Time-resolved Transient Absorption Spectra Research Articles

Structural and photophysical properties of two-dimensional lead bromide hybrid perovskite (C8H17NH3)2PbBr4

The structural versatility of organic compounds imparts manifold properties to hybrid organic−inorganic perovskites, where two-dimensional (2D) halide perovskites are potential future platforms for light-emitting devices because of their unique quantum well configuration and dielectric confinement. Notwithstanding the numerous studies on the organic−inorganic perovskites carried out to date, precisely charactering the structure and photoluminescence mechanism remain considerable challenges. Herein, by applying Rietveld refinement, we have obtained crystal structure and atomic sites of 2D layered OA2PbBr4 perovskite, which undergoes a phase transition at 190 K with activation energies of 29.6 and 3.8 meV for the two phases, respectively. The theoretical analysis based on the crystal structure were performed to understand the nature of vibrations and band structure. Steady-state PL shows a narrow emission and a broad emission located at 409 and 487 nm, respectively. With temperature cooling down to 118 K, narrow emission gradually narrowed and enhanced. Meanwhile, intensity of broad emission first weakens and then increases. The turning point occurs near 198 K. By applying time-resolved PL (TRPL) and transient absorption (TA) spectra experiment, we observed a rapid carrier transfer from the FE to the STE state, and demonstrated the narrow and broad bands originate from free exciton and self-trapped exciton emission, respectively. And the self-trapping depth is proved to be 26.3 meV by low-temperature PL. Our findings indicate the synthesized 2D perovskite has potential applications in optoelectronic and display devices.

Photocatalytic Upgrading of Polylactic Acid Waste into Alanine under Mild Conditions.

Polylactic acid (PLA) has attracted increasing interest as a sustainable plastic because it can be degraded into CO2 and H2O in nature. However, this process is sluggish, and even worse, it is a CO2-emitting and carbon resource waste process. Therefore, it is highly urgent to develop a novel strategy for recycling post-consumer PLA to achieve a circular plastic economy. Herein, we report a one-pot photoreforming route for the efficient and selective amination of PLA waste into value-added alanine using CoP/CdS catalysts under mild conditions. Results show the alanine production rate can reach up to 2.4 mmol gcat -1 h-1, with a high selectivity (>75 %) and excellent stability. Time-resolved transient absorption spectra (TAS) reveal that CoP can rapidly extract photogenerated electrons from CdS to accelerate proton reduction, favoring hole-dominated PLA oxidation to coproduce alanine. This study offers an appealing way for upcycling PLA waste and creates new opportunities for green synthesis of amino acids.

Photocatalytic Upgrading of Polylactic Acid Waste into Alanine under Mild Conditions

AbstractPolylactic acid (PLA) has attracted increasing interest as a sustainable plastic because it can be degraded into CO2 and H2O in nature. However, this process is sluggish, and even worse, it is a CO2‐emitting and carbon resource waste process. Therefore, it is highly urgent to develop a novel strategy for recycling post‐consumer PLA to achieve a circular plastic economy. Herein, we report a one‐pot photoreforming route for the efficient and selective amination of PLA waste into value‐added alanine using CoP/CdS catalysts under mild conditions. Results show the alanine production rate can reach up to 2.4 mmol gcat−1 h−1, with a high selectivity (>75 %) and excellent stability. Time‐resolved transient absorption spectra (TAS) reveal that CoP can rapidly extract photogenerated electrons from CdS to accelerate proton reduction, favoring hole‐dominated PLA oxidation to coproduce alanine. This study offers an appealing way for upcycling PLA waste and creates new opportunities for green synthesis of amino acids.

Impact of Surface Trap States on Electron and Energy Transfer in CdSe Quantum Dots Studied by Femtosecond Transient Absorption Spectroscopy.

The presence of surface trap states (STSs) is one of the key factors to affect the electronic and optical properties of quantum dots (QDs), however, the exact mechanism of how STSs influence QDs remains unclear. Herein, we demonstrated the impact of STSs on electron transfer in CdSe QDs and triplet-triplet energy transfer (TTET) from CdSe to surface acceptor using femtosecond transient absorption spectroscopy. Three types of colloidal CdSe QDs, each containing various degrees of STSs as evidenced by photoluminescence and X-ray photoelectron spectroscopy, were employed. Time-resolved emission and transient absorption spectra revealed that STSs can suppress band-edge emission effectively, resulting in a remarkable decrease in the lifetime of photoelectrons in QDs from 17.1 ns to 4.9 ns. Moreover, the investigation of TTET process revealed that STSs can suppress the generation of triplet exciton and effectively inhibit band-edge emission, leading to a significant decrease in TTET from CdSe QDs to the surface acceptor. This work presented evidence for STSs influence in shaping the optoelectronic properties of QDs, making it a valuable point of reference for understanding and manipulating STSs in diverse QDs-based optoelectronic applications involving electron and energy transfer.

