Electron transfer in a donor-substituted acridinium dye: evidence for dynamical solvent control

Multiple Emission of Phosphonium Fluorophores Harnessed by the Pathways of Photoinduced Counterion Migration

In the emerging field of intramolecular charge transfer induced counterion migration, we report the new insights into photophysical features of luminescent donor-acceptor phosphonium dyes (D-π-)n A+ [X- ] (π=-(C6 H4 )x -). The unique connectivity of the phosphorus atom affords multipolar molecules with a variable number of arms and the electronic properties of the acceptor group. In the ion-paired form, the transition from dipolar to quadrupolar configuration enhances the low energy migration-induced band by providing the additional pathways for anion motion. The multipolar architecture, adjustable lengths of the π-spacers and the nature of counterions allow for efficient tuning of the emission and achieving nearly pure white light with quantum yields around 30 %. The methyl substituent at the phosphorus atom reduces the rate of ion migration and suppresses the red shifted bands, simultaneously improving total emission intensity. The results unveil the harnessing of the multiple emission of phosphonium fluorophores by anion migration via structure and branching of donor-acceptor arms.

Cationic Organophosphorus Chromophores: A Diamond in the Rough among Ionic Dyes

Tunable electron-accepting properties of the cationic phosphorus center, its geometry and unique preparative chemistry that allows combining this unit with diversity of π-conjugated motifs, define the appealing photophysical and electrochemical characteristics of organophosphorus ionic chromophores. This Minireview summarizes the achievements in the synthesis of the π-extended molecules functionalized with P-cationic fragments, modulation of their properties by means of structural modification, and emphasizes the important effect of cation-anion interactions, which can drastically change physical behavior of these two-component systems.

Steric and electronic control of an ultrafast isomerization

Synthetic control of the influence of steric and electronic factors on the ultrafast (picosecond) isomerization of penta-coordinate ruthenium dithietene complexes (Ru((CF3)2C2S2)(CO)(L)2, where L = a monodentate phosphine ligand) is reported. Seven new ruthenium dithietene complexes were prepared and characterized by single crystal X-ray diffraction. The complexes are all square pyramidal and differ only in the axial vs. equatorial coordination of the carbonyl ligand. Fourier Transform Infrared (FTIR) spectroscopy was used to study the ν(CO) bandshapes of the complexes in solution, and these reveal rapid exchange between two or three isomers of each complex. Isomerization is proposed to follow a Berry psuedorotation-like mechanism where a metastable, trigonal bipyramidal (TBP) intermediate is observed spectroscopically. Electronic tuning of the phosphine ligands L = PPh3, P((p-Me)Ph)3, ((p-Cl)Ph)3, at constant cone angle is found to have little effect on the kinetics or thermodynamic stabilities of the axial, equatorial and TBP isomers of the differently substituted complexes. Steric tuning of the phosphine ligands over a range of phosphine cone angles (135 < θ < 165°) has a profound impact on the isomerization process, and in the limit of greatest steric bulk, the axial isomer is not observable. Temperature dependence of the FTIR spectra was used to obtain the relative thermodynamic stabilities of the different isomers of each of the seven ruthenium dithietene complexes. This study details how ligand steric effects can be used to direct the solution state dynamics on the picosecond time scale of discrete isomers energetically separated by <2.2 kcal mol-1. This work provides the most detailed description to date of ultrafast isomerization in the ground states of transition metal complexes.

Cationic and Betaine-Type Boronated Acridinium Dyes: Synthesis, Characterization, and Photocatalytic Activity

A series of isomeric boronated acridinium dyes were obtained by reactions of 10-(4'-octyloxyphenyl) functionalized 9(10H)-acridanone derivative with lithiated phenylboronic azaesters followed by aromatization with perchloric acid. The effect of the position of boronic group attached at ortho, meta, and para positions of the 9-phenyl ring on the photophysical properties was investigated. Conversion to related betaine trifluoroborato-substituted compounds was successfully performed, and the effect of this structural change on UV-vis absorption and fluorescence spectroscopy characteristics was established. Furthermore, cyclic voltammetry studies revealed that electrochemical behavior of cationic versus betaine structures is different in terms of redox potential values as well as stability. The theoretical calculations revealed a different scheme for molecular excitation processes in B(OH)₂ versus BF₃ ^--substituted compounds as charge transfer to acridinium core is observed from N-aryl or B-aryl moiety, respectively. Obtained compounds were active as photocatalysts in selected visible-light-promoted addition reactions to unsaturated substrates.

