Coherent Vibration and Femtosecond Dynamics of the Platinum Complex Oligomers upon Intermolecular Bond Formation in the Excited State

Femtosecond time-resolved absorption and picosecond time-resolved emission measurements were carried out for highly concentrated aqueous solutions of K₂[Pt(CN)₄] to investigate excited-state dynamics of the [Pt(CN)₄²⁻] oligomers formed with metallophilic interactions. Time-resolved absorption spectra exhibit complicated dynamics that are represented with five time constants. Among them, the 90-ps and 400-ps dynamics were assigned to the S₁ → T₁ intersystem crossing of the trimer and tetramer coexisting in the solution by comparison with the fluorescence decays. Clear oscillations of transient absorption were observed in the first few picoseconds, and the frequency-detected-wavelength 2D analysis revealed that the 135-cm⁻¹ and 65-cm⁻¹ oscillations arise from the Pt–Pt stretch motions of the S₁ trimer and S₁ tetramer, respectively. The obtained time-resolved spectroscopic data provide a clear view of the excited-state dynamics of the [Pt(CN)₄²⁻] oligomers in the femto-/picosecond time region.     

Comparison of Phosphorescent Pt(II) Complexes with C^N^N vs N^N^N Chelators and Caffeine-Based NHC-co-ligands†

In this work, we explored coordination compounds featuring caffeine-based carbene co-ligands and tridentate dianionic pincer luminophores derived from 2,6-bis(1H-1,2,4-triazol-5-yl)pyridine (N), as well as from 2-phenyl-6-(1H-1,2,4-triazol-5-yl)pyridine (C), bearing either Ad (adamantyl) or tBu (tertiary butyl) substituents. The new 2-phenyl-6-(1H-1,2,4-triazol-5-yl)pyridine-based ligand precursors along with four Pt(II) complexes, namely Pt(C-tBu), Pt(C-Ad), Pt(N-tBu) and Pt(N-Ad) were characterized. Further on, the influence of the different substituents at the chelating luminophores and of the caffeine-based NHC-co-ligand on the photophysical properties (including photoluminescence quantum yields (Φ_L), excited-state lifetimes (τ), radiative (k_r), and non-radiative (k_nr) deactivation rate constants) was assessed in fluid solutions at room temperature (RT) and in frozen glassy matrices at 77 K. All four luminophores perform equivalently well within the experimental uncertainty. In deoxygenated fluid solutions at RT, photoluminescence quantum yields reaching up to 24 ± 2% and excited-state lifetimes of around 12 μs were found. The generally long excited-state lifetimes and only minor blue shift upon cooling to 77 K along with mostly well-resolved vibrational progressions point to metal-perturbed ligand-centered excited states. Notably, the yield of the complexation reaction in case of Pt(C-tBu) and Pt(C-Ad) was almost two times higher compared to Pt(N-tBu) and Pt(N-Ad). Cyclometallation is not an essential feature to achieve high photoluminescence quantum yields, but it can improve the synthetic efficiency. In summary, it can be observed that coordination chemical concepts based on natural products can lead to stable phosphorescent species with interesting excited-state properties.     

Solvent effects on the excited-state processes of protochlorophyllide: a femtosecond time-resolved absorption study

The excited-state dynamics of protochlorophyllide a, a porphyrin-like compound and, as substrate of the NADPH/protochlorophyllide oxidoreductase, a precursor of chlorophyll biosynthesis, is studied by femtosecond absorption spectroscopy in a variety of solvents, which were chosen to mimic different environmental conditions in the oxidoreductase complex. In the polar solvents methanol and acetonitrile, the excited-state dynamics differs significantly from that in the nonpolar solvent cyclohexane. In methanol and acetonitrile, the relaxation dynamics is multiexponential with three distinguishable time scales of 4.0-4.5 ps for vibrational relaxation and vibrational energy redistribution of the initially excited S1 state, 22-27 ps for the formation of an intermediate state, most likely with a charge transfer character, and 200 ps for the decay of this intermediate state back to the ground state. In the nonpolar solvent cyclohexane, only the 4.5 ps relaxational process can be observed, whereas the intermediate intramolecular charge transfer state is not populated any longer. In addition to polarity, solvent viscosity also affects the excited-state processes. Upon increasing the viscosity by adding up to 60% glycerol to a methanolic solution, a deceleration of the 4 and 22 ps decay rates from the values in pure methanol is found. Apparently not only vibrational cooling of the S1 excited state is slowed in the more viscous surrounding, but the formation rate of the intramolecular charge transfer state is also reduced, suggesting that nuclear motions along a reaction coordinate are involved in the charge transfer. The results of the present study further specify the model of the excited-state dynamics in protochlorophyllide a as recently suggested (Chem. Phys. Lett. 2004, 397, 110).     

