Excimer Formation Inhibits the Intramolecular Singlet Fission Dynamics: Systematic Tilting of Pentacene Dimers by Linking Positions

The role of excimer formation in inhibiting or enhancing the efficiency of the intramolecular singlet fission (iSF) process has been a subject of recent debate. Here, we investigated the effect of excimer formation on iSF dynamics by modifying its configuration by connecting pentacenes at various positions. Hence, pentacene dimers having slip-stacked (2,2′ BP, J-type), oblique (2,6′ BP), and facial (6,6′ BP, H-type) configurations were synthesized by covalently linking pentacenes at positions 2,2′, 2,6′, and 6,6′, respectively, with an ethynyl bridge, and their ultrafast excited-state relaxation dynamics were characterized. Femtosecond time-resolved transient absorption spectra revealed that the efficiency of iSF dynamics decreased from slip-stacked (182%) to oblique configuration (97%),whereas in the 6,6′ BP with facial configuration, strong electronic coupling led to the formation of excimers that decayed nonradiatively without formation of correlated triplet pairs. These studies reveal the formation of excimers by strong intrapentacene electronic coupling upon ultrafast excitation, preventing the efficient iSF process

Direct Observation of Cascade of Photoinduced Ultrafast Intramolecular Charge Transfer Dynamics in Diphenyl Acetylene Derivatives: Via Solvation and Intramolecular Relaxation

Interaction of light with electron donor–acceptor π-conjugated systems leading to intramolecular charge transfer (ICT) plays an essential role in transformation of light energy. Here the cascade of photoinduced ICT processes is directly observed by investigating the excited state relaxation dynamics of cyano and mono/di methoxy substituted diphenyl acetylene derivatives using femtosecond pump–probe spectroscopy and nanosecond laser flash photolysis. The femtosecond transient absorption spectra of the chromophores upon ultrafast excitation reveal the dynamics of intermediates involved in transition from initially populated Frank–Condon state to local excited state (LE). It also provides the dynamic details of the transition from the LE to the charge transfer state yielding the formation of the radical ions. Finally, the charge transfer state decays to the triplet state by geminate charge recombination. The latter dynamics are observed in the nanosecond transient absorption spectra. It is found that excited state relaxation pathways are controlled by different stages of solvation and intramolecular relaxation depending on the solvent polarity. The twisted ICT state is more stabilized (978 ps) in acetonitrile than cyclohexane where major components of transient absorption originate from the S₁ state

Investigations of the Low-Frequency Spectral Density of Cytochrome c upon Equilibrium Unfolding

The equilibrium unfolding process of ferric horse heart cytochrome c (cyt c), induced by guanidinium hydrochloride (GdHCl), was studied using UV–vis absorption spectroscopy, resonance Raman spectroscopy, and vibrational coherence spectroscopy (VCS). The unfolding process was successfully fit using a three-state model which included the fully folded (N) and unfolded (U) states, along with an intermediate (I) assigned to a Lys bound heme. The VCS spectra revealed for the first time several low-frequency heme modes that are sensitive to cyt c unfolding: γ_a (∼50 cm^–1), γ_b (∼80 cm^–1), γ_c (∼100 cm^–1), and ν_s(His-Fe-His) at 205 cm^–1. These out-of-plane modes have potential functional relevance and are activated by protein-induced heme distortions. The free energies for the N–I and the I–U transitions at pH 7.0 and 20 °C were found to be 4.6 kcal/M and 11.6 kcal/M, respectively. Imidazole was also introduced to replace the methionine ligand so the unfolding can be modeled as a two-state system. The intensity of the mode γ_b∼80 cm^–1 remains nearly constant during the unfolding process, while the amplitudes of the other low frequency modes track with spectral changes observed at higher frequency. This confirms that the heme deformation changes are coupled to the protein tertiary structural changes that take place upon unfolding. These studies also reveal that damping of the coherent oscillations depends sensitively on the coupling between heme and the surrounding water solvent

Ultrafast Intermolecular Energy Transfer in OLED Materials: Excited-State Dynamics of a Blend of Poly(vinylcarbazole) and Oxadiazole Derivative in Solution and Film States

Intermolecular energy transfer dynamics between layers of organic materials controls the efficiency of organic light-emitting diode (OLED) devices. Here, to understand the ultrafast intermolecular energy transfer dynamics occurring in widely used UV-OLED materials of poly(vinylcarbazole) (PVK, donor: hole-transporting material) and synthesized oxadiazole derivative (OXD, acceptor: electron-transporting material), their excited-state relaxation pathways are investigated in tetrahydrofuran (THF) and film. Upon addition of OXD, the nanosecond fluorescence lifetime studies of PVK exhibited efficient quenching of dynamics, supporting the occurrence of Förster resonance energy transfer from PVK to OXD with an efficiency of ∼90%. The femtosecond time-resolved absorption measurements of the mixture of PVK and OXD revealed the intermolecular resonance energy transfer with a time constant of ∼1.32 ps from all of the intermediates in the excited states of PVK to OXD. Interestingly, in the film state, the formation of exciplex beneficial for devices with a lifetime of ∼32.66 ns was observed at ∼516 nm upon excitation of PVK in a blend of OXD and PVK. Nanosecond transient absorption spectra confirmed the occurrence of energy transfer also from the triplet state of PVK. Understanding of these excited-state dynamics occurring in solution and film states is a prerequisite to design and improve the performance of OLED materials