Excited-State Dynamics Simulations of a Light-Driven Molecular Motor in Solution.

Molecular motors, where light can be transformed into motion, are promising in the design of nanomechanical devices. For applications, however, finding relationships between molecular motion and the environment is important. Here, we report the study of excited-state dynamics of an overcrowded alkene in solution using a hybrid quantum mechanics/molecular mechanics (QM/MM) approach combined with excited-state molecular dynamics simulations. Using QM/MM surface-hopping trajectories, we calculated time-resolved emission and transient absorption spectra. These show the rise of a short-lived Franck-Condon state, followed by the formation of a dark state in the first 150 fs before the molecular motor relaxes to the ground state in about 1 ps. From the analysis of radial distribution functions, we infer that the orientation of the solvent with respect to the molecular motor in the electronic excited state is similar to that in the ground state during the photoisomerization.

Frontispiece: Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor‐Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy

Unraveling the complicated photophysics of thermally activated delayed fluorescence (TADF) compounds is not a trivial task, although it is critical to improve the performance of TADF emitters in OLED. In the Review by Zhao and colleagues (DOI: 10.1002/chem.202203737), recent developments of using the transient optical spectroscopic methods to study the photophysics of TADF emitters are summarized, e. g. the manifestation of the theoretically proposed spin-vibronic coupling effect and the three-state model describing the reverse intersystem crossing (RISC) in time-resolved transient absorption spectra.

Fluorenone-based covalent organic frameworks with efficient exciton dissociation and well-defined active center for remarkable photocatalytic hydrogen evolution

Precisely modulating both the fine structures and exciton behaviors of covalent organic framework (COF)-based photocatalysts for efficient photocatalytic hydrogen evolution is still a great challenge. Herein, we report two COFs, fluorenone-based COF (FOO-COF) and fluorenyl-based COF (FO-COF), for photocatalytic H2 evolution. The results demonstrated that FOO-COF achieved a remarkable stability and hydrogen evolution rate of 119.1 mmol/h/g, surpassing those of many previously reported COF-based photocatalysts, thus resulting in an apparent quantum efficiency up to 20.5% at 435 nm. The joint observations from temperature-dependent photoluminescence, the delayed fluorescence and phosphorescence spectroscopy confirm that FOO-COF shows a stronger carrier separation and migration ability, much lower exciton binding energy, and faster dissociation speed of singlet and triplet excitons than FO-COF. More importantly, the femtosecond time-resolved transient absorption spectra (fs-TAS) revealed the great difference between the two COFs in electron trapping and exciton dissociation after Pt loading. Furthermore, theoretical calculations unveiled the feasibility of CO groups in the LUMO sites of FOO-COF as active electronic centers for anchoring the photodeposited Pt cocatalysts, due to its much lower Gibbs free energy of hydrogen adsorption and more negative charge density. We believe that the introduction of CO groups in the FOO-COF can act as both a dominant electronic collection center and partial active sites, which improve the D-A polarizability and boost the generation, separation, migration, and utilization of photogenerated carriers in the FOO-COF for achieving remarkable photocatalytic hydrogen evolution. The insights into the molecular-level mechanism in this work may guide the significance of the rational design of COF structures for advanced photocatalytic hydrogen evolution.