Organocatalyzed Controlled Radical Polymerizations

Radical polymerizations are responsible for a significant amount of the World's total polymer production. Free-radical polymerization provides a relatively inexpensive and facile route to produce bulk plastic products, however, it fails in the synthesis of precisely defined macromolecules. To address this issue, controlled radical polymerizations have been developed, which utilize a reversible deactivation mechanism for the synthesis of advanced polymeric architectures. In this chapter, we discuss the mechanisms and applications of organocatalyzed controlled radical polymerizations, specifically atom transfer radical polymerization, photo mediated reversible addition fragmentation chain-transfer polymerization, and reversible complexation mediated radical polymerization, as powerful new methods for precision polymer synthesis.

Synthesis of 9-arylalkynyl- and 9-aryl-substituted benzo[b]quinolizinium derivatives by Palladium-mediated cross-coupling reactions

9-Arylbenzo[b]quinolizinium derivatives were prepared with base-free Suzuki-Miyaura coupling reactions between benzo[b]quinolizinium-9-trifluoroborate and selected benzenediazonium salts. In addition, the Sonogashira coupling reaction between 9-iodobenzo[b]quinolizinium and the arylalkyne derivatives yielded four novel 9-(arylethynyl)benzo[b]quinolizinium derivatives under relatively mild reaction conditions. The 9-(N,N-dimethylaminophenylethynyl)benzo[b]quinolizinium is only very weakly emitting, but the emission intensity increases by a factor >200 upon protonation, so that this derivative may operate as pH-sensitive light-up probe. Photometric and fluorimetric titrations of duplex and quadruplex DNA to 9-(arylethynyl)benzo[b]quinolizinium derivatives revealed a significant binding affinity of these compounds towards both DNA forms with binding constants of Kb = 0.2-2.2 × 105 M-1.

Substituent control of the ultrafast twisted intramolecular charge transfer rate in dimethylaminochalcone derivatives

The rate of TICT relaxation in dimethylaminochalcone derivatives is shown to be controlled by acceptor strength. Variation in the charge pulling capacity of the acceptor modifies the torsional barrier along the TICT coordinate in the S₁ state, resulting in a tunable TICT relaxation rate.

Solvent Effects on the Intramolecular Charge Transfer Character of N,N-Diaryl Dihydrophenazine Catalysts for Organocatalyzed Atom Transfer Radical Polymerization

The nature of intramolecular charge transfer of N,N-diaryl dihydrophenazine photocatalysts (PCs) in different solvents is explored in context of their performance in organocatalyzed atom transfer radical polymerization (O-ATRP). PCs having a computationally predicted lowest energy excited state exhibiting charge transfer (CT) character can operate a highly controlled O-ATRP in a wide range of solvent polarities, from non-polar hexanes to highly polar N,N-dimethylacetamide. For PCs having a computationally predicted lowest energy excited state not possessing CT character, their ability to operate a controlled O-ATRP is decreased. This study confirms the importance of CT character in the excited state for N,N-diaryl dihydrophenazine PCs, and a deeper understanding of the activity of CT PCs has enabled the synthesis of polymers of low dispersity ( < 1.10) in a controlled fashion.

Probe dependence on polar solvation dynamics from fs broadband fluorescence

Solvation dynamics is remarkably independent of the probe as long as specific interactions remain similar.

Synthesis and fluorosolvatochromism of 3-arylnaphtho[1,2-b]quinolizinium derivatives

Cationic biaryl derivatives were synthesized by Suzuki-Miyaura coupling of 3-bromonaphtho[1,2-b]quinolizinium bromide with arylboronic acids. The resulting cationic biaryl derivatives exhibit pronounced fluorosolvatochromic properties. First photophysical studies in different solvents showed that the emission energy of the biaryl derivatives decreases with increasing solvent polarity. This red-shifted emission in polar solvents is explained by a charge shift (CS) in the excited state and subsequent solvent relaxation. Furthermore, the polarity of protic polar and aprotic polar solvents affects the emission energy to different extent, which indicates a major influence of hydrogen bonding on the stabilization of the ground and excited states.