Optical Investigation of Self-Aggregation of a Tetrazole-Substituted Diphenylacetylene Derivative: Steady and Excited State Dynamics in Solid and Solution State

Slow crystallization and fast precipitation of a tetrazole-substituted diphenylacetylene derivative (MPT) led to formation of solids with significantly different photoluminescence efficiencies of 0.06 and 0.33, respectively. A detailed study of the photophysical properties of solutions of MPT as a function of concentration and temperature indicated that the extent of formation of J- and H-aggregates played a significant role in determining the luminescence properties of these materials. Time-resolved emission spectroscopy showed that the lifetime of emission arising from the aggregated species was significantly higher than that of the monomer species. The long-lived emission might be due to the formation of excimer arising from the excitation of ground state J- and H-aggregates. The higher quantum yield of fluorescence in the solids obtained by fast precipitation could be attributed to the presence of increased amounts of J-aggregates similar to that observed in highly concentrated solutions (≥ 4.2 × 10–4 M). The photophysical studies of MPT in various concentrations indicate that J-aggregates are significantly more fluorescent than the H-aggregates. Transient absorption spectra measured by nanosecond laser flash photolysis indicated the formation of a triplet excited state with an absorption maximum of ∼490 nm and a quantum yield of 0.61

Vibrational Coherence Spectroscopy of the Heme Domain in the CO-Sensing Transcriptional Activator CooA

Femtosecond vibrational coherence spectroscopy was used to investigate the low-frequency vibrational dynamics of the heme in the carbon monoxide oxidation activator protein (CooA) from the thermophilic anaerobic bacterium Carboxydothermus hydrogenoformans (Ch-CooA). Low frequency vibrational modes are important because they are excited by the ambient thermal bath (kBT = 200 cm–1) and participate in thermally activated barrier crossing events. However, such modes are nearly impossible to detect in the aqueous phase using traditional spectroscopic methods. Here, we present the low frequency coherence spectra of the ferric, ferrous, and CO-bound forms of Ch-CooA in order to compare the protein-induced heme distortions in its active and inactive states. Distortions take place predominantly along the coordinates of low-frequency modes because of their weak force constants, and such distortions are reflected in the intensity of the vibrational coherence signals. A strong mode near ∼90 cm–1 in the ferrous form of Ch-CooA is suggested to contain a large component of heme ruffling, consistent with the imidazole-bound ferrous heme crystal structure, which shows a significant protein-induced heme distortion along this coordinate. A mode observed at ∼228 cm–1 in the six-coordinate ferrous state is proposed to be the ν(Fe–His) stretching vibration. The observation of the Fe–His mode indicates that photolysis of the N-terminal α-amino axial ligand takes place. This is followed by a rapid (∼8.5 ps) transient absorption recovery, analogous to methionine rebinding in photolyzed ferrous cytochrome c. We have also studied CO photolysis in CooA, which revealed very strong photoproduct state coherent oscillations. The observation of heme-CO photoproduct oscillations is unusual because most other heme systems have CO rebinding kinetics that are too slow to make the measurement possible. The low frequency coherence spectrum of the CO-bound form of Ch-CooA shows a strong vibration at ∼230 cm–1 that is broadened and up-shifted compared to the ν(Fe–His) of Rr-CooA (216 cm–1). We propose that the stronger Fe–His bond is related to the enhanced thermal stability of Ch-CooA and that there is a smaller (time dependent) tilt of the histidine ring with respect to the heme plane in Ch-CooA. The appearance of strong modes at ∼48 cm–1 in both the ferrous and CO-bound forms of Ch-CooA is consistent with coupling of the heme doming distortion to the photolysis reaction in both samples. Upon CO binding and protein activation, a heme mode near 112 ± 5 cm–1 disappears, probably indicating a decreased heme saddling distortion. This reflects changes in the heme environment and geometry that must be associated with the conformational transition activating the DNA-binding domain. Protein-specific DNA binding to the CO-bound form of Ch-CooA was also investigated, and although the CO rebinding kinetics are significantly perturbed, there are negligible changes in the low-frequency vibrational spectrum of the heme

Investigation of the Low Frequency Dynamics of Heme Proteins: Native and Mutant Cytochrome P450_cam and Redox Partner Complexes

