Borderline class II/III ligand-centered mixed valency in a porphyrinic molecular rectangle

Inorganic Chemistry

Volumn 44, Issue 16, 2005, Pages 5789-5797

  Dinolfo, Peter H ^a   Suk, Joong Lee ^a   Coropceanu, Veaceslav ^b   Brédas, Jean Luc ^b   Hupp, Joseph T ^a  

^a NORTHWESTERN UNIVERSITY   (United States)

^b GEORGIA INSTITUTE OF TECHNOLOGY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 23844432584     PISSN: 00201669     EISSN: None     Source Type: Journal    
DOI: 10.1021/ic050834o     Document Type: Article

References (41)

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14. An energetic and geometric subtlety may exist here. Recall that van der Waals forces (responsible for assembly collapse) are exceptionally short-range forces. If they are counterbalanced by an electrostatic interaction sufficient enough to separate the porphyrin ligands slightly, significant further separation should occur, with minimal additional loss of favorable van der Waals interactions, because of the geometric stabilization (covalent bond stabilization) obtainable by restraightening the two bent ethynyl pyridine moieties. The fully expanded conformation then leads to smaller-than-otherwise- expected ligand/ligand electrostatic repulsions (and charge-state differences in repulsion) for the 2-, 3-, and 4- assemblies.
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33. The non-Liptay first-derivative contribution can appear regardless of whether vibronic coupling is important. Its origin is in the double-welled ground potential energy surface that characterizes symmetrical class II mixed-valence species. In the presence of a tiny applied field, essentially equal numbers of MV molecules will be oriented with dipole moments parallel to the field vs antiparallel to it. In the presence of a large field, one orientation will be significantly energetically stabilized and the other will be destabilized. Consequently, the intervalence band for one will be slightly blue-shifted, and the intervalence band for the other red-shifted. This is the origin of the standard Stark spectral broadening effect that appears as a second-derivative contribution to the electroabsorbance spectrum. A large field, however, can shift a significant portion of the population of one orientation (the energetically destabilized one) to the other orientation (the energetically stabilized one), with the shift being accomplished simply by intramolecular electron transfer. The field, therefore, generates more of the blue-shifted form. The blue shift appears in the difference spectrum (Stark spectrum) as a first-derivative contribution. The effect ought to show up, to a degree, whenever a completely symmetrical double-welled system is examined. In a low dielectric matrix, such as a glass used for Stark measurements, however, the mixed-valence species and its counterion will tend to pair. Ion pairing is well-known to introduce an energetic asymmetry in mixed-valence molecules because the ion preferentially interacts with the more highly charged half of the molecule. In extreme cases, the asymmetry can reach a few hundred millielectronvolts. (See, for example: Blackbourn, R. L.; Dong, Y.; Lyon, L. A.; Hupp, J. T. Inorg. Chem. 1994, 33, 4446-4452.) The non-Liptay blue-shift effect, therefore, will become important only when the external field is sufficient to offset the pairing-induced redox asymmetry. The system examined here may well be such a case since the charge of the mixed-valence assembly is low and the cation and anion sizes are relatively large. On the other hand, the effects of the external field are also low: about 24 meV for the conditions in Figure 6, if the molecule is perfectly aligned and a 3.4 Å charge-transfer distance is assumed. Notably, the spectrum measured at a quarter of this field strength yields the same values (within a few percent, i.e., smaller than the fitting uncertainties) for dipole moment changes and polarizability changes. Nevertheless, the values obtained here for polarizability changes probably should not be taken too seriously.
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