Minimum energy paths in the excited and ground states of short protonated Schiff bases and of the analogous polyenes

A theoretical study of the minimum energy paths (MEP) for the first excited state S-1 and for the ground state S-0 of two short-chain protonated Schiff bases (the protonated s-cis 1-iminium-2-propene H2C=CH-CH=NH2 + and the protonated tZt 1-iminium-2,4-pentadiene H2C=CH-CH=CH-CH=NH2 +) and of the two analogous polyenes (s-cis butadiene H2C=CH-CH=CH2 and tZt hexatriene H2C=CH-CH=CH-CK=CH2) is reported. The geometries have been optimized at the CAS-SCF level and the energetics have been refined via single-point computations at the multi-reference MP2 level. It is demonstrated that the photochemistry of the protonated Schiff bases and of the analogous polyenes is driven by two different S-1 excited states, the spectroscopic 1B ionic state for the protonated Schiff bases and the covalent 2A(1) excited state for the analogous polyenes. This difference induces different electronic and geometric features in the computed MEP and conical intersections which depend on the nature of the involved excited states. The conical intersections between S-1/S-0 in polyenes have a covalent tetraradicaloid nature and are characterized by the development of a typical (-CH-)(3) 'kink' in the carbon moiety, while the conical intersection points in the protonated Schiff bases have a twisted double bond. Thus in the protonated Schiff bases, the photoproducts arise only from isomerization processes, while in the analogous polyenes, the photoproducts arise from various types of recoupling schemes of the electrons of the quasi-tetraradical conical intersection point, leading to a more complex photoreactivity pattern. (C) 1998 Elsevier Science S.A. All rights reserved.