Ionic liquids: structure and photochemical reactions

Ionic liquids are subjects of intense current interest within the physical chemistry community. A great deal of progress has been made in just the past five years toward identifying the factors that cause these salts to have low melting points and other useful properties. Supramolecular structure and organization have emerged as important and complicated topics that may be key to understanding how chemical reactions and other processes are affected by ionic liquids. New questions are posed, and an active debate is ongoing regarding the nature of nanoscale ordering in ionic liquids. The topic of reactivity in ionic liquids is still relatively unexplored; however, the results that have been obtained indicate that distributed kinetics and dynamical heterogeneity may sometimes, but not always, be influencing factors.

Charge-Transfer-Induced Twisting of the Nitro Group

Excited-state relaxation dynamics of 2-amino-7-nitrofluorene (ANF) and 2-dimethylamino-7-nitrofluorene (DMANF) has been investigated in two aprotic solvents, namely acetonitrile and DMSO using femtosecond transient absorption spectroscopic technique. Following photoexcitation to the highly dipolar excited singlet (S1) state, ANF and DMANF undergo mainly two concomitant relaxation processes, namely dipolar solvation and conformational relaxation via twisting of the nitro group to an orthogonal configuration with respect to the aromatic plane. Viscosity dependence of the relaxation dynamics of the S1 states of both ANF and DMANF suggests no involvement of the twisting motion of the amino or dimethylamino group in the charge-transfer process. The twisting of the nitro group is found to be a friction affected diffusive motion, which does not associate with any further charge transfer. The results presented in this paper resolve experimentally the dynamics of the twisting motion of the nitro group for the first time.

Ultrafast electron-transfer and solvent adiabaticity effects in viologen charge-transfer complexes

We report ultrafast electron transfer (ET) in charge-transfer complexes that shows solvent relaxation effects consistent with adiabatic crossover models of nonadiabatic ET. The complexes of either dimethyl viologen (MV) or diheptyl viologen (HV) with 4,4'-biphenol (BP) (MVBP or HVBP complexes) have identical charge-transfer spectra and kinetics in ethylene glycol with approximately 900 fs ET decay. We assign this decay time as largely due to adiabatic control of a predicted approximately 40 fs nonadiabatic ET. The MVBP decay in methanol of 470 fs is reduced in mixtures having low (2-20%) concentrations of acetonitrile to as short as 330 fs; these effects are associated with faster relaxation time in methanol and its mixtures. In contrast, HVBP has much longer ET decay in methanol (730 fs) and mixture effects that only reduce its decay to 550 fs. We identify the heptyl substituent as creating major perturbations to solvent relaxation times in the methanol solvation shell of HVBP. These charge-transfer systems have reasonably well-defined geometry with weak electronic coupling where the electronic transitions are not dependent on intramolecular motions. We used a nonadiabatic ET model with several models for adiabatic crossover predictions to discuss the small variation of energy gap with solvent and the ET rates derived from adiabatic solvent control. A time correlation model of solvent relaxation was used to define the solvent relaxation times for this case of approximately zero-barrier ET.

9-Donor-Substituted Acridizinium Salts: Versatile Environment-Sensitive Fluorophores for the Detection of Biomacromolecules

The absorption and steady-state emission properties of a series of N-alkyl- and N-aryl-9-aminoacridizinium derivatives and two 9-sulfanyl-substituted acridizinium derivatives were investigated. The N-alkyl derivatives and the 9-methylsulfanylacridizinium have an intense intrinsic fluorescence (phi(f) = 0.2-0.6), whereas the N-aryl-substituted compounds are virtually nonfluorescent in liquid solutions (phi(f) < or = 0.01). The emission intensity of the latter compounds significantly increases with increasing viscosity of the medium. It is demonstrated that the excited-state deactivation of the N-aryl-9-aminoacridizinium derivatives is due to two nonradiative processes: (i) torsional relaxation by rotation about the N-aryl bond and (ii) an electron-transfer process from an electron-donor substituted phenyl ring to the photoexcited acridizinium chromophore. The binding of several representative acridizinium derivatives to double-stranded DNA was studied by the spectrophotometric titrations and linear dichroism spectroscopy. The results give evidence that the prevailing binding mode is intercalation with binding constants in the range (0.5-5.0) x 10(5) M(-1) (in base pairs). Notably, the binding of most of the N-aryl-9-aminoacridizinium derivatives leads to a fluorescence enhancement by a factor of up to 50 upon binding to the biomacromolecules. Moreover, the addition of selected proteins, namely albumins, to N-(halogenophenyl)-9-aminoacridizinium ions in the presence of an anionic surfactant (sodium dodecyl sulfate) results in a 20-fold fluorescence enhancement. In each case, the emission enhancement is supposed to result from the hindrance of the torsional relaxation in the corresponding binding site of the biomacromolecule, which in turn suppresses the excited-state deactivation pathway.

