation of bipyrrole 7.31 with benzaldehyde, followed by oxidation with DDQ (Scheme 7.3.1). In a similar way, the rosarins 7.37-7.39 could also be prepared in good yield. The preparation of rosarins thus bears formal analogy to the Rothemund-Adler-type methodology used to prepare meso-substituted porphyrins. " In marked contrast to what is observed under these latter porphyrin-forming conditions, the rosarins are obtained as 24 jt-electron, non-aromatic 4n 7r-electron species. According to the Franck notation, therefore, the rosarins are formally [24]hexaphyrins-(1.0.1.0.1.0). 

Chemical enhancement is typically explained by the CT mechanism. When a molecule is adsorbed on a metal surface, new electronic states are formed due to chemisorption. The new electronic states may serve as resonant intermediate states in Raman scattering. If the Fermi level of the metal is located between the Highest Occupied Molecular Orbital (HOMO) Lowest Unoccupied Molecular Orbital (LUMO) in energy, CT excitations may likely occur at lower energy than intrinsic intramolecular excitations of the adsorbate [55-58]. According to Albrechts notation [55], in the CT mechanism via Albrechts A term Franck-Condon term) only the totally symmetric modes are resonantly enhanced when the laser excitation is close to an allowed electronic transition, and only one excited state is involved. The resonance Raman effects for vibrational modes that are non-totally symmetric, are usually observed when these modes couple two excited states of the chromophore. The product of the symmetry of both excited states should be equal or contain the non-totally symmetry. This mechanism is known as the Herzberg-Teller mechanism or B mechanism in Albrechts notation. 

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