Triplet Charge-separated State Research Articles

Probing Charge Management across the π-Systems of Nanographenes in Regioisomeric Electron Donor-Acceptor Architectures.

Inspired by light-induced processes in nature to mimic the primary events in the photosynthetic reaction centers, novel functional materials combine electron donors and acceptors, i.e., (metallo)porphyrins (ZnP) and fullerenes (C60), respectively, with emerging materials, i.e., nanographenes. We utilized hexa-peri-hexabenzocoronene (HBC) due to its versatility regarding functionalization and physicochemical properties, to construct three regioisomeric ZnP-HBC-C60 conjugates, which foster geometrical diversity by arranging ZnP and C60 in ortho-, meta-, and para-positions to each other. The corresponding hexaarylbenzene (HAB) motifs, with an interrupted π-system, were also prepared. Transient absorption measurements disclosed the fast population of charge transfer as well as singlet and triplet charge-separated states. With the help of density functional theory (DFT) calculations, we further conceive the communication across the HBCs and HABs. This work reveals the impact of both the geometrical arrangement with respect to through-space versus through-bond interactions and the structural rigidity/flexibility on the charge management across the different π-systems.

Magnetic-Field-Induced Modulation of Charge-Recombination Dynamics in a Rosarin-Fullerene Complex.

Charge-recombination processes are critical for photovoltaic applications and should be suppressed for efficient charge transport. Here, we report that an applied magnetic field (0-1 T) can be used control the charge-recombination dynamics in an expanded rosarin-C60 complex. In the low magnetic field regime (<100 mT), the charge-recombination rate slows down due to hyperfine coupling, as inferred from transient absorption spectroscopic analyses. In contrast, in the high field regime, i.e., over 500 mT, the charge-recombination rate recovers and increases because the Δg mechanism facilitates spin conversion to a triplet charge-separated state (S to T0 ) that undergoes rapid charge-recombination to a localized rosarin triplet state. Therefore, we highlight the charge-recombination rate and the localized triplet state population can be modulated by the magnetic field in charge donor/acceptor non-covalent complexes.

Long-lived triplet charge-separated state in naphthalenediimide based donor-acceptor systems.

1,4,5,8-Naphthalenediimides (NDIs) are widely used motifs to design multichromophoric architectures due to their ease of functionalisation, their high oxidative power and the stability of their radical anion. The NDI building block can be incorporated in supramolecular systems by either core or imide functionalization. We report on the charge-transfer dynamics of a series of electron donor–acceptor dyads consisting of a NDI chromophore with one or two donors linked at the axial, imide position. Photo-population of the core-centred π–π* state is followed by ultrafast electron transfer from the electron donor to the NDI. Due to a solvent dependent singlet–triplet equilibrium inherent to the NDI core, both singlet and triplet charge-separated states are populated. We demonstrate that long-lived charge separation in the triplet state can be achieved by controlling the mutual orientation of the donor–acceptor sub-units. By extending this study to a supramolecular NDI-based cage, we also show that the triplet charge-separation yield can be increased by tuning the environment.

Competing ultrafast photoinduced electron transfer and intersystem crossing of [Re(CO)_3(Dmp)(His124)(Trp122)]^+ in Pseudomonas aeruginosa azurin: a nonadiabatic dynamics study

We present a computational study of sub-picosecond nonadiabatic dynamics in a rhenium complex coupled electronically to a tryptophan (Trp) side chain of Pseudomonas aeruginosa azurin, a prototypical protein used in the study of electron transfer in proteins. To gain a comprehensive understanding of the photoinduced processes in this system, we have carried out vertical excitation calculations at the TDDFT level of theory as well as nonadiabatic dynamics simulations using the surface hopping including arbitrary couplings (SHARC) method coupled to potential energy surfaces represented with a linear vibronic coupling model. The results show that the initial photoexcitation populates both singlet metal-to-ligand charge transfer (MLCT) and singlet charge-separated (CS) states, where in the latter an electron was transferred from the Trp amino acid to the complex. Subsequently, a complex mechanism of simultaneous intersystem crossing and electron transfer leads to the sub-picosecond population of triplet MLCT and triplet CS states. These results confirm the assignment of the sub-ps time constants of previous experimental studies and constitute the first computational evidence for the ultrafast formation of the charge-separated states in Re-sensitized azurin.

Remarkable Dependence of the Final Charge Separation Efficiency on the Donor-Acceptor Interaction in Photoinduced Electron Transfer.