Ultrafast Excited State Relaxation Dynamics of Branched Donor-π-Acceptor Chromophore: Evidence of a Charge-Delocalized State

Excited-state dynamics and complete transient absorption features of the trimer tris-4,4',4' '-(4-nitrophenyleethynyl)triphenylamine and the monomer 4-N,N-(dimethylamino)-4'-nitrotolane have been obtained from femtosecond pump-probe spectroscopy. The measurements are carried out to understand the mechanism behind enhanced two-photon absorption cross-sections of branched systems over their linear counterparts. Absorption and emission transition dipole moments of monomer and trimer in toluene have suggested that the emitting state of trimer is different from the monomer and probably is arising from the charge-delocalized C(3) symmetry state. Ultrafast transient absorption measurements on these molecules have spectroscopically validated the presence of an initial electron delocalized state with the C(3) symmetry state in the trimer molecule. The results have shown that there is a slower rate of internal conversion from the C(3) symmetry state to intramolecular charge transfer of trimer suggesting a barrier between them. Also, presence of a charge-stabilized state and involvement of a nonemissive state in the excited-state deactivation has been observed for both monomer and trimer.

Twisting Dynamics in the Excited Singlet State of Michler's Ketone

Ultrafast relaxation dynamics of the excited singlet (S(1)) state of Michler's ketone (MK) has been investigated in different kinds of solvents using a time-resolved absorption spectroscopic technique with 120 fs time resolution. This technique reveals that conversion of the locally excited (LE) state to the twisted intramolecular charge transfer (TICT) state because of twisting of the N,N-dimethylanilino groups with respect to the central carbonyl group is the major relaxation process responsible for the multi-exponential and probe-wavelength-dependent transient absorption dynamics of the S1 state of MK, but solvation dynamics does not have a significant role in this process. Theoretical optimization of the ground-state geometry of MK shows that the dimethylanilino groups attached to the central carbonyl group are at a dihedral angle of about 51 degrees with respect to each other because of steric interaction between the phenyl rings. Following photoexcitation of MK to its S1 state, two kinds of twisting motions have been resolved. Immediately after photoexcitation, an ultrafast "anti-twisting" motion of the dimethylanilino groups brings back the pretwisted molecule to a near-planar geometry with high mesomeric interaction and intramolecular charge transfer (ICT) character. This motion is observed in all kinds of solvents. Additionally, in solvents of large polarity, the dimethylamino groups undergo further twisting to about 90 degrees with respect to the phenyl ring, to which it is attached, leading to the conversion of the ICT state to the TICT state. Similar characteristics of the absorption spectra of the TICT state and the anion radical of MK establish the nearly pure electron transfer (ET) character of the TICT state. In aprotic solvents, because of the steep slope of the potential energy surface near the Franck-Condon (FC) or LE state region, the LE state is nearly nonemissive at room temperature and fluorescence emission is observed from only the ICT and TICT states. Alternatively, in protic solvents, because of an intermolecular hydrogen-bonding interaction between MK and the solvent, the LE region is more flat and stimulated emission from this state is also observed. However, a stronger hydrogen-bonding interaction between the TICT state and the solvent as well as the closeness between the two potential energy surfaces due to the TICT and the ground states cause the nonradiative coupling between these states to be very effective and, hence, cause the TICT state to be weakly emissive. The multi-exponentiality and strong wavelength-dependence of the kinetics of the relaxation process taking place in the S1 state of MK have arisen for several reasons, such as strong overlapping of transient absorption and stimulated emission spectra of the LE, ICT, and TICT states, which are formed consecutively following photoexcitation of the molecule, as well as the fact that different probe wavelengths monitor different regions of the potential energy surface representing the twisting motion of the excited molecule.

