Abstract
This short review outlines the main results obtained by our research group over the last 15 years in the field of porphyrins and metal porphyrins for second order nonlinear optics (NLO). This overview aims to provide a general framework of the key factors which affect the second order NLO response of porphyrin chromophores. The pivotal role of the porphyrin ring as π-conjugated linker, the nature of the metal center, the substitution pattern which features the geometrical arrangement of donor and acceptor substituents in the different classes of porphyrin NLO-phores, as well as the aggregation phenomena and the role of solvents are addressed in detail.
Highlights
The expression nonlinear optics (NLO) defines the optical phenomena caused by the interaction of a strong oscillating electromagnetic field with molecules or bulk materials, with emission of new electromagnetic fields that differ from the incident ones in frequency, in phase or in other optical properties.When a laser beam interacts with a molecule, the perturbation is expressed by Equation (1): Pi = Pi + αij Ej + βijk Ej Ek + γijkl Ej Ek El + . . . (1)where Pi is the dipole moment induced by the perturbating optical field E which polarizes the electronic charge. aij is the linear polarizability, while βijk and γijkl are the quadratic and cubic hyperpolarizabilities respectively
This series of investigations has pointed out the ambivalent role of the polarizable porphyrin ring which, already in the ground state, behaves as a donor or acceptor depending on the nature of the substituent either in the β-pyrrolic or meso position, showing a charge-transfer process between the porphyrin core and the substituent as the origin of the second order NLO response
When the substituent is an electron donor a significant increase of the electron donor properties is observed when the substituent is in the β-pyrrolic or meso position, probably due to an auxiliary donor effect of the electron rich porphyrin ring
Summary
The expression nonlinear optics (NLO) defines the optical phenomena caused by the interaction of a strong oscillating electromagnetic field (such as the one associated with an intense and coherent laser light) with molecules or bulk materials, with emission of new electromagnetic fields (i.e., new light) that differ from the incident ones in frequency, in phase or in other optical properties. We suggest that the high and positive value of EFISH β1907 originates from an increase of the effective strength of the donor group due to the interaction with the very electron-rich π system of the porphyrin macrocycle, which produces a reduction of the ground state polarization with an increase of the second order NLO response [18]. With this view there is no involvement of the π core of the porphyrin ring in the excitation process controlling the second order NLO response
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