Ultrafast Decay Dynamics of N-Ethylpyrrole Excited to the S1 Electronic State: A Femtosecond Time-Resolved Photoelectron Imaging Study

N-ethylpyrrole is one of ethyl-substituted derivatives of pyrrole and its excited-state decay dynamics has never been explored. In this work, we investigate ultrafast decay dynamics of N-ethylpyrrole excited to the S₁ electronic state using a femtosecond time-resolved photoelectron imaging method. Two pump wavelengths of 241.9 and 237.7 nm are employed. At 241.9 nm, three time constants, 5.0±0.7 ps, 66.4±15.6 ps and 1.3±0.1 ns, are derived. For 237.7 nm, two time constants of 2.1±0.1 ps and 13.1±1.2 ps are derived. We assign all these time constants to be associated with different vibrational states in the S₁ state. The possible decay mechanisms of different S₁ vibrational states are briefly discussed.     

Femtosecond dynamics of excited-state intramolecular proton transfer in <Emphasis Type="Italic">o</Emphasis>-tosylaminobenzaldehyde

Dynamics of excited-state intramolecular proton transfer (ESIPT) in o-tosylaminobenzaldehyde has been studied by femtosecond absorption spectroscopy with a time resolution of 30 fs. The characteristic time of this process is ∼100 fs. Differential absorption rate curves exhibit oscillations that are consistent with theoretically predicted ESIPT-promoting vibrational modes. Efficient nonradiative deactivation with a rate constant of 6.25 × 10¹⁰ s⁻¹ occurs in the excited product of proton transfer, with internal rotation followed by intersystem crossing being one of the feasible deactivation pathways.     

Ultrafast pump-probe study of excited-state charge-transfer dynamics in umecyanin from horseradish root

We have applied femtosecond pump-probe spectroscopy to investigate the excited-state dynamics of umecyanin from horseradish roots, by exciting its 600-nm ligand-to-metal charge-transfer band with a 15-fs pulse and probing over a broad range in the visible region. The decay of the pump-induced ground-state bleaching is modulated by clearly visible oscillations and occurs exponentially with a time constant depending on the observed spectral component of the transmission difference signal, ranging from 270 fs up to 700 fs. The slower decaying process characterizes the spectral component corresponding to the metal-to-ligand charge-transfer transition. The excited-state decay rate is significantly lower than in other blue copper proteins, probably because of the larger energy gap between ligand- and metal-based orbitals in umecyanin. Wavelength dependence of the recovery times could be due to either the excitation of several transitions or the occurrence of intramolecular vibrational relaxation within the excited state. We also find evidence of a hot ground-state absorption, at 700 nm, persisting for several picoseconds. The vibrational coherence induced by the ultrashort pump pulse allows vibrational activity to be observed, mainly in the ground state, as expected in a system with fast excited-state decay. However, we find evidence of a rapidly damped oscillation, which we assign to the excited state. Finally, the Fourier transform of the oscillatory component of the signal presents additional bands in the low-frequency region which are assigned to collective motions of the protein.     

Excited-State THz Vibrations in Aggregates of PtII Complexes Contribute to the Enhancement of Near-Infrared Emission Efficiencies**

The exploration of deactivation mechanisms for near-infrared(NIR)-emissive organic molecules has been a key issue in chemistry, materials science and molecular biology. In this study, based on transient absorption spectroscopy and transient grating photoluminescence spectroscopy, we demonstrate that the aggregated Pt^II complex 4H (efficient NIR emitter) exhibits collective out-of-plane motions with a frequency of 32 cm⁻¹ (0.96 THz) in the excited states. Importantly, similar THz characteristics were also observed in analogous Pt^II complexes with prominent NIR emission efficiency. The conservation of THz motions enables excited-state deactivation to proceed along low-frequency vibrational coordinates, contributing to the suppression of nonradiative decay and remarkable NIR emission. These novel results highlight the significance of excited-state vibrations in nonradiative processes, which serve as a benchmark for improving device performance.     