Charge Shift and Triplet State Formation in the 9-Mesityl-10-methylacridinium Cation

The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium ion to the orthogonally sited mesityl group in polar solution. The resultant charge-transfer (CT) state fluoresces in modest yield and decays on the nanosecond time scale. The LE and CT states reside in thermal equilibrium at ambient temperature; decay of both states is weakly activated in fluid solution, but decay of the CT state is activationless in a glassy matrix. Analysis of the fluorescence spectrum allows precise location of the relevant energy levels. Intersystem crossing competes with radiative and nonradiative decay of the CT state such that an acridinium-like, locally excited triplet state is formed in both fluid solution and a glassy matrix. Phosphorescence spectra position the triplet energy well below that of the CT state. The triplet decays via first-order kinetics with a lifetime of ca. 30 micros at room temperature in the absence of oxygen but survives for ca. 5 ms in an ethanol glass at 77 K. The quantum yield for formation of the LE triplet state is 0.38 but increases by a factor of 2.3-fold in the presence of iodomethane. The triplet reacts with molecular oxygen to produce singlet molecular oxygen in high quantum yield. In sharp contradiction to a recent literature report, there is no spectroscopic evidence to indicate the presence of an unusually long-lived CT state.

Ultrafast Intermolecular Hydrogen Bond Dynamics in the Excited State of Fluorenone

Steady-state fluorescence and time-resolved absorption measurements in pico- and femtosecond time domain have been used to investigate the dynamics of hydrogen bond in the excited singlet (S(1)) state of fluorenone in alcoholic solvents. A comparison of the features of the steady-state fluorescence spectra of fluorenone in various kinds of media demonstrates that two spectroscopically distinct forms of fluorenone in the S(1) state, namely the non-hydrogen-bonded (or free) molecule as well as the hydrogen-bonded complex, are responsible for the dual-fluorescence behavior of fluorenone in solutions of normal alcoholic solvents at room temperature (298 K). However, in 2,2,2-trifluoroethanol (TFE), a strong hydrogen bond donating solvent, emission from only the hydrogen-bonded complex is observed. Significant differences have also been observed in the temporal evolution of the absorption spectroscopic properties of the S(1) state of fluorenone in protic and aprotic solvents following photoexcitation using 400 nm laser pulses. An ultrafast component representing the solvent-induced vibrational energy relaxation (VER) process has been associated with the dynamics of the S(1) state of fluorenone in all kinds of solvents. However, in protic solvents, in addition to the VER process, further evolution of the spectroscopic and dynamical properties of the S(1) state have been observed because of repositioning of the hydrogen bonds around the carbonyl group. In normal alcohols, two different kinds of hydrogen-bonded complex of the fluorenone-alcohol system with different orientations of the hydrogen bond with respect to the carbonyl group and the molecular plane of fluorenone have been predicted. On the other hand, in TFE, formation of only one kind of hydrogen-bonded complex has been observed. These observations have been supported by theoretical calculations of the geometries of the hydrogen-bonded complexes in the ground and the excited states of fluorenone. Linear correlation between the lifetimes of the equilibration process occurring because of repositioning of the hydrogen bonds and Debye or longitudinal relaxation times of the normal alcoholic solvents establish the fact that, in weakly hydrogen bond donating solvents, the hydrogen bond dynamics can be described as merely a solvation process. Whereas, in TFE, hydrogen bond dynamics is better described by a process of conversion between two distinct excited states, namely, the non-hydrogen-bonded form and the hydrogen-bonded complex.

S2 Fluorescence and Ultrafast Relaxation Dynamics of the S2 and S1 States of a Ketocyanine Dye

Dynamics of the excited singlet (both the S2 and S1) states of a ketocyanine dye, namely, 2,5-bis[(2,3-dihydroindolyl)-propylene]-cyclopentanone (KCD), have been investigated in different kinds of media using steady-state absorption and emission as well as femtosecond transient absorption spectroscopic techniques. Steady-state fluorescence measurements, following photoexcitation of KCD to its second excited singlet state, reveal dual fluorescence (emission from both the S2 and S1 states) behavior. Although the intensity of the S2 --> S0 fluorescence is weaker than that of the S1 --> S0 fluorescence in solutions at room temperature (298 K), the former becomes as much as or more intense than the latter in rigid matrixes at 77 K. The lifetime of the S2 state is short and varies between 0.2 and 0.6 ps in different solvents. After its creation, the S2 state undergoes two simultaneous processes, namely, S2 --> S0 fluorescence and S2 --> S1 internal conversion. Time-resolved measurements reveal the presence of an ultrafast component in the decay dynamics of the S1 state. A good correlation between the lifetime of this component and the longitudinal relaxation times (tauL) of the solvents suggests that this component arises due to solvation in polar solvents. More significant evolution of the spectroscopic properties of the S1 state in alcoholic solvents in the ultrafast time domain has been explained by the occurrence of the repositioning of the hydrogen bonds around the carbonyl group in the excited state of KCD. In 2,2,2-trifluoroethanol, a strongly hydrogen bond donating solvent, it has even been possible to establish the existence of two distinct forms of the S1 state, namely, the non-hydrogen-bonded (or free) molecule and the hydrogen-bonded complex.

