Electron transfer rate modulation with mid-IR in butadiyne-bridged donor-bridge-acceptor compounds.

Controlling electron transfer (ET) processes in donor-bridge-acceptor (DBA) compounds by mid-IR excitation can enhance our understanding of the ET dynamics and may find practical applications in molecular sensing and molecular-scale electronics. Alkyne moieties are attractive to serve as ET bridges, as they offer the possibility of fast ET and present convenient vibrational modes to perturb the ET dynamics. Yet, these bridges introduce complexity because of the strong torsion angle dependence of the ET rates and transition dipoles among electronic states and a shallow torsion barrier. In this study, we implemented ultrafast 3-pulse laser spectroscopy to investigate how the ET from the dimethyl aniline (D) electron donor to the N-isopropyl-1,8-napthalimide (NAP) electron acceptor can be altered by exciting the CC stretching mode (νCC) of the butadiyne bridge linking the donor and acceptor. The electron transfer was initiated by electronically exciting the acceptor moiety at 400 nm, followed by vibrational excitation of the alkyne, νCC, and detecting the changes in the absorption spectrum in the visible spectral region. The experiments were performed at different delay times t1 and t2, which are the delays between UV-mid-IR and mid-IR-Vis pulses, respectively. Two sets of torsion-angle conformers were identified, one featuring a very fast mean ET time of 0.63 ps (group A) and another featuring a slower mean ET time of 4.3 ps (group B), in the absence of the mid-IR excitation. TD-DFT calculations were performed to determine key torsion angle dependent molecular parameters, including the electronic and vibrational transition dipoles, transition frequencies, and electronic couplings. To describe the 3-pulse data, we developed a kinetic model that includes a locally excited, acceptor-based S2 state, a charge separated S1 state, and their vibrationally excited counterparts, with either excited νCC (denoted as S1Atr, S1Btr, S2Atr, and S2Btr, where tr stands for the excited triplet bond, νCC) or excited daughter modes of the νCC relaxation (S1Ah, S1Bh, S2Ah, and S2Bh, where h stands for vibrationally hot species). The kinetic model was solved analytically, and the species-associated spectra (SAS) were determined numerically using a matrix approach, treating first the experiments with longer t1 delays and then using the already determined SAS for modeling the experiments with shorter t1 delays. Strong vibronic coupling of νCC and of vibrationally hot states makes the analysis complicated. Nevertheless, the SAS were identified and the ET rates of the vibrationally excited species, S2Atr, S2Btr and S2Bh, were determined. The results show that the ET rate for the S2A species is ca. 1.2-fold slower when the νCC mode is excited. The ET rate for species S2B is slower by ca. 1.3-fold if the compound is vibrationally hot and is essentially unchanged when the νCC mode is excited. The SAS determined for the tr and h species resemble the SAS for their respective precursor species in the 2-pulse transient absorption experiments, which validates the procedure used and the results.

Macrocyclic Chromium(III) Catecholate Complexes.

The synthesis and structural, electrochemical, spectroscopic, and magnetic characterizations of CrIII(HMC) catecholate and semiquinonate complexes are reported herein, where HMC is 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. cis-[Cr(HMC)(Cat)]+ complexes (Cat = catecholate, [1]+; tetrachlorocatecholate, [2]+; and 3,5-di-tert-butylcatecholate, [3]+) were prepared from the reaction between appropriate catechol and [CrIII(HMC)Cl2]Cl reduced in situ by zinc. Chemical oxidation of [3]+ by FcPF6 resulted in cis-[Cr(HMC)(SQ)]2+ ([3]2+, SQ = 3,5-di-tert-butylsemiquinonate). Single crystal X-ray diffraction studies revealed the cis-chelation of the Cat/SQ ligand around the Cr metal center and confirmed the Cat/SQ nature of the ligands. Reversible oxidations of Cat to SQ were observed in the cyclic voltammograms of [1]+-[3]+, while the CrIII center remains redox inactive. The absorption spectrum of the SQ complex [3]2+ exhibits an intense spin-forbidden transition in solution. Time-delayed phosphorescence spectra recorded at 77 K revealed that all catecholate complexes emit from the 2E state, while [2]+ also emits from the 2T1 state. Temperature-dependent magnetic susceptibility measurements indicate the Cat complexes exist as S = 3/2 systems, while the SQ complex behaves as an S = 1 system, resulting from strong antiferromagnetic coupling of the S = 3/2 Cr center with the S = 1/2 SQ radical. Density functional theory (DFT) shows the similarities between the SOMOs of [1]+ and [2]+ and differences in their LUMOs in the ground state.

Spectroelectrochemical and Computational Analysis of a Series of Cycloaddition-Retroelectrocyclization-Derived Donor-Acceptor Chromophores.