Vibrational Spectra and Molecular Vibrational Behaviors of All-Carboatomic Rings,cyclo[18]carbon and Its Analogues

The vibrational spectra of cyclo[18]carbon and its analogues, cyclo[2n]carbon (n=3 to 15), were carefully simulated and characterized. The in-plane C−C stretching vibrations shows strong rigidity, while out-of-plane motions seem to be extremely flexible. The solvation effect can enhance signal strengths of the vibrational spectra, but does not evidently change the shape of the spectral curves. The infrared and Raman spectra of cyclo[2n]carbons are quite sensitive to ring size in the range of n=3 to 7, while the size only modestly affects peak positions and strengths for larger rings. Molecular dynamic trajectories show that the fluctuation period of the skeleton of cyclo[18]carbon is basically constant at different temperatures, and they are all about 300 fs. With increase of simulation temperature, the ring distortion due to thermal motion is notable and becomes much stronger. However, neither ring breaking nor isomerization in cyclo[18]carbon is observed during the simulations untill 298.15 K.     

Excited-State Aromaticity of Gold(III) Hexaphyrins and Metalation Effect Investigated by Time-Resolved Electronic and Vibrational Spectroscopy

Expanded porphyrins with appropriate metalation provide an excellent opportunity to study excited-state aromaticity. The coordinated metal allows the excited-state aromaticity in the triplet state to be detected through the heavy-atom effect, but other metalation effects on the excited-state aromaticity were ambiguous. Herein, the excited-state aromaticity of gold(III) hexaphyrins through the relaxation dynamics was revealed via electronic and vibrational spectroscopy. The S_Q states of gold [26]- and [28]-hexaphyrins showed interconvertible absorption and IR spectra with those of counterparts in the ground-state, indicating aromaticity reversal. Furthermore, while the T₁ states of gold [28]-hexaphyrins also exhibited reversed aromaticity according to Baird's rule, the ligand-to-metal charge-transfer state of gold [26]-hexaphyrins contributed by the gold metal showed non-aromatic features arising from the odd-number of π-electrons.     

Probing Effect of Solvation on Photoexcited Quadrupolar Donor-Acceptor-Donor Molecule via Ultrafast Raman Spectroscopy

The symmetric and quadrupolar donor-acceptordonor (D-A-D) molecules usually exhibit excitedstate charge redistribution process from delocalized intramolecular charge transfer (ICT) state to localized ICT state. Direct observation of such charge redistribution process in real-time has been intensively studied via various ultrafast time-resolved spectroscopies. Femtosecond stimulated Raman spectroscopy (FSRS) is one of the powerful methods which can be used to determine the excited state dynamics by tracking vibrational mode evolution of the specific chemical bonds within molecules. Herein, a molecule, 4, 4′-(buta-1, 3-diyne-1, 4-diyl)bis(N, N-bis(4-methoxyphenyl)aniline), that consists of two central adjacent alkyne (-C≡C-) groups as electron-acceptors and two separated, symmetric N, N-bis(4-methoxyphenyl)aniline at both branches as electron-donors, is chosen to investigate the excited-state photophysical properties. It is shown that the solvation induced excited-state charge redistribution in polar solvents can be probed by using femtosecond stimulated Raman spectroscopy. The results provide a fundamental understanding of photoexcitation induced charge delocalization/localization properties of the symmetric quadrupolar molecules with adjacent vibrational markers located at central position.

     

Red and yellow solvates of chloro­(2,2′:6′,2′′‐terpyridine)platinum(II) chloride and Pt⋯Pt inter­actions

Crystallization of chloro­(2,2′:6′,2′′‐terpyridine)platinum(II) chloride from dimethyl sulfoxide yields a red polymorph, [PtCl(C₁₅H₁₁N₃)]Cl·C₂H₆OS, (I), which exhibits stacking along the a axis through pairs of Pt⋯Pt(−x, −y, −z) inter­actions of 3.3155 (8) Å. The cations are further associated through close Pt⋯Pt(1 − x, −y, −z) distances of 3.4360 (8) Å. Recrystallization from water gives a mero­hedrally twinned yellow–orange dihydrate form, [PtCl(C₁₅H₁₁N₃)]Cl·2H₂O, (II), with pairwise short Pt⋯Pt(1 − x, 2 − y, −z) contacts of 3.3903 (5) Å but no long‐range stacking through the crystals. Inter­pair Pt⋯Pt(−x, 2 − y, −z) distances between cation pairs in the hydrate are 4.3269 (5) Å.     

