Donor Bridge Research Articles

Nonlinear optical chromophores based on Dewar's rules: enhancement of electro-optic activity by introducing heteroatoms into the donor or bridge.

In this work, we investigated the enhancement of the electro-optic response by introducing electron-rich heteroatoms as additional donors into the donor or bridge of a conventional second-order nonlinear optical chromophore. A series of chromophores C2-C4 based on the same tricyanofuran acceptor (TCF) but with different heteroatoms in the alkylamino phenyl donor (C2 or C3) or thiophene bridge (C4) have been synthesized and systematically investigated. Density functional theory calculations suggested that chromophores C2-C4 had a smaller energy gap and larger first-order hyperpolarizability (β) than traditional chromophore C1 due to the additional heteroatoms. Single crystal structure analyses and optimized configurations indicate that the rationally introduced heteroatom group would bring larger β and weaker intermolecular interactions which were beneficial for translating molecular β into macro-electro-optic activity in electric field poled films. The electro-optic coefficient of poled films containing 25 wt% of these new chromophores doped in amorphous poly-carbonate afforded values of 83 and 91 pm V(-1) at 1310 nm for chromophores C3 and C4, respectively, which are two times higher than that of the traditional chromophore C1 (39 pm V(-1)). High r33 values indicated that introducing heteroatoms to the donor and bridge of a conventional molecular structure can efficiently improve the electron-donating ability, which improves the β. The long-chain on the donor or bridge part, acting as the isolation group, may reduce inter-molecular electrostatic interactions, thus enhancing the macroscopic EO activity. These results, together with good solubility and compatibility with the polymer, show the new chromophore's potential application in electro-optic devices.

Molecular wires--impact of π-conjugation and implementation of molecular bottlenecks.

In this review we highlight recent progress in the field of photochemically and thermally induced electron transport through molecular bridges as integrative parts of electron donor-bridge-acceptor conjugates. The major emphasis is hereby on the design and the modular composition of the bridges. To this end, we will demonstrate that control over attenuation factors and reorganization energies, on one hand, as well as electronic and electron-vibration couplings, on the other hand, enables tuning electron transport over distances as short as 3.5 Å and as large as 50 Å by up to nine orders of magnitude. In terms of electron transport, the maximum extreme is given by carbon-bridged oligo-p-phenylenevinylenes of different lengths, while a zinc tetraphenylporphyrin free base tetraphenylporphyrin dyad constitutes the minimum extreme.

Watching the dynamics of electrons and atoms at work in solar energy conversion.

The photochemical reactions performed by transition metal complexes have been proposed as viable routes towards solar energy conversion and storage into other forms that can be conveniently used in our everyday applications. In order to develop efficient materials, it is necessary to identify, characterize and optimize the elementary steps of the entire process on the atomic scale. To this end, we have studied the photoinduced electronic and structural dynamics in two heterobimetallic ruthenium-cobalt dyads, which belong to the large family of donor-bridge-acceptor systems. Using a combination of ultrafast optical and X-ray absorption spectroscopies, we can clock the light-driven electron transfer processes with element and spin sensitivity. In addition, the changes in local structure around the two metal centers are monitored. These experiments show that the nature of the connecting bridge is decisive for controlling the forward and the backward electron transfer rates, a result supported by quantum chemistry calculations. More generally, this work illustrates how ultrafast optical and X-ray techniques can disentangle the influence of spin, electronic and nuclear factors on the intramolecular electron transfer process. Finally, some implications for further improving the design of bridged sensitizer-catalysts utilizing the presented methodology are outlined.

Ultrafast charge transfer dynamics in supramolecular Pt(II) donor-bridge-acceptor assemblies: the effect of vibronic coupling.

Thanks to major advances in laser technologies, recent investigations of the ultrafast coupling of nuclear and electronic degrees of freedom (vibronic coupling) have revealed that such coupling plays a crucial role in a wide range of photoinduced reactions in condensed phase supramolecular systems. This paper investigates several new donor-bridge-acceptor charge-transfer molecular assemblies built on a trans-Pt(II) acetylide core. We also investigate how targeted vibrational excitation with low-energy IR light post electronic excitation can perturb vibronic coupling and affect the efficiency of electron transfer (ET) in solution phase. We compare and contrast properties of a range of donor-bridge-acceptor Pt(II) trans-acetylide assemblies, where IR excitation of bridge vibrations during UV-initiated charge separation in some cases alters the yields of light-induced product states. We show that branching to multiple product states from a transition state with appropriate energetics is the most rigid condition for the type of vibronic control we demonstrate in our study.