The unprecedented dependence of final charge separation efficiency as a function of donor-acceptor interaction in covalently-linked molecules with a rectilinear rigid oligo-p-xylene bridge has been observed. Optimization of the donor-acceptor electronic coupling remarkably inhibits the undesirable rapid decay of the singlet charge-separated state to the ground state, yielding the final long-lived, triplet charge-separated state with circa 100% efficiency. This finding is extremely useful for the rational design of artificial photosynthesis and organic photovoltaic cells toward efficient solar energy conversion.

Photoinduced intercomponent excited-state decays in a molecular dyad made of a dinuclear rhenium(I) chromophore and a fullerene electron acceptor unit.

A novel molecular dyad, 1, made of a dinuclear {[Re2(μ-X)2(CO)6(μ-pyridazine)]} component covalently-linked to a fullerene unit by a carbocyclic molecular bridge has been prepared and its redox, spectroscopic, and photophysical properties - including pump-probe transient absorption spectroscopy in the visible and near-infrared region - have been investigated, along with those of its model species. Photoinduced, intercomponent electron transfer occurs in 1 from the thermally-equilibrated, triplet metal/ligand-to-ligand charge-transfer ((3)MLLCT) state of the dinuclear rhenium(I) subunit to the fullerene acceptor, with a time constant of about 100 ps. The so-formed triplet charge-separated state recombines in a few nanoseconds by a spin-selective process yielding, rather than the ground state, the locally-excited, triplet fullerene state, which finally decays to the ground state by intersystem crossing in about 290 ns.

Near-Infrared Photoelectrochemical Conversion via Photoinduced Charge Separation in Supramolecular Complexes of Anionic Phthalocyanines with Li+@C60

Two phthalocyanines possessing carboxylate groups ((TBA)4H2Pc·1 and (TBA)4H2Pc·2) form 1:2 supramolecular complexes with lithium cation-encapsulated C60 (Li(+)@C60) [H2Pc·1(4-)/(Li(+)@C60)2 and H2Pc·2(4-)/(Li(+)@C60)2] in a polar mixed solvent. From the UV-vis spectral changes, the binding constants (K) were estimated as ca. 10(12) M(-2). Upon the photoexcitation of constructed supramolecular complexes, photoinduced electron transfer occurred to form the charge-separated (CS) state. The lifetime of the CS state was determined to be 1.2 ms for H2Pc·2(4-)/(Li(+)@C60)2, which is the longest CS lifetime among the porphyrinoid/fullerene supramolecular complexes. H2Pc·1(4-)/(Li(+)@C60)2 also afforded the long-lived CS state of 1.0 ms. The spin state of the long-lived CS states was determined to be a triplet, as indicated by the EPR signal at g = 4. The reorganization energy (λ) and the electronic coupling term were determined to be λ = 1.70 eV, V = 0.15 cm(-1) from the temperature dependence of the rate constant for the charge recombination of the CS state of H2Pc·1(4-)/(Li(+)@C60)2. The energy of the CS state (0.49 eV) is much smaller than the reorganization energy, indicating that the back-electron-transfer process is located in the Marcus normal region. The small electronic coupling term results from the spin-forbidden back electron transfer due to the triplet CS state. Supramolecular complexes of anionic zinc phthalocyanines with Li(+)@C60 were also prepared and investigated. The ZnPc·4(4-)/Li(+)@C60 supramolecular nanoclusters were assembled on the optically transparent electrode (OTE) of nanostructured SnO2 (OTE/SnO2) to construct the dye-sensitized solar cell. The IPCE (incident photon-to-photocurrent efficiency) values of OTE/SnO2/(ZnPc·4(4-)/Li(+)@C60)n were much higher than the sum of the two IPCE values of the individual systems OTE/SnO2/(Li(+)@C60)n and OTE/SnO2/(ZnPc·4(4-))n, covering the near-infrared region.

Binding of scandium ions to metalloporphyrin-flavin complexes for long-lived charge separation.