Solvent Adiabaticity Effects on Ultrafast Electron Transfer in Viologen Charge Transfer Complexes

Charge recombination (CR) kinetics following photoinduced charge transfer are measured by optical transient absorption for complexes of dimethyl viologen and diheptyl viologen with 4,4'-biphenol (MVBP and HVBP) in methanol. Exponential time constants for MVBP and HVBP are 480 and 790 fs, respectively. Kinetic differences cannot be rationalized with a standard equilibrium nonadiabatic rate formula using parameters obtained from linear absorption and resonance Raman measurements, which give nearly indistinguishable results for the two complexes. Solvent relaxation times and adiabaticities of MVBP are calculated using a full solvation correlation function approach. This analysis suggests that the smaller CR rate of HVBP is due to solvent reorganization differences, and is consistent with a greater adiabatic contribution for HVBP than MVBP. We conjecture that interactions between the diheptyl aliphatic groups of HVBP and the local solvent structure are responsible for the CR differences.

Solvent-Dependent Dynamics of the MQ•→ReII Excited-State Electron Transfer in [Re(MQ+)(CO)3(dmb)]2+

The Re-->MQ(+) MLCT excited state of [Re(MQ(+))(CO)(3)(dmb)](2+) (MQ(+) = N-methyl-4,4'-bipyridinium, dmb = 4,4'-dimethyl-2,2'-bipyridine), which is populated upon 400-nm irradiation, was characterized by picosecond time-resolved IR and resonance Raman spectroscopy, which indicate large structural differences relative to the ground state. The Re-->MQ(+) MLCT excited state can be formulated as [Re(II)(MQ*)(CO)(3)(dmb)](2+). It decays to the ground state by a MQ*-->Re(II) back-electron transfer, whose time constant is moderately dependent on the molecular nature of the solvent, instead of its bulk parameters: formamides approximately DMSO approximately MeOH (1.2-2.2 ns) < THF, aliphatic nitriles (3.2-3.9 ns) << ethylene-glycol approximately 2-ethoxyethanol (4.2-4.8 ns) < pyridine (5.7 ns) < MeOCH(2)CH(2)OMe (6.9 ns) < PhCN (7.5 ns) < MeNO(2) (8.6 ns) <<< CH(2)Cl(2), ClCH(2)CH(2)Cl (25.9-28.9 ns). An approximate correlation was found between the back-reaction rate constant and the Gutmann donor number. Temperature dependence of the decay rate measured in CH(2)Cl(2), MeOH, and BuCN indicates that the inverted MQ*-->Re(II) back-electron transfer populates a manifold of higher vibrational levels of the ground state. The solvent dependence of the electron transfer rate is explained by solvent effects on inner reorganization energy and on frequencies of electron-accepting vibrations, by interactions between the positively charged MQ(+) pyridinium ring and solvent molecules in the electron-transfer product, that is the [Re(MQ(+))(CO)(3)(dmb)](2+) ground state.

Solvent dynamical control of electron-transfer rates in mixed-valence complexes observed by infrared spectral line shape coalescence

Rate constants for intramolecular electron transfer within the intervalence charge transfer (-1) states of the complexes [{Ru3O(OAc)6(L)(CO)}2(mu-pz)] (where L= 4-(dimethylamino)pyridine (1), pyridine (2), 3-cyanopyridine (3), or 4-cyanopyridine (4) and pz = pyrazine) were determined by coalescence of infrared (IR) vibrational spectral line shapes in seven solvents. The electron-transfer times (kET-1) show a strong correlation with solvent relaxation times determined in separate ultrafast time-resolved fluorescence experiments. The best comparison is found with the parameter t1e, which is ascribed to inertial solvent relaxation. The IR spectra of these mixed-valence complexes are thus a steady-state spectral probe of ultrafast, dynamic solvent relaxation processes which are otherwise only accessible using laser-pumped, ultrafast time-resolved measurements.