Photoinduced mixed valency in zinc porphyrin dimer of triruthenium cluster dyads.

The preparation, electrochemistry, and spectroscopic characterization of three new species, (ZnTPPpy)Ru3O(OAc)6(CO)-pz-Ru3O(OAc)6(CO)L, where ZnTPPpy = zinc(II) 5-(4-pyridyl)-10,15,20-triphenylporphyin, L = pyridyl ligand, and pz = pyrazine, are reported. These porphyrin-coordinated Ru3O–BL–Ru3O (BL = bridging ligand) dyads are capable of undergoing intramolecular electron transfer from the photoexcited Zn porphyrin to Ru3O donor–bridge–acceptor dimer systems. Seven reversible redox processes are observed in the cyclic voltammograms of the newly synthesized dyads, showing no significant electrochemical interaction between the redox active porphyrin and the pyrazine-bridged ruthenium dimer of Ru3O trimers. From the electrochemical behavior of the dyads, large comproportionation constants (Kc = 6.0 × 10(7) for L = dmap) were calculated from the reduction potentials of the Ru(III)Ru(III)Ru(II) clusters, indicating a stable mixed-valence state. Electronic absorption spectra of the singly reduced mixed-valence species show two intervalence charge transfer (IVCT) bands assigned within the Brunschwig–Creutz–Sutin semiclassical three-state model as metal-to-bridge and metal-to-metal in character. The progression from most to least delocalized mixed-valence dimer ions, as determined by the divergence of the IVCT bands and in agreement with electrochemical data, follows the order of L = 4-dimethylaminopyridine (dmap) > pyridine (py) > 4-cyanopyridine (cpy). These systems show dynamic coalescence of the infrared spectra in the ν(CO) region of the singly reduced state. This sets the time scale of electron exchange at <10 ps. The electron transfer from the S1 excited state of the coordinated porphyrin to the dimer is predicted to be thermodynamically favorable, with ΔGFET(0) ranging from −0.54 eV for L = dmap to −0.62 eV for L = cpy. Observation of IVCT band growth under continual photolysis (λexc = 568 nm) confirms a phototriggered intramolecular electron transfer process resulting in a strongly coupled singly reduced mixed-valence species.

Dimeric nickel(II) and copper(II) complexes of the pentadentate N3S2 chelating agents derived from S-alkyl/aryl esters of dithiocarbazic acid

The helical dimeric copper(II) complex, [Cu(dapsbz)]2 (dapsbz=doubly deprotonated form of the 2,6-diacetylpyridine Schiff base of S-benzyldithiocarbazate) has been synthesized and structurally characterized. Each Schiff base in the dimer acts as a doubly negatively charged N3S2 pentadentate chelating agent, providing three donor atoms, viz. the pyridine nitrogen atom, the azomethine nitrogen atom and the thiolate sulfur atom, to one copper ion and the azomethine nitrogen atom and thiolate sulfur atom to the second copper(II) ion. The stereochemistry adopted by each copper(II) ion in the dimer is five-coordinate, approximately square-pyramidal, with a CuN3S2 coordination core. In the dimeric nickel(II) complex, [Ni(dapsme)]2 (dapsme=doubly deprotonated form of the 2,6-diacetylpyridine Schiff base of S-methyldithiocarbazate), the Schiff base also coordinates with the two nickel(II) ions as pentadentate N3S2 chelating agent, but here the pyridine nitrogen atom of each ligand acts as a bridging donor atom. The azomethine nitrogen and thiolate sulfur atoms of one arm of a ligand coordinate with one nickel ion whereas the azomethine nitrogen and thiolate sulfur atoms of the other arm of the same ligand coordinate with a second nickel atom, the stereochemistry adopted by each nickel atom in the dimer being distorted octahedral with a NiN4S2 coordination sphere.

Dependence of reaction rates for bidirectional PCET on the electron donor-electron acceptor distance in phenol-Ru(2,2'-bipyridine)₃²⁺ dyads.

