Charge Transfer or J‑Coupling?
Assignment of
an Unexpected Red-Shifted Absorption Band in a Naphthalenediimide-Based
Metal–Organic Framework
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Abstract
We investigate and assign a previously reported unexpected
transition
in the metal–organic framework Zn<sub>2</sub>(NDC)<sub>2</sub>(DPNI) (<b>1</b>; NDC = 2,6-naphthalenedicarboxylate, DPNI
= dipyridyl-naphthalenediimide) that displays linear arrangements
of naphthalenediimide ligands. Given the longitudinal transition dipole
moment of the DPNI ligands, J-coupling seemed possible. Photophysical
measurements revealed a broad, new transition in <b>1</b> between
400 and 500 nm. Comparison of the MOF absorption spectra with that
of a charge transfer (CT) complex formed by manual grinding of DPNI
and H<sub>2</sub>NDC led to the assignment of the new band in <b>1</b> as arising from an interligand CT. Constrained density functional
theory utilizing a custom long-range-corrected hybrid functional was
employed to determine which ligands were involved in the CT transition.
On the basis of relative oscillator strengths, the interligand CT
was assigned as principally arising from π-stacked DPNI/NDC
dimers rather than the alternative orthogonal pairs within the MOF