Vibrational coherence spectroscopy (VCS) is used to investigate the low-frequency dynamics of camphor-free and camphor-bound cytochrome P450cam (CYP 101) and its L358P mutant. The low-frequency heme vibrations are found to be perturbed upon binding to the electron transfer partner putidaredoxin (Pdx). A strong correlation between the “detuned” vibrational coherence spectrum, which monitors frequencies between 100 and 400 cm−1, and the lower frequency part of the Raman spectrum is also demonstrated. The very low frequency region ≤200 cm−1, uniquely accessed by open-band VCS measurements, reveals a mode near 103 cm−1 in P450cam when camphor is not present in the distal pocket. This reflects the presence of a specific heme distortion, such as saddling or ruffling, in the substrate-free state where water is coordinated to the low-spin iron atom. Such distortions are likely to retard the rate of electron transfer to the substrate-free protein. The presence of strong mode near ∼33 cm−1 in the camphor-bound form suggests a significant heme-doming distortion, which is supported by analysis using normal coordinate structural decomposition. Pdx also displays a strong coherent vibration near 30 cm−1 that in principle could be involved in vibrational resonance with its electron transfer target. A splitting of the 33 cm−1 feature and intensification of a mode near 78 cm−1 appear when the P450cam/Pdx complex is formed. These observations are consistent with vibrational mixing and heme geometric distortions upon Pdx binding that are coincident with the increased thiolate electron donation to the heme. The appearance of a mode near 65 cm−1 in the coherence spectra of the L358P mutant is comparable to the mode at 78 cm−1 seen in the P450cam/Pdx complex and is consistent with the view that the heme and its environment in the L358P mutant are similar to the Pdx-bound native protein. Resonance Raman spectra are presented for both P450cam and the L358P mutant and the changes are correlated with an increased amount of thiolate electron donation to the heme in the mutant sample

Photoinduced Dynamics of Guanosine Monophosphate in Water from Broad-Band Transient Absorption Spectroscopy and Quantum-Chemical Calculations

Guanosine monophosphate (GMP) in aqueous solutions has been studied with femtosecond broad-band transient absorption spectroscopy and by quantum-mechanical calculations. The sample was excited at 267 or 287 nm and probed between 270 and 1000 nm with 100 fs resolution, for various pH values between 2 and 7. At pH 2, when the guanine ring is ground-state protonated (GMPH+), we observe isosbestic behavior indicating state-to-state relaxation. The relaxation is biexponential, τ1 = 0.4 ps, τ2 = 2.2 ps, and followed by slower internal conversion with τ3 = 167 ps. For nonprotonated GMP in the pH range 7−4, we find biexponential decay in the region 400−900 nm (τ1 = 0.22 ps, τ2 = 0.9 ps), whereas, between 270 and 400 nm, the behavior is triexponential with one growing, τ1 = 0.25 ps, and two decaying, τ2 = 1.0 ps, τ3 = 2.5 ps, components. The excited-state evolution is interpreted with the help of quantum-chemical calculations, performed at the time-dependent PBE0 level accounting for bulk solvent effects and specific solvation. The computed dynamics involves La and Lb bright excited states, whereas the n0π* and πσ* dark excited states play a minor role. Independent of the pH, the photoinduced evolution involves ultrafast Lb→La conversion (τba ≪ 100 fs) and exhibits the presence of a wide planar plateau on La. For neutral GMP a barrierless path connects this region to a conical intersection (CI) with the ground state, giving an account of the ultrafast decay of this species. For protonated GMPH+ the system evolves into a stable minimum La min characterized by out-of-plane displacement of NH and CH groups, which explains the longer (167 ps) fluorescence lifetime

Investigations of the Low Frequency Modes of Ferric Cytochrome <i>c</i> Using Vibrational Coherence Spectroscopy

Femtosecond vibrational coherence spectroscopy is used to investigate the low frequency vibrational dynamics of the electron transfer heme protein, cytochrome <i>c</i> (cyt <i>c</i>). The vibrational coherence spectra of ferric cyt <i>c</i> have been measured as a function of excitation wavelength within the Soret band. Vibrational coherence spectra obtained with excitation between 412 and 421 nm display a strong mode at ∼44 cm^–1 that has been assigned to have a significant contribution from heme ruffling motion in the electronic ground state. This assignment is based partially on the presence of a large heme ruffling distortion in the normal coordinate structural decomposition (NSD) analysis of the X-ray crystal structures. When the excitation wavelength is moved into the ∼421–435 nm region, the transient absorption increases along with the relative intensity of two modes near ∼55 and 30 cm^–1. The intensity of the mode near 44 cm^–1 appears to minimize in this region and then recover (but with an opposite phase compared to the blue excitation) when the laser is tuned to 443 nm. These observations are consistent with the superposition of both ground and excited state coherence in the 421–435 nm region due to the excitation of a weak porphyrin-to-iron charge transfer (CT) state, which has a lifetime long enough to observe vibrational coherence. The mode near 55 cm^–1 is suggested to arise from ruffling in a transient CT state that has a less ruffled heme due to its iron d⁶ configuration