A homologous series of three donor-bridge-acceptor molecules in which a phenolic unit is attached covalently to a Ru(bpy)3(2+) (bpy =2,2'-bipyridine) complex via rigid rod-like p-xylene spacers was investigated. Photoexcitation at 532 nm in the presence of a large excess of methyl viologen leads to rapid (<10 ns) formation of Ru(bpy)3(3+). When imidazole base is present in CH3CN solution, intramolecular electron transfer from the phenol to Ru(bpy)3(3+) occurs, and this is coupled to proton transfer from the phenol to imidazole. All mechanistic possibilities for this proton-coupled electron transfer (PCET) process are considered, and based on a combination of kinetic and thermodynamic data, one arrives at the conclusion that electron and proton release by the phenol occur in concert. By varying the number of p-xylene bridging units, it then becomes possible to investigate the dependence of the reaction rates for concerted proton-electron transfer (CPET) on the phenol-Ru(bpy)3(3+) distance. A distance decay constant of 0.87 ± 0.09 Å(-1) is obtained. This is one of the largest β values reported for electron transfer across oligo-p-phenylene-based molecular bridges, but it is still relatively close to what was determined for "simple" (i. e., not proton-coupled) electron transfer across oligo-p-xylenes. Bidirectional CPET plays a key role in photosystem II. Understanding the distance dependence of such reactions is of interest, for example, in the context of separating protons and electrons across artificial membranes in order to build up charge gradients for light-to-chemical energy conversion.

The Canadian Living Donor Exchange Programme: Success at Four Years.

The Canadian LDE Program, established in 2009 by Canadian Blood Services and the Canadian Renal Transplant community, is fully funded by government (gov’t). In general, donors (D) have travelled to recipient (R) sites for surgery with coverage of travel and accommodation costs. Match points have been allocated to maximize transplant (Tx) number,minimize travel, equity for high need patient groups, and optimal matches. HLA Laboratories standardized antibody testing such that the agreement between virtual and flow cross match testing was 98.6%. Standard criteria defined an unacceptable HLA antigen. All Tx were virtual and flow cross match negative. Match runs are performed every 4 months with optimized, computer based allocation. The maximum number of pairs was arbitrarily set at 5. There were no bridge donors by design. From Jan 2009 to Nov 2013, 468 pairs with 439 Rs and 56 non-directed Ds have registered. 240 Tx have been performed, 194 to registered Rs and 46 to wait listed Rs. 10 were from paired exchange,16 from closed chains, and 47 from domino exchanges. 28 matches are under review. 39% of Tx Rs were A, 36% O, 19% B, and 6% AB. 65% of Tx Rs had cPRA of 0-79% (51% of registered Rs), 19% had cPRA 80-94% (12% of registered Rs), and 16% had cPRA ≥95% (37% of registered Rs). 58% of proposed matches and 62% of proposed chains were completed. Tx were performed on the same day, or over 2, 3, or 4 days in 49%, 42%, 6%, or 3% respectively. The mean time from match proposal to completion of all Tx in the chain was 101 days for PE, 129 days for CC and 117 days for DC. The mean time from D registration after medical clearance to nephrectomy was 252 days for Ds registered as pairs and 134 days for non directed Ds. Donor travel was required in 54% of cases. 7 kidneys were shipped. 1 year D and R survival is 100%, graft survival 97% and mean serum creatinine 1.2 mg%.The Canadian LDE accounted for 10% of living donor renal Tx in 2012 and has been responsible for maintaining a stable number of living donor Tx in Canada as other living donor renal Tx has fallen. A successful National LDE Program is feasible with national collaboration, standardized antibody testing, workup and acceptance criteria for donors, centralized allocation software, operational oversight, dedicated central support staff, and gov’t funding of logistics for organization and travel.

Intramolecular Charge- and Energy-Transfer Rates with Reduced Modes: Comparison to Marcus Theory for Donor–Bridge–Acceptor Systems

We present a new, fully ab initio approach for computing intramolecular charge- and energy-transfer rates. Using a time-convolutionless master equation approach and parametrizing using couplings obtained using an accurate quantum chemical approach, we benchmark the approach against experimental results and Marcus theory rates for triplet energy transfer for a series of donor-bridge-acceptor systems. An important component of our analysis is the use of a projection operator scheme that parses out specific internal nuclear motions that accompany the electronic transition. Using an iterative Lanczos approach, we concentrate the coupling between the electronic and nuclear degrees of freedom into a small number of reduced harmonic modes. We find that by using only a single reduced mode termed the "primary mode", one obtains an accurate evaluation of the golden rule rate constant and insight into the nuclear motions responsible for coupling the initial and final electronic states. In particular, the primary mode reflects the irreducible representation of the donor and acceptor excited states.

