Franck-Condon Excited State Research Articles

External Electric Field Effects on Excited-State Intramolecular Proton Transfer in 4'-N,N-Dimethylamino-3-hydroxyflavone in Poly(methyl methacrylate) Films.

The external electric field effects on the steady-state electronic spectra and excited-state dynamics were investigated for 4'-N,N-(dimethylamino)-3-hydroxyflavone (DMHF) in a poly(methyl methacrylate) (PMMA) film. In the steady-state spectrum, dual emission was observed from the excited states of the normal (N*) and tautomer (T*) forms. Application of an external electric field of 1.0 MV·cm(-1) enhanced the N* emission and reduced the T* emission, indicating that the external electric field suppressed the excited-state intramolecular proton transfer (ESIPT). The fluorescence decay profiles were measured for the N* and T* forms. The change in the emission intensity ratio N*/T* induced by the external electric field is dominated by ESIPT from the Franck-Condon excited state of the N* form and vibrational cooling in potential wells of the N* and T* forms occurring within tens of picoseconds. Three manifolds of fluorescent states were identified for both the N* and T* forms. The excited-state dynamics of DMHF in PMMA films has been found to be very different from that in solution due to intermolecular interactions in a rigid environment.

Primary photophysical and photochemical processes upon UV excitation of PtBr6 2– and PtCl6 2– complexes in water and methanol

Primary photophysical and photochemical processes were studied for PtIVBr62– and PtIVCl62– complexes in water and methanol by ultrafast kinetic spectroscopy upon excitation in the band region of charge transfer from the ligand-centered group π-orbitals to the eg*-orbital of PtIV complex anion (LMCT bands). The data obtained earlier upon excitation in the region of d—d bands were compared. Irrespective of the excitation wavelength, the photochemical properties of complexes are caused by the reactions of intermediates proceeding in the picosecond time range. These intermediates were identified as PtIVBr5– upon photolysis of PtIVBr62– and, presumably, the Adamson radical pair [PtIIICl52–(C4v)...Cl•] upon photolysis of PtIVCl62–. The difference in the exciting light wavelengths has an impact only on the first step of these processes, i.e., transition from the Franck—Condon excited state to intermediates.

The Interactions with Proton-Donor Solvents and Stability of some Borondipyrrinates

The constants of acid-base equilibrium were estimated with spectroscopic method at the ground (S0) and at the fluorescent (S1fl) states. The constants of acid-base equilibrium for Franck-Condon excited states (S1F-C) were estimated from spectral shifts for basic and acid forms. This results are compared with quantum yields of photostability of complexes in neutral solvents. The mechanisms of complexes protonation are discussed dependence from the structure of ligands.

Photoisomerization of Silyl-Substituted Cyclobutadiene Induced by σ → π* Excitation: A Computational Study

Photoinduced chemical processes upon Franck-Condon (FC) excitation in tetrakis(trimethylsilyl)-cyclobutadiene (TMS-CBD) have been investigated through the exploration of potential energy surface crossings among several low-lying excited states using the complete active space self-consistent field (CASSCF) method. Vertical excitation energies are also computed with the equation-of-motion coupled-cluster model with single and double excitations (EOM-CCSD) as well as the multireference Møller-Plesset (MRMP) methods. Upon finding an excellent coincidence between the computational results and experimental observations, it is suggested that the Franck-Condon excited state does not correspond to the first π-π* single excitation state (S1, 1(1)B1 state in terms of D2 symmetry), but to the second (1)B1 state (S3), which is characterized as a σ-π* single excitation state. Starting from the Franck-Condon region, a series of conical intersections (CIs) are located along one isomerization channel and one dissociation channel. Through the isomerization channel, TMS-CBD is transformed to tetrakis(trimethylsilyl)-tetrahedrane (TMS-THD), and this isomerization process could take place by passing through a "tetra form" conical intersection. On the other hand, the dissociation channel yielding two bis(trimethylsilyl)-acetylene (TMS-Ac) molecules through further stretching of the longer C-C bonds might be more competitive than the isomerization channel after excitation into S3 state. This mechanistic picture is in good agreement with recently reported experimental observations.