A porphyrin-flavin-linked dyad and its zinc and palladium complexes (MPor-Fl: 2-M, M=2 H, Zn, and Pd) were newly synthesized and the X-ray crystal structure of 2-Pd was determined. The photodynamics of 2-M were examined by femto- and nanosecond laser flash photolysis measurements. Photoinduced electron transfer (ET) in 2-H2 occurred from the singlet excited state of the porphyrin moiety (H2 Por) to the flavin (Fl) moiety to produce the singlet charge-separated (CS) state (1) (H2 Por(.+) -Fl(.-) ), which decayed through back ET (BET) to form (3) [H2 Por]*-Fl with rate constants of 1.2×10(10) and 1.2×10(9) s(-1) , respectively. Similarly, photoinduced ET in 2-Pd afforded the singlet CS state, which decayed through BET to form (3) [PdPor]*Fl with rate constants of 2.1×10(11) and 6.0×10(10) s(-1) , respectively. The rate constant of photoinduced ET and BET of 2-M were related to the ET and BET driving forces by using the Marcus theory of ET. One and two Sc(3+) ions bind to the flavin moiety to form the Fl-Sc(3+) and Fl-(Sc(3+) )2 complexes with binding constants of K1 =2.2×10(5) M(-1) and K2 =1.8×10(3) M(-1) , respectively. Other metal ions, such as Y(3+) , Zn(2+) , and Mg(2+) , form only 1:1 complexes with flavin. In contrast to 2-M and the 1:1 complexes with metal ions, which afforded the short-lived singlet CS state, photoinduced ET in 2-Pd⋅⋅⋅Sc(3+) complexes afforded the triplet CS state ((3) [PdPor(.+) -Fl(.-) (Sc(3+) )2 ]), which exhibited a remarkably long lifetime of τ=110 ms (kBET =9.1 s(-1) ).

Photoinduced electron transfer in a charge-transfer complex formed between corannulene and Li+@C60 by concave-convex π-π interactions.

A charge-transfer (CT) complex was formed between corannulene (C20H10) and lithium ion-encapsulated [60]fullerene (Li(+)@C60) with the binding constant KG = 1.9 × 10 M(-1) by concave-convex π-π CT interactions in benzonitrile at 298 K, exhibiting a broad CT absorption extended to the NIR region. Femotosecond laser excitation of the C20H10/Li(+)@C60 CT complex resulted in the singlet charge-separated (CS) state, (1)(C20H10(•+)/Li(+)@C60(•-)), which decayed with the lifetime of 1.4 ns. Nanosecond laser excitation of Li(+)@C60 resulted in intermolecular electron transfer (ET) from C20H10 to the triplet excited state of Li(+)@C60 [(3)(Li(+)@C60)*] to produce the triplet CS state (3)(C20H10(•+)/Li(+)@C60(•-)). The distance between two electron spins in the triplet CS state was estimated to be 10 Å from the zero-field splitting pattern observed by EPR measurements at 4 K. The triplet CS state decayed to the ground state via intramolecular back electron transfer (BET). The CS lifetime was determined to be 240 μs in benzonitrile at 298 K. The temperature dependence of the rate constant of BET afforded the reorganization energy (λ = 1.04 eV) and the electronic coupling term (V = 0.0080 cm(-1)). The long lifetime of triplet CS state results from the spin-forbidden BET process and a small V value.

Long-Lived Charge Separation and Applications in Artificial Photosynthesis

Researchers have long been interested in replicating the reactivity that occurs in photosynthetic organisms. To mimic the long-lived charge separations characteristic of the reaction center in photosynthesis, researchers have applied the Marcus theory to design synthetic multistep electron-transfer (ET) systems. In this Account, we describe our recent research on the rational design of ET control systems, based on models of the photosynthetic reaction center that rely on the Marcus theory of ET. The key to obtaining a long-lived charge separation is the careful choice of electron donors and acceptors that have small reorganization energies of ET. In these cases, the driving force of back ET is located in the Marcus inverted region, where the lifetime of the charge-separated state lengthens as the driving force of back ET increases. We chose porphyrins as electron donors and fullerenes as electron acceptors, both of which have small ET reorganization energies. By linking electron donor porphyrins and electron acceptor fullerenes at appropriate distances, we achieved charge-separated states with long lifetimes. We could further lengthen the lifetimes of charge-separated states by mixing a variety of components, such as a terminal electron donor, an electron mediator, and an electron acceptor, mimicking both the photosynthetic reaction center and the multistep photoinduced ET that occurs there. However, each step in multistep ET loses a fraction of the initial excitation energy during the long-distance charge separation. To overcome this drawback in multistep ET systems, we used designed new systems where we could finely control the redox potentials and the geometry of simple donor-acceptor dyads. These modifications resulted in a small ET reorganization energy and a high-lying triplet excited state. Our most successful example, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), can undergo a fast photoinduced ET from the mesityl (Mes) moiety to the singlet excited state of the acridinium ion moiety (Acr(+)) with extremely slow back ET. The high-energy triplet charge-separated state is located deep in the Marcus inverted region, and we have detected the structural changes during the photoinduced ET in this system using X-ray crystallography. To increase the efficiency of both the light-harvesting and photoinduced ET, we assembled the Acr(+)-Mes dyads on gold nanoparticles to bring them in closer proximity to one another. We can also incorporate Acr(+)-Mes molecules within nanosized mesoporous silica-alumina. In contrast to the densely assembled dyads on gold nanoparticles, each Acr(+)-Mes molecule in silica-alumina is isolated in the mesopore, which inhibits the bimolecular back ET and leads to longer lifetimes in solution at room temperature than the natural photosynthetic reaction center. Acr(+)-Mes and related compounds act as excellent organic photocatalysts and facilitate a variety of reactions such as oxygenation, bromination, carbon-carbon bond formation, and hydrogen evolution reactions.