The [2+2] cyclcoaddition (CA) and subsequent retroelectrocyclization (RE) reactions are useful in constructing nonplanar donor-acceptor chromophores that exhibit nonlinear optical properties and intramolecular charge-transfer transitions. However, both the infrared (IR) and visible-near IR (vis-NIR) spectroelectrochemical responses of CA-RE-derived chromophores are rarely explored in depth. Reported in this contribution is a comprehensive IR and vis-NIR spectroelectrochemical study of the CA-RE adducts of DMAP-C2n-NAPiPr of both tetracyanoethene (TCNE) and tetracyanoquinodimethane (TCNQ) and companion time-dependent density functional theory (TD-DFT) analysis of the bands observed. Specifically, DMAP-C2n-NAPiPr (1a, n = 1; 1b n = 2; DMAP = N,N-dimethylaniline; NAPiPr = N-isopropyl-1,8-naphthalimide) react with TCNE to yield the tetracyanobutadiene (TCBD) derivatives (2a and 2b, respectively) and with TCNQ to yield the dicyanoquinodimethane (DCNQ) derivatives (3a and 3b, respectively). IR spectroelectrochemical studies showed the emergence/intensification of new CN stretches upon reductions. Ultraviolet-vis-NIR (UV-vis-NIR) spectroelectrochemical study of 3 revealed a partial bleach of the charge-transfer (CT) bands, originally appearing in the neutral species, and the emergence of new CT bands originating from NAPiPr to the reduced DCNQ moiety. UV-vis-NIR spectroelectrochemical study of 2, surprisingly, indicated a very minimal change upon reductions. Dynamic changes were observed in the mid-IR absorption for C≡C and C≡N for both 2 and 3, indicative of enhanced asymmetry and the formation of ion pairs on the dicyano bridge. DFT and TD-DFT analyses were used to obtain the semi-quantitative pictures of the frontier orbitals of 1-3 and elucidate the origin of the transient features observed spectroelectrochemically for the 1e- and 2e- reduced species.

Symmetry controlled photo-selection and charge separation in butadiyne-bridged donor-bridge-acceptor compounds.

Electron transfer (ET) in donor-bridge-acceptor (DBA) compounds depends strongly on the structural and electronic properties of the bridge. Among the bridges that support donor-acceptor conjugation, alkyne bridges have attractive and unique properties: they are compact, possess linear structure permitting access to high symmetry DBA molecules, and allow torsional motion of D and A, especially for longer bridges. We report conformation dependent electron transfer dynamics in a set of novel DBA compounds featuring butadiyne (C4) bridge, N-isopropyl-1,8-napthalimide (NAP) acceptors, and donors that span a range of reduction potentials (trimethyl silane (Si-C4-NAP), phenyl (Ph-C4-NAP), and dimethyl aniline (D-C4-NAP)). Transient mid-IR absorption spectra of the C[triple bond, length as m-dash]C bridge stretching modes, transient spectra in the visible range, and TD-DFT calculations were used to decipher the ET mechanisms. We found that the electronic excited state energies and, especially, the transition dipoles (S0 → Sn) depend strongly on the dihedral angle (θ) between D and A and the frontier orbital symmetry, offering an opportunity to photo-select particular excited states with specific ranges of dihedral angles by exciting at chosen wavelengths. For example, excitation of D-C4-NAP at 400 nm predominantly prepares an S1 excited state in the planar conformations (θ ∼ 0) but selects an S2 state with θ ∼ 90°, indicating the dominant role of the molecular symmetry in the photophysics. Moreover, the symmetry of the frontier orbitals of such DBA compounds not only defines the photo-selection outcome, but also determines the rate of the S2 → S1 charge separation reaction. Unprecedented variation of the S2-S1 electronic coupling with θ by over four orders of magnitude results in slow ET at θ ca. 0° and 90° but extremely fast ET at θ of 20-60°. The unique features of high-symmetry alkyne bridged DBA structures enable frequency dependent ET rate selection and make this family of compounds promising targets for the vibrational excitation control of ET kinetics.

Unsymmetrical Bis-Alkynyl Complexes Based on Co(III)(cyclam): Synthesis, Ultrafast Charge Separation, and Analysis.

Donor-bridge-acceptor (D-B-A) systems with a polarizable bridge can afford rapid photoinduced electron transfer dynamics that may be susceptible to rate modulation by infrared excitation. We describe the synthesis, characterization, and electronic structure of a class of readily assembled D-B-A structures linked by a cobalt cyclam bridge. The reaction between [Co(cyclam)Cl2]Cl and 4-ethynyl-N-isopropyl-1,8-naphthalimide (HC2NAPiPr) yields [Co(cyclam)(C2NAPiPr)Cl]Cl (1), which reacts with LiC2Y at -78 °C to afford [Co(cyclam)(C2NAPiPr)(C2D)]Cl with D as C6H4-4-NMe2 (2a), NAPiPr (2b), Ph (2c), and C6H4-4-N(4-MeOPh)2 (2d). Molecular structures of 1 and 2a were established using single-crystal X-ray diffraction, while the redox properties and fluorescence profiles of compounds 1 and 2 were examined using voltammetric and steady-state emission techniques, respectively. The electronic structures and photophysical properties of these compounds were studied using density functional theory and time-dependent density functional theory methods. The excited-state dynamics of compounds 1, 2a, and 2d were explored using femtosecond transient absorption spectroscopy with 400 nm excitation and detection in both the visible and mid-IR spectral regions. Formation of a long-lived excited state was complete within 20 ps of excitation in all three compounds. Ultrafast spectral changes observed in 2a and 2d within the first 20 ps indicated the formation of a charge separated state (CS state, D+-B-A-) with characteristic times of less than 0.1 and 0.25 ps, respectively. The CS state undergoes rapid charge recombination (8 ps in 2a and 4 ps in 2d). The CS dynamics is facilitated by the Co center, which mixes the bright NAP-centered electronic state with a pure CS state. The mixing strength depends on the donor energetics and conformation, which significantly influences the charge transfer dynamics in 2a and 2d.