Absorption Spectra and Electron Injection Study of the Donor Bridge Acceptor Sensitizers by Long Range Corrected Functional

Ground state geometries have been computed using Density Functional Theory (DFT) at B3LYP/6-31G(d,p) level of theory. The excitation energies and spectroscopic parameters have been computed using Long range Corrected (LC) hybrid functional by Time Dependent Density Functional Theory (TDDFT) with LC-BLYP level of theory. The Polarizable Continuum Model (PCM) has been used for evaluating bulk solvent effects at all stages. The efficient materials have been predicted and electron injection (ΔGinject), electron coupling constant ( |VRP| ) and Light Harvesting Efficiency (LHE) has been discussed. By elongating the bridge all these three parameters ΔGinject, |VRP| and LHE enhanced which revealed that new designed sensitizers would be efficient.

Correction to “Superexchange Contributions to Distance Dependence of Electron Transfer/Transport: Exchange and Electronic Coupling in Oligo(para-Phenylene)- and Oligo(2,5-Thiophene)-Bridged Donor–Bridge–Acceptor Biradical Complexes”

Correction to “Superexchange Contributions to Distance Dependence of Electron Transfer/Transport: Exchange and Electronic Coupling in Oligo(<i>para-</i>Phenylene)- and Oligo(2,5-Thiophene)-Bridged Donor–Bridge–Acceptor Biradical Complexes”

Binuclear tungsten carbonyl nitrosyls: Comparison with valence isoelectronic Cr2(NO)2(CO)n

Density functional theory predicts an unbridged structure for W2(NO)2(CO)8 with a W–W single bond distance of ∼3.2Å analogous to the experimental structures for the isoelectronic species Cr2(NO)2(CO)8 and M2(CO)10 (M=Mn, Tc, Re). The lowest energy structure of the unsaturated W2(NO)2(CO)7 is an unbridged structure linking a W(NO)(CO)4 fragment and a W(NO)(CO)3 fragment through a formal WW double bond similar to Cr2(NO)2(CO)7. For W2(NO)2(CO)6, the lowest energy structure has two bridging CO groups and a WW distance of ∼2.7Å suggesting a formal triple bond. However, an unbridged W2(NO)2(CO)6 structure with a WW triple bond distance of ∼2.8 Å lies within ∼2kcal/mol of this structure suggesting a fluxional system. The energy surface of the even more unsaturated W2(NO)2(CO)5 is more complicated with low energy singlet structures having four-electron donor bridging η2-μ-CO groups, five-electron donor bridging η2-μ-NO groups, or no bridging groups combined with a short WW triple bond distance. The lowest energy structure for the carbonyl-rich W2(NO)2(CO)9 has a bridging NO group and a long ∼4.0 ÅW⋯W distance suggesting the absence of a direct bond. However, loss of CO from W2(NO)2(CO)9 to give W2(NO)2(CO)8 requires only ∼4kcal/mol suggesting that W2(NO)2(CO)9 is not a viable species.

Cross‐Conjugation vs. Linear Conjugation in Donor–Bridge–Acceptor Nitrophenol Chromophores

AbstractThe influence of cross‐conjugation vs. linear conjugation on the electronic communication between donor and acceptor groups in phenol(ate)–bridge–nitrobenzene chromophores was investigated by solution and gas‐phase absorption spectroscopy, fluorescence spectroscopy, and quantum chemical calculations. The compounds studied include, among others, geminally and trans‐substituted diethynylethenes prepared by stepwise Sonogashira cross‐coupling reactions, and the cross‐conjugated analogue of stilbene. A butadiyne‐bridged donor–acceptor chromophore was prepared by an unsymmetrical Pd‐catalysed coupling between a chloroalkyne and a terminal alkyne. While the linearly conjugated chromophores showed a strong and redshifted charge‐transfer (CT) absorption maximum upon deprotonation in solution, a new redshifted absorption was either absent or present as a weak shoulder for the cross‐conjugated derivatives. Calculations on the nonplanar 1,1‐diaryl‐substituted ethylene derivative revealed that the S0–S1 absorption has a very low oscillator strength due to insignificant coupling between the donor and acceptor ends. The energy of this CT absorption for a nonsolvated molecule is smaller than that of the linearly conjugated stilbene analogue by 0.4 eV. This result cannot directly be inferred from the solution studies. A gas‐phase absorption spectrum of this chromophore was measured by action spectroscopy, which only revealed a higher‐energy absorption band. Based on calculations, this band was assigned to a higher‐lying π–π* transition. For the planar diethynylethene chromophores, cross‐conjugation leads to a decrease of 0.3 eV in the excitation energy, according to the calculations. Finally, in contrast to the linearly conjugated chromophores, the cross‐conjugated ones did not show any fluorescence, which may be due to deexcitation via the low‐lying CT “dark” state.