How disorder controls the kinetics of triplet charge recombination in semiconducting organic polymer photovoltaics.

Recent experiments by Rao et al. (Nature, 2013, 500, 435-439) indicate that recombination of triplet charge-separated states is suppressed in organic polymer-fullerene based bulk-heterojunction (BHJ) photovoltaic cells exhibiting a high degree of crystallinity in the fullerene phase relative to systems with more disorder. In this paper, we use a series of Frenkel-exciton lattice models to rationalize these results in terms of wave-function localization, interface geometry, and density of states. In one-dimensional co-linear and co-facial models of the interface, increasing local energetic disorder in one phase localizes the interfacial triplet charge-transfer ((3)CT) states and increases the rate at which these states relax to form lower-energy triplet excitons. In two dimensional BHJ models, energetic disorder within the fullerene phase plays little role in further localizing states pinned to the interface. However, inhomogeneous broadening introduces strong coupling between the interfacial (3)CT and nearby fullerene triplet excitons and can enhance the decay of these states in systems with higher degrees of energetic disorder.

Long-Range Charge Separation in a Ferrocene–(Zinc Porphyrin)–Naphthalenediimide Triad. Asymmetric Role of 1,2,3-Triazole Linkers

New dyad and triad systems based on a zinc porphyrin (ZnP), a naphthalenediimide (NDI), and a ferrocene (Fc) as molecular components, linked by 1,2,3-triazole bridges, ZnP-NDI (3) and Fc-ZnP-NDI (4), have been synthesized. Their photophysical behavior has been investigated by both visible excitation of the ZnP chromophore and UV excitation of the NDI unit. Dyad 3 exhibits relatively inefficient quenching of the ZnP singlet excited state, slow charge separation, and fast charge recombination processes. Excitation of the NDI chromophore, on the other hand, leads to charge separation by both singlet and triplet quenching pathways, with the singlet charge-separated (CS) state recombining in a subnanosecond time scale and the triplet CS state decaying in ca. 90 ns. In the triad system 4, primary formation of the Fc-ZnP+-NDI– charge-separated state is followed by a secondary hole shift process from ZnP to Fc. The product of the stepwise charge separation, Fc+-ZnP-NDI–, undergoes recombination to the ground state in 1.9 μs. The charge-separated states are always formed more efficiently upon NDI excitation than upon ZnP excitation. DFT calculations on a bridge–acceptor fragment show that the bridge is expected to mediate a fast donor-to-bridge-to-acceptor electron cascade following excitation of the acceptor. More generally, triazole bridges may behave asymmetrically with respect to photoinduced electron transfer in dyads, kinetically favoring hole-transfer pathways triggered by excitation of the acceptor over electron-transfer pathways promoted by excitation of the donor.

Long-Lived Singlet and Triplet Charge Separated States in Small Cyclophane-Bridged Triarylamine–Naphthalene Diimide Dyads

Two different dyads containing a triarylamine (TAA) donor and a naphthalene-1,8:4,5-bis(dicarboximide) (NDI) acceptor bridged by either a [2.2]- or a [3.3]paracyclophane (CP) were synthesized. These dyads show a high population of long-lived charge separated (CS) singlet and triplet states. The lifetimes of these different spin states vary only by 1 order of magnitude. This unique situation is a consequence of both a large electronic coupling V and a large exchange coupling 2J. The population of the different CS spin states and therefore the charge recombination (CR) and intersystem crossing (ISC) kinetics were monitored by standard ns-transient absorption spectroscopy. Together with fs-transient absorption spectroscopy supported by electrochemistry, steady state fluorescence and steady state absorption spectroscopy a detailed model of the photoinduced processes was derived.

