Excited state distortions in a charge transfer state of a donor–acceptor [2]rotaxane

Abstract

The charge transfer excited state of a mechanically interlocked [2]rotaxane (R(4+)) with a donor 1,5-dioxynaphthalene (DNP) unit in the rod and the acceptor cyclobis(paraquat-p-phenylene) (CBPQT(4+)) ring component, along with the analogous non-interlocked [2]pseudorotaxane (P(4+)), is studied by resonance Raman spectroscopy and electronic absorption spectroscopy. Resonance Raman excitation profiles are obtained, calculated quantitatively using time-dependent theoretical methods, and interpreted with the assistance of DFT calculations. The active vibrational modes are consistent with an electron transfer from the HOMO centered on the DNP unit to the LUMO on the CBPQT(4+) ring. Displacement vectors of highly distorted modes agree with the bonding changes predicted from the MO nodal pattern. Subtle changes in the frequency of some modes in the free components compared with those in R(4+) are observed. The largest distortions are found for modes involving ring breathing in the DNP unit of the rod and the paraquat units of the CBPQT(4+) ring. The individual mode contributions to the vibrational reorganization energy, as well as the total vibrational reorganization energy, are calculated. Very similar values of λ(v) for R(4+) and P(4+) are calculated (∼2910 cm(-1)), indicating that the mechanical stoppers in the interlocked system do not significantly affect the excited state properties of R(4+) compared with P(4+).

Ultrafast Conformational Dynamics of Electron Transfer in ExBox4+⊂Perylene

“…Given the calculated structural differences and the evidence from the fsTA and FSRS spectra, we assert that the frequency shifts arise from vibrationally mediated conformational relaxation of the p -phenylene ring. Similar distortions along the extended bipyridinium unit backbone were observed in the ∼1640 cm –1 pyridinium ν quad stretch CBPQT 4+ charge-transfer complex already discussed, suggesting some reorganization along the long axis of the cyclophane upon CT there as well. In that shorter system, however, there is no central p -phenylene on the long axis, so the nature of the reorganization is different from that considered here.…”

“…Understanding what happens to these systems upon sequential photodriven charge-transfer events constitutes an important fundamental step toward reversibly storing multiple charges. One early investigation involving CBPQT 4+ has already addressed photoinduced charge transfer (CT) from 1,5-bis[2-(2-(2-hydroxyethoxy)ethoxy)ethoxy]naphthalene ( BHEEEN ). Upon photoexcitation of CBPQT 4+ ⊂ BHEEEN , CT is observed from the HOMO of the guest to the LUMO of the CBPQT 4+ host, as inferred from the resonance Raman spectrum.…”

Cucurbit[8]uril Mediated Donor–Acceptor Ternary Complexes: A Model System for Studying Charge-Transfer Interactions

“…Consequently, only a few donor–acceptor pairs proved suitable for a straightforward mix-and-match approach . With the use of rational design principles, donor–acceptor pairs have been covalently or topologically tethered together and thus aforementioned complications were greatly reduced. − However, arriving at such molecules is both time-consuming and suffers from synthetic challenges when the investigation of multiple donor–acceptor pair interactions is desired and therefore often limits the scope to specific example cases. Moreover, the energetic contribution of CT to the Gibbs free binding energy cannot be obtained through a tethering strategy as the donor and acceptor have been irreversibly linked to each other.…”

Structural Reorganizations Control Intermolecular Conductance and Charge Trapping in Paraquat‐Tetraphenylborate Inverse Photochemical Cell

“…To address these questions, we perform detailed experimental and theoretical (quantum mechanical) analysis of the alteration of structural and electronic properties of PQ and BPh 4 upon the formation of a complex between them. PQ is a viologen derivative with an easily ionized electronic structure allowing for the utilization of this molecule in optoelectronic switches (1), electrochromic molecular probes (4), charge‐separating moieties of photovoltaic (PV) devices (3) and molecular memory units (5). The second component of the complex, BPh 4 , is a rigid core‐shell molecule, consisting of an easily ionizing atom, surrounded by a rigid shell with delocalized electronic structure.…”