Synthesis and Theoretical and Photophysical Study on a Series of Neutral Ruthenium(II) Complexes with Donor–Metal–Accepter Configuration

In order to construct a new type of ruthenium(II) terpyridine complexes with activated triplet metal-centered (3MC) states, as well as stabilized triplet metal-to-ligand charge transfer (3MLCT) states, conducive to fine emissive performances, Ru-1, Ru-2, Ru-3, and Ru-4 were synthesized. Compared with the [Ru(terpyridine)2]2+ prototype (0.25 ns), this series of ruthenium(II) terpyridine complexes exhibit lengthened excited state lifetime (43.3 ns for Ru-1, 52.7 ns for Ru-2, 43.6 ns for Ru-3, and 53.4 ns for Ru-4). Interfragment charge transfer analysis illustrates the electron transfer direction of the four complexes, manifesting their intramolecular charge transfer characteristic. When excited, their lowest-lying triplet states are assigned as 3MLCT based on spin-density surface distribution. The singlet excited states and 3MLCT states were thoroughly studied by UV-visual absorption and nanosecond time-resolved transient absorption spectra, respectively. Photoluminescence spectra revealed their weak broadband near-infrared emission at room temperature and red phosphorescence at 77 K. The low molecular weight and the good thermal stability make Ru-1 and Ru-2 suitable for vaporization coating, while the fine solubility in common organic solvents makes Ru-3 and Ru-4 suitable for solution processing. Furthermore, the intrinsic electroneutrality and favorable energy levels endow them with new potential to be applied in the optoelectronic field.

Solvent-dependent dual emission processes in charge-transfer excited states of phenothiazine-triphenyltriazine conformers

The solvent effects were studied on charge-transfer excited states in a donor–acceptor-type luminescent material, phenothiazine-triphenyltriazine, which has two distinct conformers: quasi-coplanar and perpendicular conformers. The solvent dielectric permittivity dependence of the steady-state photoluminescence spectra using solvent mixtures of toluene and tetrahydrofuran (THF) indicated that the two conformers have nearly identical excited-state dipole moments. The time-resolved photoluminescence and transient absorption spectra in toluene and THF showed different solvent dependences for the two conformers. A detailed analysis of these time-resolved spectra revealed solvent-dependent emission processes in the two conformers.

Truxene-linked coumarin-corrole triad: Synthesis, photophysical properties and application in Triplet−Triplet annihilation upconversion

It is crucial to develop organic triplet photosensitizers (PSs) with excellent performance for upconversion applications. Herein, we synthesized a novel truxene-linked coumarin-corrole triads (C-Trux-Corr) and investigated the photophysical properties by steady-state UV–Vis absorption and fluorescence spectroscopies as well as nanosecond time-resolved transient absorption spectroscopy. The triad displays an efficient intramolecular singlet energy transfer from coumarin moiety to the corrole moiety. Nanosecond time-resolved transient difference absorption spectra indicate that the triplet excited states of C-Trux-Corr are distributed on the corrole moiety. The triplet lifetimes of C-Trux-Corr and Trux-C are 195.2 μs and 45.9 μs, respectively. The triplet state lifetime of C-Trux-Corr is significantly prolonged and it was applied as an organic triplet photosensitizer in the triplet-triplet upconversion system with an upconversion quantum yield of 2.93%, which is higher than that the corrole itself (1.27%) by more than 2-fold.

Integrating polyarylether-COFs with TiO2 nanofibers for enhanced visible-light-driven CO2 reduction in artificial photosynthesis

Artificial photosynthesis is considered a promising strategy to mitigate environmental issues and energy crises. Herein, one-dimensional oxide–semiconductor (TiO2-nanofiber, TNF) and polyarylether-based covalent organic framework (COF-318) were composed to form a visible-light responsive heterojunction (COF-318/TNF) for artificial photosynthesis by simultaneously triggering CO2 reduction and H2O oxidation. Without additional sacrificial agents, photosensitizers or cocatalysts, the optimal sample COF-318/TNF-15 exhibited CO2-to-CO conversion efficiency of 70.1 ± 0.7 µmol∙g−1∙h−1, 12.7 times higher than that of COF-318. The molar ratio of produced CO to O2 was about 2:1, suggesting H2O was the electron donor in the photocatalytic process. Photoelectrochemical measurements and time-resolved transient absorption spectra illustrated the high separation/transfer efficiency of photo-generated charges in COF-318/TNF-15, which was attributed to the construction of heterojunction and the specific one-dimensional property of TiO2 nanofibers. During the CO2 photoreduction, HCOO* was detected as the key intermediate for CO production according to the in-situ FT-IR analysis. This work provides a new route for the rational design of organic–inorganic heterogeneous photocatalysts for visible-light-driven CO2 reduction.

A-PET and Weakened Triplet-Triplet Annihilation Self-Quenching Effects in Benzo-21-Crown-7-Functionalized Diiodo-BODIPY.