Long-lived long-distance photochemically induced spin-polarized charge separation in β,β′-pyrrolic fused ferrocene-porphyrin-fullerene systems

The exceptionally long lived charge separation previously observed in a β,β′-pyrrolic-fused ferrocene-porphyrin-fullerene triad (lifetime 630 μs) and related porphyrin-fullerene dyad (lifetime 260 μs) is attributed to the production of triplet charge-separated states. Such molecular excited-state spin polarization maintained over distances of up to 23 A is unprecedented and offers many technological applications. Electronic absorption and emission spectra, femtosecond and nanosecond time-resolved transient absorption spectra, and cyclic voltammograms of two triads and four dyads are measured and analyzed to yield rate constants, donor–acceptor couplings, free-energy changes, and reorganization energies for charge-separation and charge-recombination processes. Production of long-lived intramolecular triplet states is confirmed by electron-paramagnetic resonance spectra at 77–223 K, as is retention of spin polarization in π-conjugated ferrocenium ions. The observed rate constants were either first predicted (singlet manifold) or later confirmed (triplet manifold) by a priori semiclassical kinetics calculations for all conceivable photochemical processes, parameterized using density-functional theory and complete-active-space self-consistent-field calculations. Identified are both a ps-timescale process attributed to singlet recombination and a μs-timescale process attributed to triplet recombination.

Mimicking Photosynthetic Antenna‐Reaction‐Center Complexes with a (Boron Dipyrromethene)3–Porphyrin–C60 Pentad

A highly efficient functional mimic of the photosynthetic antenna-reaction-center complexes has been designed and synthesized. The model contains a zinc(II) porphyrin (ZnP) core, which is connected to three boron dipyrromethene (BDP) units by click chemistry, and to a C(60) moiety using the Prato procedure. The compound has been characterized using various spectroscopic methods. The intramolecular photoinduced processes of this pentad have also been studied in detail with steady-state and time-resolved absorption and emission spectroscopic methods, both in polar benzonitrile and nonpolar toluene. The BDP units serve as the antennae, which upon excitation undergo singlet-singlet energy transfer to the porphyrin core. This is then followed by an efficient electron transfer to the C(60) moiety, resulting in the formation of the singlet charge-separated state (BDP)(3)-ZnP(·+) -C(60)(·-) , which has a lifetime of 476 and 1000 ps in benzonitrile and toluene, respectively. Interestingly, a slow charge-recombination process (k(CR)(t)=2.6×10(6) s(-1)) and a long-lived triplet charge-separated state (τ(CS)(T)=385 ns) were detected in polar benzonitrile by nanosecond transient measurements.

Photoinduced Electron Transfer in Photorobust Coumarins Linked with Electron Donors Affording Long Lifetimes of Triplet Charge‐Separated States

Electron donor (D) substituted 3-ethoxycarbonylcoumarin (CM) derivatives [D-CM: D = 4-diphenylaminophenyl (DPA), 4-diethylaminophenyl (DEA), 4-dimethylaminophenyl (DMA), and 2-methyl-4-dimethylaminophenyl (MeDMA)] are synthesized and characterized. Photoinduced electron transfer (ET) from the D moiety to the acceptor (CM) and back electron transfer (BET) are investigated by femtosecond and nanosecond laser flash photolysis measurements. Femtosecond laser excitation at 355 nm of a deaerated acetonitrile (MeCN) solution of D-CM shows generation of the singlet charge-separated (CS) state [(1)(D(.+)-CM(.-))] by ET from D to the singlet excited state of the CM moiety ((1)CM*), and this is followed by rapid decay within 3 ns to afford the triplet excited state (D-(3)CM*). Nanosecond laser excitation of a deaerated MeCN solution of D-CM results in formation of the triplet CS state by ET from D to (3)CM*. The quantum yield of formation of the triplet CS state [(3)(DPA(.+)-CM(.-))] in the presence of iodobenzene (PhI) in deaerated MeCN increases with increasing concentration of PhI to reach 27 % at 0.5 M PhI. The triplet CS state decays by bimolecular BET because of the long CS lifetime by unimolecular BET. Formation of the long-lived triplet CS state was confirmed by electron spin resonance (ESR) measurements. The photorobust nature of DPA-CM is demonstrated by multiple laser pulse excitation (>1000 times) at 355 nm. The photoinduced ET and BET rate constants of a series of D-CM are thoroughly analyzed in light of the Marcus theory of electron transfer.