Ultrafast Structural Dynamics of Biomolecules Examined by Multiple‐Mode 2D IR Spectroscopy: Anharmonically Coupled Motions are in Harmony

Quantum chemical computations, molecular dynamics simulations, and linear and nonlinear infrared spectral simulations are carried out for four representative biomolecules: cellobiose, alanine tripeptide, L ‐α‐glycerylphosphorylethanolamine, and the DNA base monomer guanine. Anharmonic transition frequencies and anharmonicities for the molecules in vacuum are evaluated. Instantaneous normal‐mode analysis is performed and the vibrational frequency distribution correlations are examined for the molecules solvated in TIP3P water. Many local and regional motions of the biomolecules are predicted to be anharmonically coupled and their vibrational frequencies are predicted to be largely correlated. These coupled and correlated vibrational motions can be easily visualized by pairwise cross peaks in the femtosecond broadband two‐dimensional infrared (2D IR) spectra, which are simulated using time‐domain third‐order nonlinear response functions. A network of distinctive spectral profiles of the 2D IR cross peaks, including peak orientations and positive and negative signal patterns, are shown to be intimately connected with the couplings and correlations. The results show that the vibrational couplings and correlations, driven by solvent interactions and also by intrinsic vibrational interactions, are vibrational mode dependent and thus chemical group dependent, and form the structural and dynamical basis of the anharmonic vibrators that are ubiquitous in biomolecules.     

E−Z Isomerization in Guanidine: Second-order Saddle Dynamics,Non-statisticality,and Time-frequency Analysis

Our recent work on the E−Z isomerization reaction of guanidine using ab initio chemical dynamics simulations [Rashmi et al., Regul. Chaotic Dyn. 2021 , 26, 119] emphasized the role of second-order saddle ( SOS ) in the isomerization reaction; however, we could not unequivocally establish the non-statistical nature of the dynamics followed in the reaction. In the present study, we performed thousands of on-the-fly trajectories using forces computed at the MNDO level to investigate the influence of second-order saddle in the E−Z isomerization reaction of guanidine and the role of intramolecular vibrational energy redistribution (IVR) on the reaction dynamics. The simulations reveal that while majority of the trajectories follow the traditional transition state pathways, 15 % of the trajectories follow the SOS path. The dynamics was found to be highly non-statistical with the survival probabilities of the reactants showing large deviations from those obtained within the RRKM assumptions. In addition, a detailed analysis of the dynamics using time-dependent frequencies and the frequency ratio spaces reveal the existence of multiple resonance junctions that indicate the existence of regular dynamics and long-lived quasi-periodic trajectories in the phase space associated with non-RRKM behavior.     

Coherent nuclear wavepacket motions in ultrafast excited-state intramolecular proton transfer: sub-30-fs resolved pump-probe absorption spectroscopy of 10-hydroxybenzo[h]quinoline in solution

