Excited State Decay Dynamics Research Articles

Structural Dynamics of Phenyl Azide in Light-Absorbing Excited States: Resonance Raman and Quantum Mechanical Calculation Study

The excited state structural dynamics of phenyl azide (PhN3) after excitation to the light absorbing S2(A′), S3(A′), and S6(A′) states were studied using the resonance Raman spectroscopy and complete active space self-consistent field calculations. The vibrational spectra and the UV absorption bands were assigned on the basis of the Fourier transform (FT)-Raman, FT-infrared measurements, the density-functional theory computations and the normal mode analysis. The A-, B-, and C-bands resonance Raman spectra in cyclohexane, acetonitrile, and methanol solvents were, respectively, obtained at 273.9, 252.7, 245.9, 228.7, 223.1, and 208.8 nm excitation wavelengths to probe the corresponding structural dynamics of PhN3. The results indicated that the structural dynamics in the S2(A′), S3(A′), and S6(A′) states were significantly different. The crossing points of the potential energy surfaces, S2S1(1) and S2S1(2), were predicted to play a key role in the low-lying excited state decay dynamics, in accordance with Kasha's rule, and N7=N8 dissociation. Two decay channels initiated from the Franck-Condon region of the S2(A′) state were predicted: the radiative S2,min→S0 radiative decay and the S2→S1 internal conversion through the crossing points S2S1(1)/S2S1(2).

Anomalous Size-Dependent Excited-State Relaxation Dynamics of NanoGUMBOS

The synthesis, characterization, and ultrafast spectroscopy of size-selected nanospheres of ruthenium bipyridine–bis(pentafluoroethylsulfonyl)imide ([Ru(bipy)3][BETI]2) in water are reported. These studies represent the first experimental evidence of phonons with nanosecond lifetimes in organometallic nanomaterials. Thermally stable, crystalline nanoparticles of [Ru(bipy)3][BETI]2 are derived from a group of uniform materials based on organic salts (GUMBOS). Excited-state relaxation dynamics are studied using pump–probe time-resolved transient absorption spectroscopy, and the results are compared to corresponding measurements of aqueous Ru(bipy)3Cl2. The nanoGUMBOS show spectral shifts and size-dependent relaxation dynamics for nanoparticle diameters varying from 20 to 100 nm, characterized by excited-state decay dynamics similar to those of the precursor dye at higher pump pulse energies with an additional pathway attributed to intermolecular energy transfer, where all lifetimes increase with increasing ...

Direct evidence for secondary interactions in planar and nonplanar aromatic π-conjugates and their photophysical characteristics in solid-state assemblies.

Direct evidence for non-covalent secondary interactions in planar and nonplanar aromatic π-conjugates and their solid-state assemblies is established. A series of horizontally, vertically, and radially expanded oligo(phenylenevinylene)s (H-OPVs, V-OPVs, and R-OPVs, respectively) were designed with a fixed π-core and variable alkyl chain lengths on the periphery. Single-crystal structures of the OPVs were resolved to trace the secondary interactions that direct the solid-state self-organization and molecular packing of the chromophores. The H-OPVs were found to be planar, and they did not show any secondary interactions in the crystal lattices. The V-OPVs and R-OPVs were found to be nonplanar and to exhibit multiple CH/π hydrogen-bonding interactions among aryl hydrogen donors and acceptors. The enthalpies of the melting and crystallization transitions revealed that the planar H-OPVs are highly crystalline compared with the nonplanar R-OPVs and V-OPVs. Polarized light microscopy studies revealed the formation of one-dimensional nematic mesophases in H-OPVs. The absolute solid-state photoluminescence quantum yields (PLQYs) of the OPVs were determined using an integrating sphere setup. The highly packed H-OPVs showed low PLQYs compared with those of the weakly packed V-OPVs and R-OPVs. Time-resolved fluorescence decay measurements revealed that the excited-state decay dynamics of highly packed H-OPVs was much faster with respect to their low PLQYs. The decay profiles were found to be relatively slow (with higher life time (τ)) in the V-OPVs and R-OPVs. A field-effect transistor (FET) device was constructed for an OPV sample that showed a hole carrier mobility in the range of 10(-5) cm(2) V(-1) s(-1). The present investigation thus provides a new opportunity to trace the role of secondary interactions on π-conjugated mesophase self-assemblies and their solid-state emission and FET devices, more specifically based on OPV chromophores.