Weakening the triplet–triplet annihilation (TTA) self-quenching effect induced by sensitizers remains a tremendous challenge due to the very few investigations carried out on them. Herein, benzo-21-crown-7 (B21C7)-functionalized 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (DIBDP) was synthesized to investigate the influences of huge bulks and electron-rich cavities of B21C7 moieties on the fluorescence emission and triplet-state lifetimes of DIBDP moieties. Density functional theory (DFT)/time-dependent DFT (TDDFT) computable results preliminarily predicted that B21C7 moieties had influences on the fluorescence emissions of DIBDP moieties but not on their localization of triplet states of B21C7-functionalized DIBDP (B21C7-DIBDP). The UV–vis absorption spectra, fluorescence emission spectra, and cyclic voltammograms verified that there was an electron-transfer process from the B21C7 moiety to the DIBDP moiety in B21C7-DIBDP. However, the calculated results of ΔGCS and ECS values and nanosecond time-resolved transient absorption spectra demonstrated that the electron-transfer process from the B21C7 moiety to the DIBDP moiety in B21C7-DIBDP had direct influences on the fluorescence emission of DIBDP moieties but not on the triplet states of DIBDP moieties. The experimental values of triplet-state lifetimes of B21C7-DIBDP were obviously longer than those of DIBDP at a high concentration (1.0 × 10–5 M); however, the fitted values of intrinsic triplet-state lifetimes of B21C7-DIBDP were slightly greater than those of DIBDP in the same solvent. These results demonstrated that the steric hindrance of B21C7 moieties could weaken the TTA self-quenching effect of DIBDP moieties at a high concentration and the a-PET effect induced a proportion of the produced singlet states of DIBDP moieties and could not emit fluorescence in the form of radiation transition but they could be transformed into triplet states through intersystem crossing (ISC) processes due to the iodine atoms in the DIBDP moiety. The stronger a-PET effects in polar solvents induced smaller fluorescence quantum yields so that more singlet states of DIBDP moieties were transformed into triplet states to weaken the TTA self-quenching effects.

Photoinduced energy transfer in truxene-linked zinc porphyrin–fullerene-corrole tetrad and its application in triplet−triplet annihilation upconversion

Triplet photosensitizers (PSs) have attracted widespread interest due to their application in photocatalytic organic reactions, photodynamic therapy (PDT), photoinduced hydrogen production from water splitting, and triplet–triplet annihilation (TTA) upconversion. Herein, the triad T-P-Co and tetrad T-C60-P-Co were synthesized, characterized, and applied in triplet–triplet annihilation (TTA) upconversion. The photophysical processes were investigated with steady-state UV–visible absorption and fluorescence spectroscopies, nanosecond time-resolved transient absorption spectroscopy and electrochemical characterization. T-P-Co and T-C60-P-Co show efficient intramolecular singlet–singlet energy transfer from the porphyrin moiety to corrole/fullerene moiety. The energy transfer efficiency is nearly 100% for T-P-Co and 69.4% for T-C60-P-Co, respectively. Electrochemical investigation shows that the photo-induced intramolecular electron transfer of the excited triad T-P-Co and tetrad T-C60-P-Co is thermodynamically prohibited in toluene due to the positive ΔGCS for charge-separation. Nanosecond time-resolved transient difference absorption spectra indicate that the triplet excited states of T-P-Co and T-C60-P-Co are both distributed on the corrole moiety. The triplet lifetimes of T-P-Co and T-C60-P-Co are 86.9 μs and 73.3 μs, respectively. The triad T-P-Co and tetrad T-C60-P-Co were used as triplet photosensitizers for TTA upconversion, and upconversion quantum yields of 0.76% and 0.84% were observed, respectively.

Salt-air template synthesis of Na and O doped porous graphitic carbon nitride nanorods with exceptional photocatalytic H2 evolution activity