Chemically optimizing operational efficiency of molecular rotary motors.

Unidirectional molecular rotary motors that harness photoinduced cis-trans (E-Z) isomerization are promising tools for the conversion of light energy to mechanical motion in nanoscale molecular machines. Considerable progress has been made in optimizing the frequency of ground-state rotation, but less attention has been focused on excited-state processes. Here the excited-state dynamics of a molecular motor with electron donor and acceptor substituents located to modify the excited-state reaction coordinate, without altering its stereochemistry, are studied. The substituents are shown to modify the photochemical yield of the isomerization without altering the motor frequency. By combining 50 fs resolution time-resolved fluorescence with ultrafast transient absorption spectroscopy the underlying excited-state dynamics are characterized. The Franck-Condon excited state relaxes in a few hundred femtoseconds to populate a lower energy dark state by a pathway that utilizes a volume conserving structural change. This is assigned to pyramidalization at a carbon atom of the isomerizing bridging double bond. The structure and energy of the dark state thus reached are a function of the substituent, with electron-withdrawing groups yielding a lower energy longer lived dark state. The dark state is coupled to the Franck-Condon state and decays on a picosecond time scale via a coordinate that is sensitive to solvent friction, such as rotation about the bridging bond. Neither subpicosecond nor picosecond dynamics are sensitive to solvent polarity, suggesting that intramolecular charge transfer and solvation are not key driving forces for the rate of the reaction. Instead steric factors and medium friction determine the reaction pathway, with the sterically remote substitution primarily influencing the energetics. Thus, these data indicate a chemical method of optimizing the efficiency of operation of these molecular motors without modifying their overall rotational frequency.

Cooperative Conformational Change and Excitation Migration of Biphenyl-PMO Amorphous Film, As Revealed by Femtosecond Time-Resolved Spectroscopy

Excited state dynamics of biphenyl-bridged mesoporous organosilica (Bp-PMO) film was investigated by femtosecond transient absorption and dichroism measurements at various excitation intensities. Under the excitation condition with low intensity (ca. 0.1 μJ/pulse), the relaxation from the excited Franck–Condon state with skewed structure of the two phenyl rings to the preplanar state occurred with a time constant of 550 fs, followed by the excimer formation with two time constants of 9.0 and 140 ps. Under higher excitation condition with 1.0 μJ/pulse, very rapid excimer formation within 500 fs was observed. From the analysis of the transient absorption spectra, it was revealed that the cooperative geometrical relaxation from skewed to planar structures, in addition to the energy migration, led to the rapid excimer formation under the high excitation condition. By integrating these results with the fluorescence dynamics, the photoprimary processes in Bp-PMO, such as energy migration, annihilation, and excimer formation, were discussed.

Ultrafast Excited-State Dynamics of Donor–Acceptor Biaryls: Comparison between Pyridinium and Pyrylium Phenolates

The excited-state dynamics of two donor-acceptor biaryls that differ by the strength of the acceptor, a pyridinium or a pyrylium moiety, have been investigated using a combination of steady-state solvatochromic absorption, ultrafast fluorescence, as well as visible and infrared transient absorption spectroscopies. The negative solvatochromic behavior of pyridinium phenolate indicates that the permanent electric dipole moment experiences a decrease upon S1 ← S0 excitation, implying that the ground state possesses more zwitterionic character than the excited state. In contrast, pyrylium phenolate exhibits a weakly positive solvatochromic behavior corresponding to a small increase in the dipole moment upon excitation, implying more zwitterionic character in the excited than the ground state. Both compounds are therefore situated at different sides of the cyanine-limit structure, which has equally polar ground and excited states. Despite these differences, both molecules exhibit qualitatively similar excited-state properties. They are characterized by a very short fluorescence lifetime, increasing from about 1 to 20 ps, when varying solvent viscosity from 0.4 to 11 cP. There are, however, characteristic differences between the two compounds: The excited-state lifetimes of the pyrylium dye are shorter and also depend somewhat on polarity. The ensemble of spectroscopic data can be explained with a model where the emitting Franck-Condon excited state relaxes upon twisting around the single bond between the aryl units to a point where the excited- and ground-state surfaces are very close or intersect. After internal conversion to the ground state, the distorted molecule relaxes back to its equilibrium planar configuration, again largely dependent upon solvent viscosity. However, in this case, the kinetics for the pyrylium dye are slower than for the pyridinium dye and the polar solvent-induced acceleration is significantly stronger than in the excited state. This difference of kinetic behavior between the two compounds is a direct consequence of the change of the electronic structure from a normal to an overcritical merocyanine evidenced by steady-state spectroscopy.