On the effect of the nature of the bridge on oxidative or reductive photoinduced electron transfer in donor–bridge–acceptor systems

Photoinduced electron transfer is a topical issue in chemistry. In multicomponent donor-bridge-acceptor systems, electron transfer is usually discussed within the frame of superexchange theory, which takes into account electronic coupling mediated by virtual states involving bridge orbitals. However, the schematization used for superexchange in thermal electron transfer processes is not suitable to immediately understand some intriguing aspects of photoinduced charge separation and recombination processes, which are only uncovered by analyzing the virtual states involved in forward and backward excited-state electron transfer. In particular, for oxidative photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward charge separation can efficiently mediate charge recombination via the hole-transfer superexchange route, whereas for reductive photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward process can efficiently mediate charge recombination via electron-transfer superexchange. As a consequence, to obtain long-lived charge-separated states upon oxidative photoinduced electron transfer in donor-bridge-acceptor systems it is preferable to avoid easy-to-oxidize bridges, whereas easy-to-reduce bridges should better be avoided in reductive photoinduced charge separation. These considerations, exemplified by the analysis of some literature cases, can be useful hints for the design of long-lived charge-separated states.

Construction of iridium and rhodium cyclometalated macrocycles based on p-carborane and N,N'-donor bridging ligands.

Six tetranuclear half-sandwich iridium and rhodium complexes bridged by neutral N,N'-donor pyridyl-imine ligands and 1,12-dicarbadodecaborane(12)-1,12-dicarboxylate (p-CDC) were controllably synthesized and fully characterized. The results revealed that the precursors, binuclear complexes [(Cp*M)2{1,4-bis(2-pyridylmethyleneamino)benzene}Cl2][OTf]2 (Cp* = η(5)-pentamethylcyclopentadienyl, M = Ir (2a), Rh (2b)), have different preferential configurations, and tetranuclear complexes [(Cp*M)4{1,4-bis(2-pyridylmethyleneamino)benzene}2(p-CDC)2][OTf]4 (M = Ir (3a), Rh (3b)), which were prepared using monodentate p-carborane dicarboxylate and 2a or 2b, showed highly twisted backbones. Likewise, homologous regular rectangular compounds [(Cp*M)4{1,4-{(2-C5H4N)HC=N}2-2,3,5,6-Me4C6}2(p-CDC)2][OTf]4 (M = Ir (4a), Rh (4b)) and [(Cp*M)4{1,4-{(2-C5H4N)HC=N}2-1,5-naphthalene}2(p-CDC)2][OTf]4 (M = Ir (5a), Rh (5b)) were obtained following a similar synthetic route. X-ray determination confirmed that 4b and 5b have stacking channels as well.

Electronic tuning effects via π-linkers in tetrathiafulvalene-based dyes

Four new tetrathiafulvalene (TTF)-based dyes featured with a donor–bridge–acceptor (D–π–A) structure were synthesized and characterized. All of them undergo two reversible oxidations to form stable radical cation and dication species. The electronic interactions between the TTF donor and the cyanoacrylic acid acceptor through the different π-linkers have been demonstrated by the presence of a photo-induced intramolecular charge-transfer (ICT) absorption band in the visible region. A red shift of the ICT state can be finely tuned by the degree of aromaticity and extended conjugation of π-bridges. To some extent, the oxidation potentials of these dyes are affected by the nature of π-bridges. They have been applied in organic dye-sensitized solar cells, showing relatively low power conversion efficiencies of up to 0.87% due to substantial charge recombination losses.