Photocatalyst fabrication process plays the central role in photocatalytic hydrogen (H2) evolution reaction. Herein, we synthesize Na+ functionalized porous graphitic carbon nitride nanorods (Na-CNNR) via a facile one-pot calcination method. The morphology and size of Na-CNNR are controllable by changing the amount of sodium salt; both sodium salt and air are essential to the unique structure and oxygen doping. The obtained Na-CNNR contains abundant oxygen in the graphitic carbon nitride (CN) plane. The optimized Na20-CNNR (20 wt% Na2S2O3•5H2O to dicyanamide) photocatalyst exhibits a high surface area with enhanced visible light absorption. Besides, Na20-CNNR displays fast charge transfer and high carrier separation rate characterized by photoluminescence (PL) spectroscopy and electrochemical test. Through time-resolved transient absorption spectra analysis, the trapped unreactive electron accumulation can be highly restrained, favoring efficient active electron de-trapping and transfer. The optimized Na20-CNNR sample exhibits the highest photocatalytic H2 evolution rate of 7.46 mmol/h/g under visible light irradiation (>400 nm, 100 mW/cm2), which is up to 85 times that of the bare CN and 27 times that of Na+ doped graphitic carbon nitride nanoparticles (Na20–CNNP). Meanwhile, the cyclability tests indicate that Na20-CNNR displays robust stability over 24 h.

Studies of Ultrafast Molecular Dynamics by Femtosecond Extreme Ultraviolet Absorption Spectroscopy

Transient absorption spectroscopy is one of the most widely used experimental techniques for time-resolved studies of ultrafast phenomena. Recent advances in the generation of short-wavelength pulses of ultrashort duration have made transient absorption probing of atomic core level transitions possible. In this Highlight Review, we introduce our recent work on using extreme ultraviolet (XUV) transient absorption spectroscopy to investigate the ultrafast molecular dynamics induced by intense laser fields. While investigations of intense laser-molecule interaction have primarily focused on elucidating electron dynamics, it is important to understand the vibrational dynamics that are triggered by intense laser fields because of their coupling to electron dynamics. Probing the iodine 4d core level, our XUV transient absorption studies of iodoalkanes irradiated by intense laser fields elucidate ultrafast dissociative ionization dynamics and vibrational wave packet dynamics. Time-dependent XUV spectral shifts and global analysis of time-resolved XUV transient absorption spectra can be used to reconstruct C–I dissociation dynamics and identify multiple dissociation channels. In addition, XUV transient absorption spectroscopy also unravels both small- and large-amplitude coherent vibrational motion, whose origin can be identified by analyzing the phase of the oscillatory XUV transition energies. Our studies show that XUV transient absorption spectroscopy is a powerful technique for the study of ultrafast molecular dynamics.

PH dependence of photochemical kinetics of thioxanthen-9-one from nanosecond time-resolved laser flash photolysis

pH dependent fluorescence emission of a thioxanthone-based probe has been reported recently. The potential determinant factors of pH dependence may provide important clues to design novel thioxanthone-based probes in the future. pH dependence of photochemical kinetics of thioxanthone itself was investigated in this work using nanosecond time-resolved laser flash photolysis. The nanosecond time-resolved transient absorption spectra and kinetics of TX in aqueous acetonitrile were recorded, as well as for a model reaction system including TX with diphenylamine (DPA) as a co-initiator. Besides the well-known absorption peak of 3TX*, other peaks at 417, 518, 673 and 780 nm, have been reliably attributed to major intermediates in the overall reaction between TX and DPA with photolysis, which has been confirmed to occur along a multistep process. In the strong acidic solution (pH≈3.0), TX and protonated TX ions (TXH+) coexist due to protonated equilibrium. Consequently, high proton concentration promotes the predominant decay pathway after photolysis from electron transfer to proton affinity. Subsequently, the different primary products, 3TXH+* or TX•−, proceed different secondary reaction channels. In addition, within the wide pH range from weak acid (pH=5.0) to alkaline solution (pH=13.0), the overall reaction mechanism and rates do not show visible changes.

A Real-Time Time-Dependent Density Functional Tight-Binding Implementation for Semiclassical Excited State Electron-Nuclear Dynamics and Pump-Probe Spectroscopy Simulations.

The increasing need to simulate the dynamics of photoexcited molecular systems and nanosystems in the subpicosecond regime demands new efficient tools able to describe the quantum nature of matter at a low computational cost. By combining the power of the approximate DFTB method with the semiclassical Ehrenfest method for nuclear-electron dynamics, we have achieved a real-time time-dependent DFTB (TD-DFTB) implementation that fits such requirements. In addition to enabling the study of nuclear motion effects in photoinduced charge transfer processes, our code adds novel features to the realm of static and time-resolved computational spectroscopies. In particular, the optical properties of periodic materials such as graphene nanoribbons or the use of corrections such as the "LDA+U" and "pseudo SIC" methods to improve the optical properties in some systems can now be handled at the TD-DFTB level. Moreover, the simulation of fully atomistic time-resolved transient absorption spectra and impulsive vibrational spectra can now be achieved within reasonable computing time, owing to the good performance of the implementation and a parallel simulation protocol. Its application to the study of UV/visible light-induced vibrational coherences in molecules is demonstrated and opens a new door into the mechanisms of nonequilibrium ultrafast phenomena in countless materials with relevant applications.

