Observation of Coherent Symmetry-Breaking Vibration by Polarization-Dependent Femtosecond Spectroscopy

Understanding photoinduced chemical reactions beyond the Born–Oppenheimer paradigm requires a comprehensive examination of vibronic interactions. Although femtosecond studies have unveiled the influence of vibrational modes strongly coupled to ultrafast intramolecular reactions in the excited state, they often lack direct observations of how vibrations modulate electronic properties due to the rapid disappearance of reactants. To address this gap, our research investigates the dynamics of photoexcited molecules that do not react. Specifically, we focus on the coherent librational motion of molecular transition dipole moments, discovering that the coherent libration primarily originates from symmetry-breaking components in vibronically excited vibrational modes. Symmetry breaking motion can significantly impact the excited-state dynamics of highly symmetric molecules, potentially leading to nonadiabatic transitions. In essence, the data analysis framework introduced in this study can be harnessed to uncover potential reactivity in photoexcited molecules, further enhancing our understanding of the mechanisms governing these reactions

Active Role of Proton in Excited State Intramolecular Proton Transfer Reaction

Proton transfer is one of the most important elementary reactions in chemistry and biology. The role of proton in the course of proton transfer, whether it is active or passive, has been the subject of intense investigations. Here we demonstrate the active role of proton in the excited state intramolecular proton transfer (ESIPT) of 10-hydroxybenzo­[<i>h</i>]­quinoline (HBQ). The ESIPT of HBQ proceeds in 12 ± 6 fs, and the rate is slowed down to 25 ± 5 fs for DBQ where the reactive hydrogen is replaced by deuterium. The results are consistent with the ballistic proton wave packet transfer within the experimental uncertainty. This ultrafast proton transfer leads to the coherent excitation of the vibrational modes of the product state. In contrast, ESIPT of 2-(2′-hydroxyphenyl)­benzothiazole (HBT) is much slower at 62 fs and shows no isotope dependence implying complete passive role of the proton

Multifaceted Ultrafast Intramolecular Charge Transfer Dynamics of 4‑(Dimethylamino)benzonitrile (DMABN)

Intramolecular charge transfer (ICT) of DMABN has been the subject of extensive investigations. Through the measurements of highly time-resolved fluorescence spectra (TRFS) over the whole emission region, we have examined the ICT dynamics of DMABN in acetonitrile free from the solvation dynamics and vibronic relaxation. The ICT dynamics was found to be characterized by a broad range of time scales; nearly instantaneous (<30 fs), 160 fs, and 3.3 ps. TRFS revealed that an ICT state with partially twisted geometry, ICT­(P), is formed within a few hundred femtoseconds either directly from the initial photoexcited state or via the locally excited (LE) state. The ICT­(P) state undergoes further relaxation along the intramolecular nuclear coordinate to reach the twisted ICT (TICT) state with the time constant of 4.8 ps. A conformational diversity along the rotation of the dimethylamino group was speculated to account for the observed diffusive dynamics

Excited State Intramolecular Proton Transfer and Charge Transfer Dynamics of a 2-(2′-Hydroxyphenyl)benzoxazole Derivative in Solution

Excited state intramolecular proton transfer (ESIPT) and subsequent intramolecular charge transfer (ICT) dynamics of a 2-(2′-hydroxyphenyl)benzoxazole derivative conjugated with an electron withdrawing group (HBOCE) in solutions and a polymer film has been investigated by femtosecond time-resolved fluorescence (TRF) and TRF spectra measurements without the conventional spectral reconstruction method. TRF with high enough resolution (<100 fs) reveals that the ESIPT dynamics of HBOCE in liquids proceeds by at least two time constants of ∼250 fs and ∼1.2 ps. The relative amplitude of the slower picosecond component is smaller in the polymer film than that in solution. Conformational heterogeneity in the ground state originating from the dispersion of the dihedral angle between the phenolic and benzoxazole groups is invoked to account for the dispersive ESIPT dynamics in liquids. From the TRF spectra of both the enol and keto isomers, we have identified the ICT reaction of the keto isomer occurring subsequent to the ESIPT. The ICT proceeds also by two time constants of near instantaneous and 2.7 ps. Since the ICT dynamics of HBOCE is rather close to the polar solvation dynamics, we argue that the ICT is barrierless and determined mostly by the solvent fluctuation

D−π–A-Structured Porphyrins with Extended Auxiliary π‑Spacers for Highly Efficient Dye-Sensitized Solar Cells