Triphos Iridium(III) Halide Complex Photochemistry: Triphos Arm Dissociation

Photolysis of Ir(triphos)X3 (triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane; X = Cl, Br) yields an insoluble product believed to be oligomeric [Ir(triphos)X3]n with bridging triphos and halide ligands. Refluxing pyridine (py) dissolves the insoluble photoproducts ultimately yielding the dangling triphos complexes mer-Ir(κ(2)-triphos)(py)X3. Oxidation of the P center of the dangling arm of Ir(κ(2)-triphos)(py)Cl3 yields mer-Ir(κ(2)-P,P-triphosO)(py)Cl3 (triphosO = MeC(CH2P(O)Ph2)(CH2PPh2)2), which was characterized by single-crystal X-ray diffraction. mer-Ir(κ(2)-triphos)(py)Cl3 is also formed when Ir(triphos)Cl3 is photolyzed in the presence of py (ϕ = 26%). Both mer-Ir(κ(2)-triphos)(py)Cl3 and mer-Ir(κ(2)-P,P-triphosO)(py)Cl3 photoisomerize in pyridine to their thermally unstable fac-isomers. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations suggest triphos ligand arm dissociation occurs along a triplet pathway from an initial Franck-Condon ligand-field excited state that relaxes to a Jahn-Teller axially distorted octahedral triplet with a long Ir-P bond. Subsequent triphos arm dissociation yields a distorted trigonal-bipyramidal triplet that undergoes intersystem crossing to a square pyramidal singlet.

Excited-State Dynamics in Nitro-Naphthalene Derivatives: Intersystem Crossing to the Triplet Manifold in Hundreds of Femtoseconds

Femtosecond transient absorption experiments and density functional calculations are presented for 2-methyl-1-nitronaphthalene, 2-nitronaphthalene, and 1-nitronaphthalene in cyclohexane and acetonitrile solutions. Excitation of 2-methyl-1-nitronaphthalene at 340 nm populates the Franck-Condon singlet state, which bifurcates into two barrierless decay channels with sub-200-fs lifetimes. The primary decay channel connects the Franck-Condon singlet excited state with a receiver triplet state, whereas the second, minor channel involves conformational relaxation to populate an intramolecular charge-transfer state, as previously reported for 1-nitronaphthalene (J. Chem. Phys. 2009, 113, 224518). Conversely, the experimental and computational data for 2-nitronaphthalene shows that almost the entire Franck-Condon singlet excited-state population intersystem crosses to the triplet state in less than 200 fs due to a sizable energy barrier of ca. 5 kcal/mol that must be surmounted to access the intramolecular charge-transfer state. Our results lend support to the idea that the probability of population transfer to the triplet manifold in these nitronaphthalene derivatives is controlled not only by the small energy gap between the Franck-Condon singlet excited state and the receiver triplet state but also by the region of configuration space sampled in the singlet excited-state potential energy surface at the time of excitation. It is proposed that the ultrafast intersystem crossing dynamics in these nitronaphthalene molecules most likely occurs between nonequilibrated excited states in the strongly nonadiabatic regime.