Study on the Ultrafast Process of Perovskite Nanoparticles Modified by Different Alkyl Chains.

Three kinds of perovskite nanoparticles encapsulated with different chain lengths of alkylammonium, (CH3NH3)x(CH3(CH2)3NH3)(1-x)PbBr3 (NP-C4), (CH3NH3)x(CH3(CH2)7NH3)(1-x)PbBr3 (NP-C8), and (CH3NH3)x(CH3(CH2)11NH3)(1-x)PbBr3 (NP-C12), are successfully prepared. X-ray powder diffraction experiments demonstrate that these three nanoparticles are all pure cubic phase. However, the compositions of these three nanoparticles are significantly different, as revealed by steady-state absorption spectra. NP-C4 mainly consists of 2D perovskite with m (number of unit cell layers) = 1 and 3D perovskite. Instead, NP-C8 and NP-C12 are mainly composed of 2D perovskite with m = 3, 4, and 5. Time-resolved fluorescence spectra and femtosecond transient absorption spectra suggest the presence of energy transfer from 2D perovskite to 3D perovskite in these three nanoparticles. More importantly, the energy-transfer rate gradually decreases from NP-C4 to NP-C12. This result suggests that the composition of perovskite nanoparticles and their corresponding photophysical properties can be controlled by the chain length of alkylammonium. This provides a new insight for preparing novel perovskite nanoparticles for special applications.

Acetylene and trans-Ethylene Bridged BIII -Subporphyrin Dimers.

Acetylene and trans-ethylene bridged BIII -subporphyrin dimers were synthesized by cross-coupling reactions of meso-bromo BIII subporphyrin. These dimers display perturbed and red-shifted absorption spectra reaching around 750 nm and fluorescence reaching at around 850 nm with high quantum yields of 0.39 and 0.47, respectively. DFT calculations have revealed that the HOMOs and the LUMOs of both dimers are spread over the two subporphyrin units as an indication of effective conjugation between the two subporphyrin units. The large Stokes shifts and characteristic pico-second time-resolved transient absorption spectra indicated that the S1 -states of the dimers relax with structural changes, which are larger for the trans-ethylene bridged dimer.

Photophysical properties of structural isomers of homoleptic Ir-complexes derived from xylenyl-substituted N-heterocyclic carbene ligands.

The phosphorescence properties of fac-Ir(pmp)3, mer-Ir(pmp)3, fac-Ir(dmpmp)3 and mer-Ir(dmpmp)3 (where pmp = 3-methyl-1-phenyl-2,3-dihydro-1H-imidazo[4,5-b]pyridine and dmpmp = 1-(2',6'-dimethylbiphenyl-2-yl)-3-methyl-2,3-dihydro-1H-imidazo[4,5-b]pyridine) in CH2Cl2 were investigated. At 77 K, the fac-isomers showed blue emission with a vibronic structure, while the mer-isomers showed less structured emissions. At 300 K, all complexes showed broad and markedly red-shifted emission spectra compared to those at 77 K. The quantum yields of the Ir(dmpmp)3 isomers were very low, and their emission lifetimes were very short compared to those of Ir(pmp)3. In order to understand the large differences between the photodynamic properties of Ir(pmp)3 and Ir(dmpmp)3, we performed femtosecond time-resolved transient absorption (TA) spectroscopic measurements. The TA spectra of Ir(dmpmp)3 were almost the same as those of Ir(pmp)3 at a short delay time. However, Ir(dmpmp)3 showed a new broad TA band at around 720 nm with increasing delay time. The rise time of this band was ca. 10 ps for both isomers, and this may be attributed to the geometrical change in the excited state, which is associated with the steric hindrance of the bulky dimethylphenyl substituent. Actually, Ir(dmpmp)3 showed a strong rigidochromic shift in the emission spectra with varying temperature. To understand the molecular orbitals and the energy levels, theoretical calculations were performed using density functional theory. As a result, structural displacement takes place accompanied by the fast migration of localization of excited states via intraligand charge transfer.