Control of Hydrogen Bond Strengths through Push–Pull Effects Triggered by a Remote Reaction Center: A Theoretical Study

Abstract

In an effort to manipulate the bond strengths of hydrogen bonds, we have studied a three-component chemical system consisting of a reaction center, a conjugated bridge, and a hydrogen-bonding site. Protonation of the reaction center triggers intramolecular charge transfer from the hydrogen-bonding site, altering its affinity to bind to an acceptor. Previously, we had found that this communication (signal transduction) between the reaction center and the hydrogen-bonding site does not necessarily die out with increasing length of the conjugated bridge. In certain cases, this signal transduction is maintained-and even amplified-over long distances (I. Chao, T.-S. Hwang, Angew. Chem. 2001, 113, 2775-2777; Angew. Chem. Int. Ed. 2001, 40, 2703-2705). In this study we report the results of an extensive theoretical investigation of this problem to provide insights into this intriguing phenomenon. In the systems we investigated it was found that the push-pull process between the hydrogen-bonding site and the protonatable reaction center was mediated with the greatest facility by conjugated bridges with low-lying pi and pi* orbitals.

Theoretical Study of the Remote Control of Hydrogen Bond Strengths in Donor–Bridge–Acceptor Systems: Principles for Designing Effective Bridges with Substituent Tuning

“…To focus on the extents of ICT in different protonated π‐conjugated systems, the relevant orbital energies of fully substituted (CRCR) n bridges are also presented in Figure 4. If low HOMO and LUMO energies are preferred for ICT, as we suggested previously,6b the predicted order of ICT is (C(CN)C(CN)) 2 m >(CHCHC(CN)C(CN)) m >(CHC(CN)) 2 m >(CFCF) 2 m >(CHCHCFCF) m >(CHCF) 2 m . This order coincides with the trend of Δ E b(P) for a given m .…”

“…Protonation is used in this study to control the binding strength of designed D–B–A (pyrrole–bridge–imine) conjugated systems. The design strategy for the bridge (low bridge HOMO/LUMO) mentioned in a previous study6b was used to construct cyano and fluoro vinyl‐based systems. When the substitution positions are properly chosen, the cyano group, which can result in a low bridge HOMO/LUMO, is a better substituent than the fluoro group with respect to facilitating the long‐range protonation‐induced ICT as proposed.…”

“…The (NN) n bridges had low‐lying π‐HOMO orbitals, so the LUMO of iminium interacted primarily with the bridge π‐LUMO, and the resultant two‐component LUMO energy decreased as the bridge lengthened. With this simple model, it became clear that the low‐lying π‐HOMO and π‐LUMO of the azo bridges were beneficial for remote protonation‐induced ICT 6b. As it is difficult to synthesize purely nitrogen‐based π‐conjugated bridges, in this article, we explore the possibility of achieving signal‐maintaining effects with substituted carbon‐based bridges.…”

“…The influence of substituent effects on hydrogen‐bonded complexes is also well documented 14–15. Previously, we have tested the notion that bridges with low‐lying π‐HOMO and π‐LUMO could be beneficial for charge transfer with three‐component systems bearing (CHCH) n and (CFCF) n bridges 6b. The cyano group is more electron‐withdrawing than the fluoro group.…”

The switching of a Rhenium(i) complex from turn-off to turn-on sensor system through protein binding

“…The positive charge on the Re(CO) 3 (N-N) moiety also provides an additional electrostatic force on the amide binding site. 41 The addition of different inorganic anions (as tetrabutylammonium salts) to the complexes 1a and 1b is found to cause quenching of the luminescence intensity but the extent of quenching depends on the nature of the anion. Fig.…”