Structural dynamics of phenylisothiocyanate in the light-absorbing excited states: Resonance Raman and complete active space self-consistent field calculation study

The excited state structural dynamics of phenyl isothiocyanate (PITC) after excitation to the light absorbing S2(A'), S6(A'), and S7(A') excited states were studied by using the resonance Raman spectroscopy and complete active space self-consistent field method calculations. The UV absorption bands of PITC were assigned. The vibrational assignments were done on the basis of the Fourier transform (FT)-Raman and FT-infrared measurements, the density-functional theory computations, and the normal mode analysis. The A-, B-, and C-bands resonance Raman spectra in cyclohexane, acetonitrile, and methanol solvents were, respectively, obtained at 299.1, 282.4, 266.0, 252.7, 228.7, 217.8, and 208.8 nm excitation wavelengths to probe the corresponding structural dynamics of PITC. The results indicated that the structural dynamics in the S2(A'), S6(A'), and S7(A') excited states were very different. The conical intersection point CI(S2/S1) were predicted to play important role in the low-lying excited state decay dynamics. Two major decay channels were predicted for PITC upon excitation to the S2(A') state: the radiative S(2,min) → S0 transition and the nonradiative S2 → S1 internal conversion via CI(S2/S1). The differences in the decay dynamics between methyl isothiocyanate and PITC in the first light absorbing excited state were discussed. The role of the intersystem crossing point ISC(S1/T1) in the excited state decay dynamics of PITC is evaluated.

Single-Dot Spectroscopy of Zinc-Blende CdSe/CdS Core/Shell Nanocrystals: Nonblinking and Correlation with Ensemble Measurements

Here we report the first series of phase-pure zinc-blende CdSe/CdS core/shell quantum dots (QDs) with reproducibly controlled shell thickness (4-16 monolayers), which are nonblinking (≥95% 'on' time) in single-exciton regime for the entire series. These unique QDs possess well-controlled yet simple excited-state decay dynamics at both single-dot and ensemble levels, extremely small nonblinking volume threshold, if any, and unique 'on' and 'off' probability statistics. The outstanding optical properties of the QDs at the single-dot level were found to be correlated well with their ensemble properties. These small and bright nonblinking QDs offer promising technical application prospect in both single-dot and ensemble levels. The consistent and reproducible experimental results shed new light on the mechanisms of blinking of QDs.

Structural dynamics of 4‐cyanobenzaldehyde in S2(ππ*) state

The excited state structural dynamics of 4‐cyanobenzaldehyde (p‐CNB) were studied by using the resonance Raman spectroscopy and the quantum mechanical calculations. The experimental A‐ and B‐band absorptions were, respectively, assigned to the major nO → π3* and π2 → π3* transitions according to the B3LYP‐TD/6‐31G(d) and CIS/6‐31G(d) computations, and the resonance Raman spectra. It was determined that the actual S2(π2π3) state was in energy lower than S3(π1π3), which was just opposite to the B3LYP‐TD/6‐31G(d) calculated order of the S2(π1π3) and S3(π2π3). The vibrational assignments were carried out for the A‐ and B‐band resonance Raman spectra. The B‐band resonance Raman intensities of p‐CNB were dominated by the C2–C3/C5–C6 symmetric stretch mode ν8, the overtones nν8 and their combination bands with the ring C–H bend mode ν17, the C9–N10 stretch mode ν6, the C7–O8 stretch mode ν7 and the remaining modes. The conical intersection between S1(nOπ3) and S2(π2π3) states of p‐CNB was determined at complete active space self‐consistent field (CASSCF)(8,7)/6‐311G(d,p) level of theory. The B‐band short‐time structural dynamics and the corresponding decay dynamics of p‐CNB were obtained by analysis of the resonance Raman intensity pattern and CASSCF computations. The resonance Raman spectra indicated that CI[S1(nOπ3)/S2(π1π2π3π4)] located nearby the Franck–Condon region. The excited state decay dynamics evolving from the S2, FC(π2π3) to the S1(nOπ3) state was proposed. Copyright © 2013 John Wiley & Sons, Ltd.

