Quadratic Hyperpolarizability Enhancement ofpara-Substituted Pyridines upon Coordination to Organometallic Moieties:  The Ambivalent Donor or Acceptor Role of the Metal

Abstract

Coordination of para-substituted pyridines 4-X-C 5 H 4 N (where X ) NMe 2 , CMe 3 , H, COMe, CN) to metal carbonyl moieties such as "cis-M(CO) 2 Cl" (M ) Rh(I), Ir(I)) and "fac-Os(CO) 3 Cl 2 " produces an enhancement up to about 2 orders of magnitude of the quadratic hyperpolarizability β λ of the free pyridine. This effect is due either to a red shift of the intraligand charge-transfer (ILCT) transition upon coordination (when X is a strong electron donor) or to a significant effect on the metal-to-ligand charge-transfer (MLCT) transitions (when X is a strong electron acceptor). In this latter case the quadratic hyperpolarizability may assume a negative sign, due to the negative value of ∆µ eg upon excitation. Therefore we confirmed that as already shown by metal carbonyl moieties such as "M(CO) 5 " (M ) Cr, W), a soft metal center displays, from the point of view of the perturbation of the quadratic hyperpolarizability of pyridines, an ambivalent acceptor or donor role. The quadratic hyperpolarizability of complexes of more π-delocalized para-substituted pyridine ligands (where X ) trans-or trans,trans-(CHdCH) n C 6 H 4 -4′-NMe 2 with n ) 1 or 2) involving the same carbonyl Rh(I), Ir(I), and Os(II) moieties corresponds to an enhancement with respect to the free ligands of about 1.5-2.3 times that when n ) 1 and 1.5-1.7 times that when n ) 2. This enhancement, which is relatively low when compared to simple pyridines, is dependent upon the length of the π-delocalized bridge (it decreases with increasing n) and upon the nature of the metal center (oxidation state and ligand coordination sphere). Some of the complexes investigated show significant values (between 400 × 10 -30 and 600 × 10 -30 D cm 5 esu -1 ) of the product µβ 0 .

Exceptional Molecular Hyperpolarizabilities in Twisted π‐Electron System Chromophores

“…Hyperpolarizabilities were measured at 1907 nm in CH 2 Cl 2 by electric field-induced second harmonic generation (EFISH) [22] and track the same concentration-dependence as observed in the optical and PGSE spectra. Thus, the TCM-1 and TCM-2 mb values show pronounced concentration dependence due to the aforementioned aggregation effects (Figure 4).…”

Synthesis, Electronic Characterisation and Significant Second‐Order Non‐Linear Optical Responses of meso‐Tetraphenylporphyrins and Their ZnII Complexes Carrying a Push or Pull Group in the β Pyrrolic Position

“…[13,14] The values of EFISH β 1.907 increase upon introduction of a π-delocalised linker between the β pyrrolic position of the porphyrin ring and the electron-withdrawing nitro group (Table 5). As in structurally related pseudo-linear, organometallic, push-pull chromophores [42] and in structurally related push-pull phthalocyanines, [41] EFISH β 1.907 increases slightly upon increasing the length of the linker or by substitution of a triple bond with an (E)-double bond. We have also shown that EFISH β 1.907 always decreases slightly on going from the free porphyrin to its Zn II complex, with the exception of the couple 9 and 10, which show comparable values (Table 5).…”

“…It follows that a charge-transfer process involving the π linker, which may control the second-order NLO response, is probably located quite parallel to this axis, as in traditional two-dimensional, pseudo-linear, organometallic push-pull chromophores. [42] If so, the component of β vec along the dipole moment axis, that is EFISH β 1.907 (see Experimental Section), should be quite coincident with β vec , which is the vectorial part of the quadratic hyperpolarizability tensor. EFISH β 1.907 turns out to be small for porphyrin 3 and for the Zn II complexes 2 and 4, with the electron-withdrawing group linked directly to the β pyrrolic position ( Table 5).…”

A Bridging Double Bond as an Electron Acceptor for Optical Nonlinearity of Furan‐Containing [n.2]Cyclophenes

“…[11] The photophysical properties and the mb values for 1 as well as the reference compound 9 are summarized in Table 1. The absorption and emission spectra of selected [n.2]cyclophenes (1 a, 1 c, 1 e, and 1 f) are shown in Figure 1 (the spectra of the other compounds can be found in the Supporting Information).…”

“…Presumably, the strain may be relieved significantly when the tethering chain length Table. [g] Ref. [11]. increases from three to four methylene groups.…”