Excited-State Dynamics of Ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate

The photophysics and excited-state dynamics of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) in solvents of varying polarity and viscosity have been studied using femtosecond transient absorption spectroscopy. Global analysis of the time-resolved spectra revealed three processes occurring in an optically excited molecule. The sequence of reactions begins with a transition from an initially excited Franck-Condon state to the nonrelaxed intramolecular charge transfer (ICT(NR)) state which is associated with a partial electron transfer. This process is followed by an additional relaxation to a more relaxed intramolecular charge transfer (ICT(R)) state with stronger charge transfer character and flattened geometry. The lifetime of the flattened charge transfer form (ICT(R)) shortens from 200 to 300 ps in medium polar solvents down to 10 ps in strongly polar solvents. On the other hand, increase of viscosity by 1 order of magnitude leads to deceleration of processes involving twist of the donor and acceptor moieties by a factor of approximately 2.5. Observation of long-lived fluorescence of EAADCy in medium polar solvent suggests that charge transfer is possible only from a hot Franck-Condon state, but not from a relaxed locally excited state which exhibits short-wavelength fluorescence on a nanosecond time scale.

Impact of Water on the Cis–Trans Photoisomerization of Hydroxychalcones

The photochromism of a 2-hydroxychalcone has been studied in CH3CN and H2O/CH3OH (1/1, v/v), as well as in analogous deuterated solvents using steady-state (UV-vis absorption, (1)H and (13)C NMR) and time-resolved (ultrafast transient absorption and nanosecond flow flash photolysis) spectroscopies. Whereas the irradiation of trans-chalcone (Ct) under neutral pH conditions leads to the formation of the same final chromene derivative (B) in both media, two distinct photochemical mechanisms are proposed in agreement with thermodynamic and kinetic properties of the chemical reaction network at the ground state. Following light excitation, the first steps are identical in acetonitrile and aqueous solution: the Franck-Condon excited state rapidly populates the trans-chalcone singlet excited state (1)Ct* (LE), which evolves into a twisted state (1)P*. This excited state is directly responsible for the photochemistry in acetonitrile in the nanosecond time scale (16 ns) leading to the formation of cis-chalcone (Cc) through a simple isomerization process. The resulting cis-chalcone evolves into the chromene B through a tautomerization process in the ground state (τ = 10 ms). Unlike in acetonitrile, in H2O/CH3OH (1/1, v/v), the P* state becomes unstable and evolves into a new state attributed to the tautomer (1)Q*. This state directly evolves into B in one photochemical step through a consecutive ultrafast tautomerization process followed by electrocyclization. This last case represents a new hypothesis in the photochromism of 2-hydroxychalcone derivatives.

Structural Dynamics of a Noncovalent Charge Transfer Complex from Femtosecond Stimulated Raman Spectroscopy

Femtosecond stimulated Raman spectroscopy is used to examine the structural dynamics of photoinduced charge transfer within a noncovalent electron acceptor/donor complex of pyromellitic dianhydride (PMDA, electron acceptor) and hexamethylbenzene (HMB, electron donor) in ethylacetate and acetonitrile. The evolution of the vibrational spectrum reveals the ultrafast structural changes that occur during the charge separation (Franck-Condon excited state complex → contact ion pair) and the subsequent charge recombination (contact ion pair → ground state complex). The Franck-Condon excited state is shown to have significant charge-separated character because its vibrational spectrum is similar to that of the ion pair. The charge separation rate (2.5 ps in ethylacetate and ∼0.5 ps in acetonitrile) is comparable to solvation dynamics and is unaffected by the perdeuteration of HMB, supporting the dominant role of solvent rearrangement in charge separation. On the other hand, the charge recombination slows by a factor of ∼1.4 when using perdeuterated HMB, indicating that methyl hydrogen motions of HMB mediate the charge recombination process. Resonance Raman enhancement of the HMB vibrations in the complex reveals that the ring stretches of HMB, and especially the C-CH(3) deformations are the primary acceptor modes promoting charge recombination.