Primary Photodynamics of the Green/Red-Absorbing Photoswitching Regulator of the Chromatic Adaptation E Domain from Fremyella diplosiphon

Phytochromes are red/far-red photosensory proteins that utilize the photoisomerization of a linear tetrapyrrole (bilin) chromophore to detect the red to far-red light ratio. Cyanobacteriochromes (CBCRs) are distantly related cyanobacterial photosensors with homologous bilin-binding GAF domains, but they exhibit greater spectral diversity. Different CBCR subfamilies have been described, with spectral sensitivity varying across the near-ultraviolet and throughout the visible spectrum, but all known CBCRs utilize photoisomerization of the bilin 15,16-double bond as the primary photochemical event. The first CBCR discovered was RcaE, responsible for tuning light harvesting to the incident color environment (complementary chromatic adaptation) in Fremyella diplosiphon. The green/red RcaE photocycle has recently been described in detail. We now extend this analysis by examining femtosecond photodynamics using ultrafast transient absorption techniques with broadband detection and multicomponent global analysis. Excited-state dynamics in both directions are significantly slower than those recently published for the red/green CBCR NpR6012g4. In the forward reaction, the primary Lumi-G photoproduct arises from the longer-lived excited-state populations, leading to a low photoproduct quantum yield. Using dual-excitation wavelength interleaved pump-probe spectroscopy, we observe multiphasic excited-state dynamics in the forward reaction ((15Z)Pg → (15E)Pr), which we interpret as arising from ground-state inhomogeneity with different tautomers of the PCB chromophore. The reverse reaction ((15E)Pr → (15Z)Pg) is characterized via pump-probe spectroscopy and also exhibits slow excited-state decay dynamics and a low photoproduct yield. These results provide the first description of excited-state dynamics for a green/red CBCR.

Dispersion studies of optical nonlinearity and excited state dynamics in cyclo[4]naphthobipyrroles

Herein we present results from the dispersion studies of nonlinear optical (NLO) properties of three novel cyclo[4]naphthobipyrroles at wavelengths of 600nm, 640nm, 680nm, and 800nm using picosecond (ps) pulses. NLO coefficients were extracted from Z-scan measurements using ∼1.5ps pulses. Large two-photon absorption cross sections in the range of 0.2–5.2×104 GM and n2 values of ∼10−16cm2/W were recorded for these moieties. The excited state decay dynamics were investigated using degenerate pump-probe experiments with ∼70 femtosecond pulses near 600nm. Double exponential fits of the pump-probe data suggested two decay times for all the molecules investigated.

Excited State Structure and Dynamics of the Neutral and Anionic Flavin Radical Revealed by Ultrafast Transient Mid-IR to Visible Spectroscopy

Neutral and anionic flavin radicals are involved in numerous photochemical processes and play an essential part in forming the signaling state of various photoactive flavoproteins such as cryptochromes and BLUF domain proteins. A stable neutral radical flavin has been prepared for study in aqueous solution, and both neutral and anion radical states have been stabilized in the proteins flavodoxin and glucose oxidase. Ultrafast transient absorption measurements were performed in the visible and mid-infrared region in order to characterize the excited state dynamics and the excited and ground state vibrational spectra and to probe the effect of the protein matrix on them. These data are compared with the results of density functional theory calculations. Excited state decay dynamics were found to be a strong function of the protein matrix. The ultrafast electron transfer quenching mechanism of the excited flavin moiety in glucose oxidase is characterized by vibrational spectroscopy. Such data will be critical in the ongoing analysis of the photocycle of photoactive flavoproteins.