Zn­(II)-porphyrin dyes (SGT-030 and SGT-031) with extended auxiliary π-spacers in the donor (D) part have been prepared and applied to dye-sensitized solar cells (DSSCs). The porphyrin dyes contained the same D–ethynyl–zinc porphyrinyl (ZnP)–ethynyl–benzothiadiazole-acceptor platform, but their donor groups varied from phenylene (Ph) in SGT-053 as a reference dye to the thieno­[3,2-b]­benzothiophene (TBT) and 4-hexyl-4H-thieno­[3,2-b]­indole (TI) moieties in SGT-030 and SGT-031, respectively. The effects of the extended auxiliary π-spacer in the D−π–A-structured porphyrin sensitizers on the molecular and photovoltaic properties were investigated via photophysical and electrochemical experiments as well as theoretical calculations. With the trend in conjugation length (Ph TBT ≈ TI) and the donating ability of the π-spacer (Ph TBT TI), the absorption maxima and molecular absorptivity increased in the order SGT-053 (Ph) (TBT) (TI). The incorporation of TBT and TI promoted significant enhancements in the light-harvesting properties by reducing the energy gap and efficiently improving electronic communication. The DSSCs based on SGT-030 (10.80%) and SGT-031 (10.89%) with coadsorption of 4-(3,6-bis­(4-((2-ethylhexyl)­oxy)­phenyl)-9H-carbazol-9-yl)­benzoic acid in conjunction with the [Co­(bpy)3]2+/3+-based electrolyte showed better power conversion efficiency than that of SGT-053 (9.10%). Electrochemical impedance spectroscopy analysis unveiled that the difference in Jsc and Voc originates mainly from the twisted orientation between D and ZnP by the introduction of TBT and TI. This result indicated that the introduction of an extended auxiliary π-spacer in the donor part is a rational molecular design approach to improve photovoltaic performance by enhancing the light-harvesting ability and hindering charge recombination on the TiO2 photoanode

Excitation Energy Transfer within Covalent Tetrahedral Perylenediimide Tetramers and Their Intermolecular Aggregates

Perylenediimides (PDIs) offer a number of attractive characteristics as alternatives to fullerenes in organic photovoltaics (OPVs), including favorable orbital energetics, high extinction coefficients in the visible spectral region, photostability, and the capacity to self-assemble into ordered nanostructures. However, energy transfer followed by charge separation in PDI assemblies must kinetically out-compete excimer formation that limits OPV performance. We report on the excitation energy transfer (EET) rate in a covalently linked PDI tetramer in which the PDI chromophores are arranged in a tetrahedral geometry about a tetraphenyladamantane core. Transient absorption spectroscopy of the tetramer in CH₂Cl₂ reveals a laser intensity-dependent fast absorption decay component indicative of singlet–singlet annihilation resulting from intramolecular EET. Femtosecond fluorescence anisotropy measurements show that the EET time constant τ = 6 ps, which is similar to that predicted for a through-space Förster EET mechanism. Concentration-dependent steady-state spectroscopic studies reveal the formation of intermolecular aggregates of the tetramers in toluene. The aggregates are formed by cofacial π-stacking interactions between PDIs of neighboring tetramers. Transient absorption spectra of the aggregated tetramers in toluene solution demonstrate long-lived excited-state decay dynamics (τ ∼ 30 ns) in agreement with previous observations of PDI excimers

Coherent Nuclear Wave Packets Generated by Ultrafast Intramolecular Charge-Transfer Reaction

Intramolecular charge-transfer (ICT) dynamics, including reaction coordinates, structural changes, and reaction rate, has been noted experimentally and theoretically. Here we report the ICT dynamics of laurdan investigated by time-resolved fluorescence at extreme time resolution of 30 fs. A single high-frequency coherent nuclear wave-packet motion on the product potential surface is observed through the modulation of the fluorescence intensity in time. Theory and experiment show that this vibrational mode involves large displacement of the carbon atoms in the naphthalene backbone, which indicates that the naphthalene backbone coordinates are strongly coupled to the ICT reaction of laurdan, not the twisting or planarization of the dimethylamino group

Dynamics of Photoinduced Intramolecular and Intermolecular Electron Transfers in Ligand-Conjugated Ir(III)–Re(I) Photocatalysts

We report the electron transfer (ET) dynamics in a series of Ir(III)–Re(I) photocatalysts where two bipyridyl ligands of Ir and Re moieties are conjugated at the meta (m)- or para (p)-position of each side. Femtosecond transient absorption (TA) measurements identify the intramolecular ET (IET) dynamics from the Ir to Re moiety, followed by the formation of one-electron-reduced species (OERS) via the intermolecular ET with a sacrificial electron donor (SED). The IET rate depends on the bridging ligand (BL) structures (∼25 ps for BLmm/mp vs ∼68 ps for BLpm/pp), while the OERS formation happens on an even slower time scale (∼1.4 ns). Connecting the Re moiety at the meta-position of the bipyridyl of the Ir moiety can restrict the rotation around a covalent bond between two bipyridyl ligands by steric hindrances and facilitate the IET process. This highlights the importance of BL structures on the ET dynamics in photocatalysts

Perturbation of Electronic States and Energy Relaxation Dynamics in a Series of Phenylene Bridged Zn^II Porphyrin Dimers

We have investigated the perturbed electronic states and energy relaxation dynamics of a series of phenylene bridged ZnII porphyrin dimers to reveal the effects of phenylene bridges by using computational and time-resolved spectroscopic methods. The electronic states of ZnII porphyrin dimers are largely perturbed by the interchromophore interactions that can be controlled by the phenylene bridges; linking position and number of phenyl rings. On the basis of our observations, we can gain further insight into the dipole−dipole interactions and through-bond/through-space electronic exchange interactions between the neighboring porphyrin moieties, which provides a firm basis for further understanding the modification of photophysical properties caused by the phenylene bridge in multiporphyrin assemblies as artificial light-harvesting apparatus