Existence of a New Emitting Singlet State of Proflavine: Femtosecond Dynamics of the Excited State Processes and Quantum Chemical Studies in Different Solvents

Proflavine (3,6-diaminoacridine) shows fluorescence emission with lifetime, 4.6 ± 0.2 ns, in all the solvents irrespective of the solvent polarity. To understand this unusual photophysical property, investigations were carried out using steady state and time-resolved fluorescence spectroscopy in the pico- and femtosecond time domain. Molecular geometries in the ground and low-lying excited states of proflavine were examined by complete structural optimization using ab initio quantum chemical computations at HF/6-311++G** and CIS/6-311++G** levels. Time dependent density functional theory (TDDFT) calculations were performed to study the excitation energies in the low-lying excited states. The steady state absorption and emission spectral details of proflavine are found to be influenced by solvents. The femtosecond fluorescence decay of the proflavine in all the solvents follows triexponential function with two ultrafast decay components (τ(1) and τ(2)) in addition to the nanosecond component. The ultrafast decay component, τ(1), is attributed to the solvation dynamics of the particular solvent used. The second ultrafast decay component, τ(2), is found to vary from 50 to 215 ps depending upon the solvent. The amplitudes of the ultrafast decay components vary with the wavelength and show time dependent spectral shift in the emission maximum. The observation is interpreted that the time dependent spectral shift is not only due to solvation dynamics but also due to the existence of more than one emitting state of proflavine in the solvent used. Time resolved area normalized emission spectral (TRANES) analysis shows an isoemissive point, indicating the presence of two emitting states in homogeneous solution. Detailed femtosecond fluorescence decay analysis allows us to isolate the two independent emitting components of the close lying singlet states. The CIS and TDDFT calculations also support the existence of the close lying emitting states. The near constant lifetime observed for proflavine in different solvents is suggested to be due to the similar dipole moments of the ground and the evolved emitting singlet state of the dye from the Franck-Condon excited state.

Electric-Field-Induced Changes in Absorption and Fluorescence of the Green Fluorescent Protein Chromophore in a PMMA Film

External electric field effects on absorption, fluorescence, and fluorescence decay of p-HBDI that is a model compound of the chromophore of GFP have been examined in a poly(methyl methacrylate) film. The electroabsorption spectrum is similar in shape to the first derivative of the absorption spectrum, which results from the difference in molecular polarizability between the ground state and the Franck-Condon excited state. The electrophotoluminescence spectrum is dominated by the corresponding fluorescence spectrum, indicating the enhancement of the fluorescence intensity in the presence of external electric fields. The direct measurements of the electric field effect on the fluorescence decay profile suggest that the field-induced deceleration of the nonradiative process contributes to the increase in the fluorescence intensity in the presence of electric fields.

Molecular Stark effect spectroscopy of diflavonol and inhomogeneous broadening of its electronic spectra in erythrocyte membranes

Using spectroscopy of the molecular Stark effect and fluorescence spectroscopy, we study the characteristics of diflavonol 3,7-dihydroxy-2,8-di(4-dimethylaminophenyl)-4H,6H-pyrano[3,2-g]chromene-4,6-dione (DFME), which demonstrates intramolecular charge and proton phototransfer. In the ground state, this dye has only one form and, in the excited state, it has two forms, i.e., normal and phototautomeric. We found that, for the normal form of DFME, the transition dipole moment that is responsible for the absorption (m a ), the dipole moment in the equilibrium ground state (μ g ), and the change of the dipole moment upon transition of the molecule in the excited Franck-Condon state (Δ a μ) are parallel. In the ground equilibrium state, the dipole moments in 1,4-dioxane and cyclohexane are equal to μg = 12.2 × 10−30 C m and μ g = 11.0 × 10−30 C m, respectively. Upon excitation, they increase by Δ a μ = 61 × 10−30 C m and Δ a μ = 50.2 × 10−30 C m in these solvents. We study the spectral characteristics of DFME in organic solvents and erythrocyte membranes. A spectral inhomogeneity of DFME in erythrocyte ghosts is found. The inhomogeneous broadening of fluorescence spectra is manifested as a long-wavelength shift of the band of the normal form of DFME by 1640 cm−1 upon excitation at the red edge of the absorption spectrum.