The Effects of Photochemical and Mechanical Damage on the Excited State Dynamics of Charge-Transfer Molecular Crystals Composed of Tetracyanobenzene and Aromatic Donor Molecules

Charge-transfer molecular crystals are structurally well-defined systems whose electron transfer dynamics can be studied using time-resolved spectroscopy. In this paper, five 1:1 complexes, consisting of 1,2,4,5-tetracyanobenzene as the electron acceptor and durene, 9-methylanthracene, naphthalene, phenanthrene, and pyrene as electron donors, are studied using time-resolved fluorescence and transient absorption in the diffuse reflectance geometry. Two different sample morphologies were studied: single crystals and powders prepared by pulverizing the crystals and diluting them with barium sulfate microparticles. Fluorescence lifetime and transient absorption measurements performed on the crystals and the powders yielded different results. The crystals typically exhibited long-lived monoexponential fluorescence decays, while the powders had shorter multiexponential decays. Exposure of both types of samples to high laser fluence was also shown to induce faster excited state decay dynamics as observed using fluorescence and diffuse reflectance. In addition to the more rapid decays, these molecular crystals exhibited relatively high photobleaching quantum yields on the order of 10(-4). Previous work that interpreted picosecond decays in the transient absorption as evidence for rapid recombination and charge dissociation should be re-evaluated based on the susceptibility of this class of compounds to mechanical and photochemical damage.

Ultrafast transient mid IR to visible spectroscopy of fully reduced flavins

The light sensing apparatus of many organisms includes a flavoprotein. In any spectroscopic analysis of the photocycle of flavoproteins a detailed knowledge of the spectroscopy and excited state dynamics of potential intermediates is required. Here we correlate transient vibrational and electronic spectra of the two fully reduced forms of flavin adenine dinucleotide (FAD): FADH(-) and FADH(2). Ground and excited state frequencies of the characteristic carbonyl modes are observed and assigned with the aid of DFT calculations. Excited state decay and ground state recovery dynamics of the two states are reported. Excited state decay occurs on the picosecond timescale, in agreement with the low fluorescence yield, and is markedly non single exponential in FADH(-). Further, an unusual 'inverse' isotope effect is observed in the decay time of FADH(-), suggesting the involvement in the radiationless relaxation coordinate of an NH or hydrogen bond mode that strengthens in the excited electronic state. Ground state recovery also occurs on the picosecond time scale, consistent with radiationless decay by internal conversion, but is slower than the excited state decay.

Femtosecond pump-probe studies of actinic-wavelength dependence in aqueous chlorine dioxide photochemistry

The actinic or photolysis-wavelength dependence of aqueous chlorine dioxide (OClO) photochemistry is investigated using femtosecond pump-probe spectroscopy. Following photoexcitation at 310, 335, and 410 nm the photoinduced evolution in optical density is measured from the UV to the near IR. Analysis of the optical-density evolution illustrates that the quantum yield for atomic chlorine production (Phi(Cl)) increases with actinic energy, with Phi(Cl)=0.16+/-0.02 for 410 nm excitation and increasing to 0.25+/-0.01 and 0.54+/-0.10 for 335 and 310 nm excitations, respectively. Consistent with previous studies, the production of Cl occurs through two channels, with one channel corresponding to prompt (<5 ps) Cl formation and the other corresponding to the thermal decomposition of ClOO formed by OClO photoisomerization. The partitioning between Cl production channels is dependent on actinic energy, with prompt Cl production enhanced with an increase in actinic energy. Limited evidence is found for enhanced ClO production with an increase in actinic energy. Stimulated emission and excited-state absorption features associated with OClO populating the optically prepared (2)A(2) surface decrease with an increase in actinic energy suggesting that the excited-state decay dynamics are also actinic energy dependent. The studies presented here provide detailed information on the actinic-wavelength dependence of OClO photochemistry in aqueous solution.

Fullerene and pi-Conjugated Oligomer Hybrids as Advanced Functional Nanomaterials

Two classes of pi-conjugated oligomer linked bis-fullerene molecules with a triad motif of C60-pi-C60 were synthesized and investigated. In the first series of C60-pi-C60 hybrids, electron-rich OPE/OPV conjugated oligomers were employed to function as the central pi-bridges and as the electron donors. Electrochemical and electronic absorption/emission spectral analyses indicate that the electronic interactions between the fullerene cage (acceptor) and the oligomer unit (donor) varied depending on the nature of the bridging unit. Complex excited state decay dynamics on the nanosecond timesccale were observed with time-resolved transient absorption spectroscopy. The second series of fullerene-oligomer hybrids use conjugated polyynes as the pi-bridges. The unique structural rigidity and solid-state reactivity of the polyyne units impart these bisfullerene molecular dumbbells interesting interfacial aggregation behavior by forming spherical nanostructures on s urfaces. This property could be useful for solid-state synthesis of fullerene based nanomaterials.