Multistate Treatments of the Electronic Coupling in Donor–Bridge–Acceptor Systems: Insights and Caveats from a Simple Model

We use a simple one-dimensional delta function electronic structure model (dfm) to investigate the results of a pair of multistate diabatization techniques (i.e., based on n states, with n ≥ 2) for linear DBA and DBBA (donor-bridge-acceptor) electron-transfer systems. In particular, we focus on the physical meaning of the couplings obtained from multistate methods and their relationship to two-state (n = 2) coupling elements. On the basis of the simple dfm approach, which allows exact as well as finite basis set treatment and has no many-electron effects, we conclude that for orthogonal diabatic states, it is difficult to assign clear physical significance to multistate matrix elements for coupling beyond nearest-neighbor contacts. The implications of these results for more complex multistate many-electron treatments are discussed. It is emphasized that physically meaningful coupling elements must involve states that are orthogonal, either explicitly or implicitly.

Coordinating Unspecified Living Kidney Donation and Transplantation Across the Blood-Type Barrier in Kidney Exchange

This article studies multicenter coordination of unspecified living kidney donation and transplantation across the blood-type barrier in kidney exchange. Important questions are whether such coordination should use domino paired donation or non simultaneous extended altruistic donor chains, what the length of the segments in such chains should be, when they should be terminated, and how much time should be allowed between matching rounds. Furthermore, it is controversial whether the different modalities should be coordinated centrally or locally and independently. Kidney exchange policies are simulated using actual data from the Dutch national kidney exchange program. Sensitivity analysis is performed on the composition of the population, the time unspecified and bridge donors wait before donating to the wait list, the time between matching rounds, and donor renege rates. Central coordination of unspecified donation and transplantation across the blood-type barrier can increase transplants by 10% (PG0.001). Especially highly sensitized and blood type O patients benefit. Sufficient time between matching rounds is essential: three-monthly exchanges result in 31% more transplants than weekly exchanges. Benefits of non simultaneous extended altruistic donor chains are limited in case of low numbers of highly sensitized patients and sufficient unspecified donors. Chains are best terminated when no further segment is part of an optimal exchange within 3 months. There is clear synergy in the central coordination of both unspecified donation and transplantation across the blood-type barrier in kidney exchange. The best configuration of a national program depends on the composition of the patient Y donor population.

Binuclear cobalt carbonyl complexes of the strong π-acceptor trifluoromethyl isocyanide

The cobalt carbonyl complexes (CF3NC)2Co2(CO)n (n=7, 6, 5, 4) and (CF3NC)Co(CO)n(n=4, 3, 2) of the strongly π-accepting trifluoromethyl isocyanide ligand have been investigated by density functional theory. The lowest energy (CF3NC)2Co2(CO)6 structures are doubly bridged structures with the global minimum having two bridging CF3NC ligands. The lowest energy unbridged (CF3NC)2Co2(CO)6 structure lies ∼19kcal/mol in energy above this global minimum. This differs from unsubstituted Co2(CO)8 for which the doubly bridged and unbridged isomers have similar energies so both can be observed experimentally. The lowest energy structures of the formally unsaturated (CF3NC)2Co2(CO)n (n=5, 4) derivatives are also doubly bridged structures. Bridging CF3NC groups are energetically preferred over bridging CO groups. Four-electron donor bridging η2-μ-CF3NC groups are found in higher energy structures. Coupling of CF3NC groups to form a coordinated bis(trifluoromethyldiimine) ligand, CF3NCCNCF3 is observed in carbonyl-rich (CF3NC)2Co2(CO)7 structures. However, such (CF3NC)2Co2(CO)7 structures do not appear to be viable since CO dissociation from the lowest energy such structure to give (CF3NC)2Co2(CO)6 is predicted to be essentially thermoneutral.

Superexchange Contributions to Distance Dependence of Electron Transfer/Transport: Exchange and Electronic Coupling in Oligo(para-Phenylene)- and Oligo(2,5-Thiophene)-Bridged Donor–Bridge–Acceptor Biradical Complexes

The preparation and characterization of three new donor-bridge-acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo(para-phenylene), β = 0.39 Å(-1) and oligo(2,5-thiophene), β = 0.22 Å(-1). This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (β) for these bridges. The work provides a direct measurement of superexchange contributions to β, with no contribution from incoherent hopping. The different β values determined for oligo(para-phenylene) and oligo(2,5-thiophene) are due primarily to the D-B energy gap, Δ, rather than bridge-bridge electronic couplings, H(BB). This is supported by the fact that the H(BB) values extracted from the experimental data for oligo(para-phenylene) (H(BB) = 11,400 cm(-1)) and oligo(2,5-thiophene) (12,300 cm(-1)) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern H(DA) and β, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges.