We have employed two third-order femtosecond spectroscopic methods, stimulated-photon-echo peak-shift (3PEPS) and transient-grating (TG) spectroscopy, to characterize solvation dynamics and interexciton-state radiationless decay in the α subunit of C-phycocyanin and in allophycocyanin. The α subunit contains a single phycocyanobilin chromophore in an isolated protein-matrix environment. Allophycocyanin contains exciton-coupled pairs of phycocyanobilins in the same type of binding site found in the α subunit. The results show that both systems exhibit a biphasic solvation response:  the inertial phase, arising from librational motions of the amino acids or included water molecules in the phycocyanobilin-binding site, contributes a 80−100-fs component to the 3PEPS profile and appears as a rapidly damped 72-cm-1 modulation of the TG signal; the diffusive phase, arising from collective protein-matrix motions, contributes a component in the TG signal and 3PEPS profile on the 5−20-ps time scale. Both systems exhibit nearly instantaneous (16-fs) components in the 3PEPS profiles that arise from intrachromophore vibrational modes. The 3PEPS profile observed with allophycocyanin exhibits additional fast decay components, with time constants of 56 and 220 fs, that apparently report the contributions to electronic dephasing arising from radiationless decay between imperfectly correlated exciton states. The TG signal evidences vibrational relaxation in the lower exciton state and incoherent energy transfer between the chromophores in a given pair. The results present complementary details on solvation and interexciton-state radiationless decay dynamics that were first observed in this laboratory using time-resolved pump−probe and anisotropy methods.

We present the first measurements of the excited-state relaxation dynamics of a bimetallic class III mixedvalence molecule. The 800 nm absorption of [Ru 2 TIEDCl 4 ] + (TIED ) tetraiminoethylenedimacrocycle) relaxes in 250 and 1000 fs to at least two different intermediate states that can be followed with transient absorption spectroscopy. These states decay in 1.3 and 11.5 ps, and the absorption of the 1.3 ps intermediate displays a large amplitude, very low frequency, highly damped vibrational coherence that completely modulates the absorption. The coherence frequency is 20 ( 5 cm -1 , and the dephasing times range from 360 to 730 fs over the wavelength range of the absorption band. The occurrence of a low-frequency coherence at room temperature, the nearly 100% modulation amplitude, and the phase properties as a function of wavelength are consistent with a nonradiative rate modulation rather than the typical impulsive mechanism that creates a coherent Franck-Condon modulation of the absorption. A nonradiative rate modulation can occur from a vibronic coupling mechanism that is created by breakdown of the Born-Oppenheimer approximation. This type of electronic state coupling likely occurs via nontotally symmetric vibrations, and this is the first time domain measure of a vibronic coupling frequency for inorganic complexes. The resonance Raman activity of the ground-state absorption is consistent with very small mode displacements for the optically connected ground and excited states, as expected for a class III molecule. Since similar nonradiative rates are measured for both the optically excited-state and intermediate-state decays, they both require similar energy gaps in the range of 5000-7000 cm -1 . With these energy gaps, we infer that vibronic coupling matrix elements from 4500 to 11 200 cm -1 can explain the observed nonradiative decay time of 250 fs. These experiments show that class III molecules, and probably many other inorganic complexes, can have fast nonradiative decay channels from vibronic coupling when electronic states are available at lower energies. Therefore, applications with such molecules require careful molecular design to compete with or reduce rates of return to the ground state.

We have reexamined our earlier report of electron transfer in the [Co(Cp) 2 |V(CO) 6 ] radical pair using ultrafast infrared transient absorption spectroscopy. The radical pair is created from the [Co(Cp) 2 + |V(CO) 6 -] ion pair by ultrafast visible charge-transfer excitation. Transient absorption experiments with <75 fs resolution reveal two major direct electron-transfer (ET) components with ∼700 fs and ∼5 ps time constants. A small ET component with a ∼75 ps time constant is due to some separation and re-formation of the radical pairs. Transient absorption experiments monitoring the recovery of the ion-pair state show that both fast components are due to ET rather than some other vibrational relaxation (VR) process in the radical state. By modeling the visible charge-transfer band, the two fast ET decay times are assigned to two ion-pair contact geometries with absorption origins different by about 1250 ( 350 cm -1 . The ∼700 fs ET lifetime depends on the vibrational quantum state of the nontotally symmetric CO stretch in the V(CO) 6 radical, where the lifetime decreases by ∼10% for the first vibrational quantum and ∼45% for the second quantum. There is no quantum effect for the second ion-pair geometry with a 5 ps ET lifetime. Standard ET rate models cannot explain the rate dependence upon vibrational quantum state for a nontotally symmetric vibration, and it may arise from a breakdown of the Condon approximation. We also find that the intramolecular vibrational redistribution (IVR) time to transfer vibrational energy from the totally symmetric CO stretch to the nontotally symmetric stretch is less than 75 fs for a 1-quantum IVR process. This is unusually fast for metal carbonyls and may be assisted by the Jahn-Teller geometry change of the radical. The 2-quantum IVR time is ∼200 fs for 800 and 700 nm charge-transfer excitation wavelengths. At excitation wavelengths of 620 and 555 nm all quantum levels show a 200 fs rise time, which is unexpected for the zero quantum level. We assign this effect to the onset of sufficient internal vibrational energy in low-frequency vibrations to cause geometric interconversion between energetically similar Jahn-Teller geometries in the V(CO) 6 radical. The 200 fs rise time is the time for the V(CO) 6 radical species to assume a stable geometry, which requires VR of low-frequency vibrations to the solvent. These results demonstrate that earlier measurements from our group on the same molecule had insufficient time resolution to observe the ultrafast ET component and thereby inferred a vibrational quantum effect in a single ET rate of longer duration.

We have employed dynamic absorption spectroscopy to monitor coherent wave packet dynamics and anisotropy decays following impulsive excitation of the B820 subunit of the LH1 light-harvesting complex, which was isolated from Rhodospirillum rubrum G9. When the lower exciton-state transition of the bacteriochlorophyll a dimer is pumped, the time-resolved pump-probe spectrum exhibits contributions from a fully Stokes shifted stimulated-emission spectrum and a nonstationary vibrational character within 40 fs of excitation. Coherent wave packet motion in both the ground state and the excited state is observed via modulations of singlewavelength transients. The photobleaching portion of the spectrum exhibits strong components only at low frequencies, 20-60 and 180 cm -1 , and a weaker component is observed at 400 cm -1 . The stimulated-emission portion of the spectrum exhibits weak modulation components at 20-60 and 180 cm -1 , but strong components are observed at fairly high frequencies: 360, 400, 470, 600, and 730 cm -1 . An anisotropy decay observed in the stimulated-emission region reports a prompt >20°tilt of the photoselected transition-dipole moment. A possible explanation for these results is that an intradimer charge-transfer event occurs on a very short time scale following optical preparation of the lower π f π* exciton state of the bacteriochlorophyll a dimer at room temperature.

The solvent environment of the phycocyanobilin chromophore bound by the alpha subunit of C-phycocyanin was probed in buffered binary solvent systems consisting of water and methanol, acetonitrile, or acetone. The focus of the work was on determining whether the inertial phase of the solvent response observed previously in the alpha subunit from femtosecond transient hole-burning spectroscopy [Riter et al. (1996) J. Phys. Chem. 100, 14198-14205] involves solvent dipoles in the bulk. Continuous absorption and fluorescence spectra at room temperature show that addition of the nonaqueous solvent results in a change in the tertiary structure of the protein so that the phycocyanobilin chromophore is unclamped and allowed to assume a cyclic conformation. At low concentrations of nonaqueous solvent, we observe a conformational equilibrium characterized by a cooperative binding of nonaqueous solvent. The phycocyanobilin chromophore exhibits a nonshifted absorption and fluorescence spectrum characteristic of its native, extended conformation in the state with bound water molecules. In the state with bound solvent molecules, the phycocyanobilin chromophore exhibits an absorption spectrum that reports a cyclic configuration, and its fluorescence is essentially quenched. The absorption and fluorescence spectra exhibit a solvatochromic response in this state, indicating that the chromophore is now exposed to the bulk solvent. Far-UV circular dichroism spectra evidence an abrupt loss of 10% of the alpha-helical character in the nonaqueous solvent concentration regime that results in exposure of the chromophore to the bulk. These results show that the ultrafast solvation response previously detected in the alpha subunit in aqueous media from femtosecond transient hole-burning spectroscopy arises solely from protein-matrix solvation dynamics.