B 2 1 ( Σ u + 1 ) excited state decay dynamics in CS2

The authors report time resolved photoelectron spectra of the B21(Σu+1) state of CS2 at pump wavelengths in the region of 200nm. In contrast to previous studies, the authors find that the predissociation dynamics is not well described by a single exponential decay. Biexponential modeling of the authors’ data reveals a rapid decay pathway (τ&amp;lt;50fs), in addition to a longer lived channel (τ∼350–650fs) that displays a marked change in apparent lifetime when the polarization of the pump laser is rotated with respect to that of the probe. Since the initially populated B21(Σu+1) state may decay to form either S(D1) or S(P3) products (the latter produced via a spin-orbit induced crossing from a singlet to a triplet electronic surface), this lifetime observation may be rationalized in terms of changes in the relative ionization cross section of these singlet and triplet states of CS2 as a function of laser polarization geometry. The experimentally observed lifetime of the longer lived channel is therefore a superposition of these two pathways, both of which decay on very similar time scales.

Excited States of Conjugated Hydrocarbons Using the Molecular Mechanics - Valence Bond (MMVB) Method: Conical Intersections and Dynamics

We developed the molecular mechanics—valence bond (MMVB) method following an original suggestion of Jean-Paul Malrieu and coworkers. By coupling a parameterized Heisenberg Hamiltonian to a standard classical force field (MM2), reliable ground and excited state geometries of conjugated hydrocarbons can be rapidly optimized. The MMVB method was central to our development of algorithms for locating conical intersections and calculating their associated decay dynamics. Here, we briefly review the chemical applications of MMVB to date, and present two new studies using the photostability of pyracylene and the excited state decay dynamics of the photochromic dihydroazulene/vinylheptafulvene (DHA/VHF) reaction.

Experimental and theoretical investigation of the AlH b 3Σ−–a 3Π electronic transition

The laser fluorescence excitation spectrum of the b 3Σ−–a 3Π (0,0) band of AlH and AlD is reported. The AlH/AlD(a 3Π) state was prepared in a free-jet supersonic expansion by the reaction of photolyzed trimethylaluminum with hydrogen or deuterium. Spectroscopic constants for the upper and lower vibronic levels were derived from fits to the measured transition wave numbers of the rotational lines. Lifetimes of J′=1 rotational/fine-structure levels of electronically excited AlH/AlD(b 3Σ−,v′=0) were determined from fluorescence decay waveforms with laser excitation on isolated rotational lines. The measured lifetimes were compared with values obtained in a theoretical treatment of the excited-state decay dynamics, wherein both radiative decay to the a 3Π state and nonradiative decay through the repulsive 1 3Σ+ state were considered. The experimental and theoretical lifetimes are in good agreement. The theoretical treatment shows that the nonradiative excited-state decay dominates over radiative decay. The observed fine-structure dependence of the lifetimes results is due to the nature of the spin–orbit coupling of the b 3Σ− state with the 1 3Σ+ continuum.

Photoisomerization of a benzopyrromethenone derivative in micelles and organic solvents

The photophysics of a benzopyrromethenone derivative have been studied using UV–visible, fluorescence, and ultrafast transient absorption spectroscopy in solvents of varying polarity and viscosity, including ethanol, TX-100 micelles, hexane, and glycerol. The fluorescence quantum yields ( Φ f) were solvent dependent and were lower in ethanol and TX-100 and higher in glycerol and hexane. The excited state decay dynamics were best fit with a triple exponential in ethanol and TX-100 and a double exponential in hexane and glycerol. The fastest component (∼2 ps) in ethanol and TX-100, which was not observed in hexane and glycerol, is attributed to photoisomerization.