The dynamics of the excited-state intramolecular proton transfer of 10-hydroxybenzo[h]quinoline (10-HBQ) and the associated coherent nuclear motion were investigated in solution by femtosecond absorption spectroscopy. Sub-picosecond transient absorption measurements revealed spectral features of the stimulated emission and absorption of the keto excited state (the product of the reaction). The stimulated emission band appeared in the 600-800-nm region, corresponding to the wavelength region of the steady-state keto fluorescence. It showed successive temporal changes with time constants of 350 fs and 8.3 ps and then disappeared with the lifetime of the keto excited state (260 ps). The spectral feature of the stimulated emission changed in the 350-fs dynamics, which was likely assignable to the intramolecular vibrational energy redistribution in the keto excited state. The 8.3-ps change caused a spectral blue shift and was attributed to the vibrational cooling process. The excited-state absorption was observed in the 400-600-nm region, and it also showed temporal changes characterized by the 350-fs and 8.3-ps components. To examine the coherent nuclear dynamics (nuclear wavepacket motion) in excited-state 10-HBQ, we carried out pump-probe measurements of the stimulated emission and absorption signals with time resolution as good as 27 fs. The obtained data showed substantially modulated signals due to the excited-state vibrational coherence up to a delay time of several picoseconds after photoexcitation. This means that the vibrational coherence created by photoexcitation in the enol excited state is transferred to the product. Fourier transform analysis indicated that four frequency components in the 200-700-cm(-1) region contribute to the oscillatory signal, corresponding to the coherent nuclear motions in excited-state 10-HBQ. Especially, the lowest-frequency mode at 242 cm(-1) is dephased significantly faster than the other three modes. This observation was regarded as a manifestation that the nuclear motion of the 242-cm(-1) mode is correlated with the structural change of the molecule associated with the reaction (the reaction coordinate). The 242-cm(-1) mode observed in excited-state 10-HBQ was assigned to a vibration corresponding to the ground-state vibration at 243 cm(-1) by referring to the results of resonance Raman measurements and density functional calculations. It was found that the nuclear motion of this lowest-frequency mode involves a large displacement of the OH group toward the nitrogen site as well as in-plane skeletal deformation that assists the oxygen and nitrogen atoms to come closer to each other. We discuss the importance of the nuclear wavepacket motion on a multidimensional potential-energy surface including the vibrational coordinate of the low-frequency modes.     

Phosphorescent Cationic Iridium (III) Complexes with 1,3,4-Oxadiazole Cyclometalating Ligands:Solvent-Dependent Excited-State Dynamics

To elucidate the nature of low-lying triplet states and the effect of ligand modifications on the excited-state properties of functional cationic iridium complexes,the solventdependent excited-state dynamics of two phosphorescent cationic iridium (III) complexes,namely[Ir (dph-oxd)₂(bpy)]PF₆( 1 ) and[Ir (dph-oxd)₂(pzpy)]PF₆( 2 ),were investigated by femtosecond and nanosecond transient absorption spectroscopy.Upon photoexcitation to the metal-to-ligand charge-transfer (MLCT) states,the excited-state dynamics shows a rapid process (τ=0.7-3 ps) for the formation of solvent stabilized ³MLCT states,which significantly depends on the solvent polarity for both 1 and 2 .Sequentially,a relatively slow process assigned to the vibrational cooling/geometrical relaxation and a long-lived phosphorescent emissive state is identified.Due to the different excited-state electronic structures regulated by ancillary ligands,the solvation-induced stabilization of the ³MLCT state in 1 is faster than that in 2 .The present results provide a better sight of excited-state relaxation dynamics of ligand-related iridium (III) complexes and solvation effects on triplet manifolds.     

Optical control of coherent lattice motions probed by femtosecond electron diffraction

We report the study of laser-induced coherent lattice motions using femtosecond electron diffraction. The oscillations of Bragg peak positions associated with a damped lattice vibration along the surface normal were directly observed in real time and with sub-milli-angstrom spatial resolution. In addition, by using a pair of optical excitation pulses and varying their time delay and relative pulse intensities, we demonstrated the successful control of coherent lattice motions.     

Theoretical study of the external electric field effect on the HeH2 (2 1A′) metastable state

A possibility of stabilization in an external homogeneous electric field on the HeH₂(2¹ A′) metastable state relative to its nonadiabatic decay into the 1 ¹A′ state is investigated theoretically. If the lowest vibrational levels of the HeH₂ (2 ¹A′) state might be stabilized, one would try to detect the radiative transition 1 ¹A′ ← 2 ¹A′ from these levels and thus to verify experimentally the existence of the HeH₂ (2 ¹A′) state. It is shown that the separation between the potential energy surfaces (PESs) of the states in question and the energy of their avoided crossing point do increase in the field. Moreover, it is shown that in the fields with strength up to 10⁷ V/cm the nonadiabatic decay of the 2 ¹A′ state along one of the vibrational modes may be closed. However, along two other modes such decay still may occur. It is noted nevertheless that the question of such stabilization possibility in inhomogeneous fields (i.e., in ionic crystal or on its surface) is still open. The effect of the field was taken into account within an analytic two-level model. The unperturbed PESs were calculated by the CASSCF method. © 1996 John Wiley & Sons, Inc.