Localized excitation effect on dipole moments of oligophenylenevinylenes in their excited Franck–Condon state

The results from electrooptical absorption measurements (EOAM) on the ground and excited Franck–Condon state dipole moments of oligophenylenevinylenes in 1,4-dioxane and cyclohexane are presented. The dipole moments of quadrupolar dyes in the ground state are not equal zero. This means that one of the functional end groups of these molecules is not on the axis or on the plane with the central part of the molecule. Such a conclusion is confirmed by the quantum-chemical and semi-empirical recalculations. Owing to disorder in the ground-state geometry of the dyes the π–π conjugated system is located on mostly the plane part of the molecule and in the main this part absorbs the light. The localized excitation effect causes the large change of the dipole moments of quadrupolar molecules upon excitation to the Franck–Condon excited state. Due to an efficient intramolecular charge transfer the new oligophenylenevinylenes must have significant non-linear optical properties.

Synthesis and molecular properties of donor–π-spacer–acceptor ynamides with up to four conjugated alkyne units

A set of push-pull ynamides with up to four conjugated triple bonds has been synthesized and the molecular properties of these new carbon rods have been characterised showing effective intramolecular charge transfers and high values for the change of the electrical dipole moment after transition from the ground to the Franck-Condon excited state.

Structural changes accompanying excited-state electron transfer in meta- and para-dialkylaminopyridines

The electronic structure of the lowest excited singlet states and molecular geometries of a series of dialkylaminopyridines (DAAPs) representing electron donor–acceptor systems were studied by photostationary and time-resolved UV–vis spectroscopic methods and quantum chemical calculations. The comparative studies allow us to rationalize dual luminescence of 4-DAAPs in terms of the TICT state model—the analysis of the electronic transition dipole moments indicates a nearly orthogonal conformation of the fluorescent ICT states. Introduction of the amino group at meta position as in 3-diisopropylaminopyridine completely changes photophysics of these pyridine derivatives: (i) the Franck-Condon excited state initially reached upon excitation and the solvent equilibrated fluorescent state are most probably of the same nature (both excited states do not correspond to a full separation of charges) and (ii) the electronic structure and geometry of the fluorescent CT states of m-DIAP are solvent dependent.

Primary Conformation Change in Bacteriorhodopsin on Photoexcitation

Ultrafast dynamics of bacteriorhodopsin (bR) has been extensively studied experimentally and theoretically. However, there are several contradictory results reported, indicating that its detailed dynamics and initial process have not yet been fully clarified. In this work, changes in the amplitude and phase of molecular vibration in the isomerization process of bR were real-time probed simultaneously at 128 different wavelengths through intensity modulation of the electronic transition. Systematic information on the transient change in continuous spectrum extending from 505 nm (2.45 eV) to 675 nm (1.84 eV) showed different dynamics in each spectral region reflecting the difference in the excited states and intermediates dominating the dynamics during the photoisomerization. Careful analysis of the transient spectral changes and spectrograms calculated from the vibrational real-time traces elucidated that the primary event just after photoexcitation is the deformation of the retinal configuration, which decays within 30 fs near the C=N bond in the protonated Schiff base. The intensity of C=N stretching mode starts to decrease before the initiation of the frequency modulation of the C=C stretching mode. The C=C stretching mode frequency was modulated by a coupled torsion around the C 13=C 14 bond, leading to the photoisomerization around the bond. This study clarified the dynamics of the C=N and C=C stretching modes working as key vibration modes in the photoisomerization of bR. Furthermore, we have elucidated the modulation and decay dynamics of the C=C stretching mode in the photoreaction starting from H (Franck-Condon excited state) followed by I (twisted excited), and J (first intermediate) states.

Theoretical Analysis of the Electronic Ground and Excited Franck-Condon State of 2-Alkylamino-(3, 5, or 6)-Methyl-4-Nitropyridine N-oxides

We theoretically characterised properties of the electronic ground and excited Franck–Condon state of 2-alkylamino-(3, 5, or 6)-methyl-4-nitropyridine N-oxides. The obtained results of time-dependent density functional theory calculations are consistent with the experimental absorption spectra. The nature of the lowest excited states of the considered Noxides molecules is discussed. Two excited singlet states (in the midst of four lowest states) have the charge-transfer character. These states have different charge-transfer contribution of the two electron donor groups to the same electron acceptor. In consequence, different conditions for the promotion of intramolecular proton transfer in excited state are created in each case.