Jet spectroscopy, structure, anomalous fluorescence, and molecular quantum beats of silylidene (H2C=Si), the simplest unsaturated silylene

The jet-cooled B̃ 1B2–X̃ 1A1 spectrum of silylidene, the simplest unsaturated silylene, has been observed for the first time. H2C=Si and D2C=Si have been produced by an electric discharge through tetramethylsilane and tetramethylsilane-d12 vapor diluted in argon at the exit of a supersonic expansion. Rotational analysis of the 000 bands yielded the following substitution structures: rs″(CSi)=1.706(5) Å, rs″(CH)=1.099(3) Å, θs″(HCH)=114.4(2)°, rs′(CSi)=1.815(5) Å, rs′(CH)=1.073(4) Å, and θs′(HCH)=133.7(1)°. The electronic transition consists primarily of strong electronically allowed perpendicular bands, but a weaker system of vibronically induced parallel bands has also been assigned. Transitions involving Δv=2 changes in the ν6 (b2) mode show up prominently in the spectrum, due to a very large change in the vibrational frequency on excitation. Silylidene has very interesting excited state decay dynamics. Anomalous S2−S0 fluorescence is observed due to the very large S2−S1 energy gap. Rotational level specific intensity anomalies are found in the laser induced fluorescence spectra. Collision-free fluorescence decay curves exhibit superimposed quantum beats for almost all the accessible rotational levels in the 000 bands of H2CSi and D2CSi. Density of states arguments lead to the conclusion that most of the beat patterns are due to coupling with high vibrational levels of the ground state, although two examples of hyperfine splittings associated with singlet–triplet interactions have also been found.

Ultrafast Studies of Excited-State Dynamics of Phthalocyanine and Zinc Phthalocyanine Tetrasulfonate in Solution

Femtosecond measurements of transient absorption, bleach, and stimulated emission are used to study the excited-state dynamics of phthalocyanine tetrasulfonate (PcS4) and zinc phthalocyanine tetrasulfonate (ZnPcS4) in solution. In water the excited-state decay process is fast and dominated by energy relaxation due to intermolecular aggregation. In dimethyl sulfoxide (DMSO) both PcS4 and ZnPcS4 exist predominantly in the monomeric form and exhibit very different dynamics from that of the aggregates. The decays are much slower and the observed processes are strongly dependent on the probe wavelength. For PcS4 in DMSO, when probed at 790 nm, the dynamics are dominated by stimulated emission which is observed for the first time in solution. At other wavelengths either transient absorption or bleach dominates the signal. All the observed dynamics can be well fit using a double-exponential function with a fast and slow component. The fast decay has a time constant of 10 ± 4 ps for both phthalocyanines while the slow decay has a time constant of 370 ps for PcS4 and 460 ps for ZnPcS4, respectively. The overall excited-state decay dynamics correlate well with the recovery of the ground electronic state, indicating that the recovery is the predominant process on this time scale. On the basis of a simple three-state kinetic model, the fast decay (10 ps) is attributed primarily to a conversion from the second to the first excited singlet state, possibly involving vibrational relaxation in S1. There might also be a small contribution from aggregates. The first excited-state S1 subsequently decays with a time constant of 130 ps for PcS4 and 160 ps for ZnPcS4, respectively. This decay is due to a combination of radiative and nonradiative relaxation from S1 to S0 and intersystem crossing from S1 to the triplet state.

Stark mapping of H2 Rydberg states in the strong-field regime with dynamical resolution.

We have acquired spectra of high Rydberg states of molecular hydrogen in a static external field, in the energy region from below the energy at which field ionization becomes classically possible (${\mathit{E}}_{\mathit{c}}$) to well above this energy. Simultaneous spectra of ionization and dissociation were acquired, thereby allowing direct information on the excited-state decay dynamics to be obtained. We have found that states with energies below ${\mathit{E}}_{\mathit{c}}$ undergo field-induced predissociation, while states with energies well above ${\mathit{E}}_{\mathit{c}}$ decay predominantly by field ionization. Field ionization and dissociation compete effectively as decay channels for states with energies in a restricted region just above ${\mathit{E}}_{\mathit{c}}$. Comparison of our ionization spectra to the results of a single-channel quantum-defect theory Stark calculation shows quantitative agreement except near curve crossings, indicating that inclusion of different core rotational state channels will be required to properly account for coupling between the Stark states. Several states in the spectra undergo pronounced changes in their dynamical properties over a narrow range of field values, which we interpret as being due to interference cancellation of the ionization rates for these states.