A stronger acceptor decreases the rates of charge transfer: ultrafast dynamics and on/off switching of charge separation in organometallic donor–bridge–acceptor systems

Abstract

The rate of intersystem crossing increases, whilst the rates of charge separation and recombination decrease in donor–CC–Pt–CC–acceptor systems with a stronger electron acceptor – as revealed by fluorescence upconversion and ultrafast IR methods.

Ultrafast Excited-State Nonadiabatic Dynamics in Pt(II) Donor–Bridge–Acceptor Assemblies: A Quantum Approach for Optical Control

“…The experimental spectrum recorded in DCM is shown as an inset for comparison with the calculated spectrum. The main features of the poorly resolved experimental spectrum are well reproduced at this level of broadening, with one intense band between 300 and 400 nm, showing a shoulder at about 325 nm and a slightly red-shifted broad band between 480 and 670 nm. This band is attributed to the charge transfer from the bridge to the acceptor, so-called L B L A CT, that describes the S 0 → S 3 and S 0 → S 4 electronic transitions, the electronic densities of which are represented in Scheme .…”

“…However, quantum simulations beyond the Born−Oppenheimer (BO) approximation considering the interplay and coupling between the active electronic excited states and the nuclear vibrational state manifold are required to model the nonadiabatic dynamics in real time and to decipher the spin-vibronic mechanism underlying the ultrafast competitive excited state decays. 27−31 Inspired by our recent rationalization of the ultrafast excited state decay in [Pt(dpybMe)Cl] (dpyb = 2,6-di(2-pyridyl-(benzene))), reference molecule for square planar Pt(II) cyclometalated complexes, we propose here a thorough exploration of the spin-vibronic mechanism at work in a DBA system recently investigated by Weinstein et al 26 bearing a PTZ donor group associated with a Pt(II) trans-acetylide bridge conjugated with a naphthalene-diimide (NDI) acceptor (Scheme 1). This work based on transient absorption, TRIR, and upconversion fluorescence spectroscopies supported by TD-DFT calculations illustrates the difficulty at rationalizing the behavior, sometimes counterintuitive, in similar complexes with modified acceptor groups A, namely, NAP of NDI.…”

“… a PTZ: phenothiazine; NDI: naphthalene-diimide. Adapted with permission from ref . Copyright 2023 Royal Society of Chemistry. .…”

“…Pioneering experiments reported by Weinstein et al , took advantage of the selectivity of the strong IR donor phenothiazine (PTZ) and acceptor 4-ethynyl-N-octyl-1,8-naphtalimide (NAP) chromophores, incorporated in Pt­(II)-based molecular assemblies, to characterize ultrafast electron transfer (subps) and to put in evidence electronic excited state branching between vibrationally hot CT and CS triplet states. Several similar systems, attractive for their synthetic versatility and adaptable electron transfer directionality, were investigated by the same group to ultimately achieve the IR-pulse bond-specific vibrational control of light-induced charge transfer in these DBA assemblies. , It has been shown that quantum yields are alterated by IR excitation of the Pt­(II) bridge vibrations, while the nature of the D group has a dramatic influence on the kinetics of the process. , Control of the kinetics by the 3 CS state activity, tailored both chemically (A) and spectroscopically (IR), has been recently demonstrated for various Pt­(II)-based DBA systems. − …”

Controlling Excited State Localization in Bichromophoric Photosensitizers via the Bridging Group

“…Many photoactive structures also incorporate a bridging group or groups between the donor and acceptor moieties, which can subsequently affect the excited state properties and electron and or energy transfer processes. ,,− For example, thiophene units are frequently found in polymers as they are well known for their conducting ability, improving π-conjugation and facilitating charge transfer. − Thiophenes have also been successfully incorporated into donor–acceptor dyes and have been shown to red-shift as well as increase the intensity of the charge-transfer transition in both organic and inorganic systems. ,,, Ethynyl bridges have also been shown to increase donor–acceptor communication in a similar manner, providing a means of connecting different moieties in a rigid, planar geometry due to the sp-hybridized carbons. ,− Robson et al studied a family of Ru­(II) terpyridine complexes with triphenylamine (TPA) donors connected through ethynyl and thiophene bridges . They found that the lowest energy state was ILCT in nature and was red-shifted by the inclusion of both ethynyl and thiophene groups.…”