Synthesis, Crystal Structure, and Photodynamics of π-Expanded Porphyrin−Fullerene Dyads Synthesized by Diels−Alder Reaction

Free base and zinc porphyrins are linked with fullerene (C(60)) through β,β'-pyrrolic positions rather than meso-positions by Diels-Alder reaction of monoanthraporphyrins (H(2)P-mA and ZnP-mA) and C(60) to afford a π-expanded free base porphyrin-fullerene dyad (H(2)P-C(60)) and its zinc porphyrin-fullerene dyad (ZnP-C(60)) in 86 and 51% yield, respectively. The X-ray crystallographic analysis of ZnP-C(60) showed two comma-shaped swirls-like structure in a unit cell. The intramolecular center-to-center and edge-to-edge distances between the porphyrin and fullerene moieties were 11.5 and 2.5 Å, respectively. The porphyrins and fullerenes were packed in layer-by-layer structure and the porphyrins lied down in parallel. The intermolecular center-to-center distances between the neighboring fullerenes were 10.252, 10.028, and 10.129 Å, which were less than typical van der Waals distance and the π-π interaction spread two-dimensionally. The energy of the charge-separated (CS) state of ZnP-C(60) (1.11 eV) determined by differential-pulse voltammetry measurements in benzonitrile is significantly lower than those of zinc porphyrin-C(60) dyads linked at meso positions because of the low oxidation potential of the π-expanded ZnP moiety. The CS energy of ZnP-C(60) in a nonpolar solvent such as toluene (1.40 V) is lower than the triplet excited state of C(60) ((3)C(60)*), enabling us to attain a long lived triplet CS state (8.1 μs) in toluene, detected by nanosecond laser flash photolysis experiments. The distance between unpaired electrons in the triplet CS state was determined by the EPR spectrum to be 9.1 Å, which agreed with the distance between a zinc atom of the porphyrin and a carbon edge of C(60) in the crystal structure of ZnP-C(60). The rate constants of photoinduced electron transfer and back electron transfer of H(2)P-C(60) and ZnP-C(60), which were determined by femtosecond and nanosecond laser flash photolysis measurements, were analyzed in light of the Marcus theory of electron transfer.

Photochemistry of nickel salen based complexes and relevance to catalysis

A series of four nickel salen complexes have been synthesised and their photochemical properties studied. These complexes emit in hexane at λem 580 nm with similar quantum efficiencies. Fluorescence decay occurs on the nanosecond time scale and its intensity is quenched by oxygen except for the complex with the shortest lifetime. For all complexes laser flash photolysis in CH2Cl2 or hexane generates a transient (lifetime on the μs time scale) that was assigned to a Ni-to-ligand electron transfer triplet excited state. In an attempt to show the relevance of photochemical data to catalysis, epoxidation of β-methylstyrene was carried out in the presence of the four complexes. It was found that the activity of the complexes correlates with the lifetime of the triplet charge-separated state. This relationship was interpreted as reflecting the intrinsic stability of the oxidised Ni ion in the different complexes, which is the common link between the photochemical and catalytic experiments.

A Time-Dependent Density Functional Study of the Singlet−Triplet Energy Gap in Charge-Separated States of Rigid Bichromophoric Molecules

Time-dependent density functional theory (TD-DFT) has been used to calculate gas-phase singlet−triplet energy gaps, ΔEST, for the charge-separated states of a series of donor−[bridge]−acceptor molecules, in which a dimethoxynaphthalene (DMN) donor and a dicyanovinyl (DCV) acceptor are connected by a rigid hydrocarbon (norbornylogous) bridge, ranging from four bonds to 13 bonds in length. Through the use of five different functionals, B3P86, B3LYP, B3PW91, BPW91, and BLYP, together with the 6-311G(d) basis set, it is found that TD-DFT theory gives a good description of the electron density distribution in the charge-separated (CS) states in that the calculated dipole moments of the CS states are in acceptable agreement with experimental data. The calculated ΔEST values for all members of the donor−[bridge]−acceptor series are positive (i.e., the singlet CS state lies higher in energy than the triplet CS state), and the distance dependence of ΔEST follows an exponential decay with increasing number, n, of b...