Absolute Resonance Raman Cross Sections Research Articles

Short-time dynamics and decay mechanism of E,E-2,4-hexadienal in the first light-absorbing S2(ππ*) state.

The initial nonadiabatic decay dynamics of E,E-2,4-hexadienal (HAL) in the light absorbing S2(ππ*) state were studied using resonance Raman spectroscopy and complete-active space self-consistent field (CASSCF) calculations. The UV and vibrational spectra were assigned on the basis of the UV absorption, Fourier transform (FT)-Raman and FT-infrared measurements, the density-functional theory computations, and the normal mode analysis. The A-band resonance Raman spectra in cyclohexane and acetonitrile were obtained at 282.4, 273.9, 266.0, 252.7, and 245.9 nm excitation wavelengths, respectively, to probe the corresponding structural dynamics of HAL. The A-band absorption cross section and the corresponding absolute resonance Raman cross sections were simulated using a simple model based on the time-dependent wave-packet theory in a Brownian oscillator model. The geometric structures of the singlet electronic excited states and their curve-crossing points were optimized at the CASSCF level of theory. The obtained short-time structural dynamics in easy-to-visualize internal coordinates were then compared with the CASSCF-predicted structural-parameter changes of S2(ππ*)S1(nπ*)-n (n = 1-4). Our results indicate that the initial population of HAL in the S2 state ramifies in or nearby the Franck-Condon (FC) region, leading to five S2(ππ*) → S1(nπ*) internal conversion pathways due to the flexibility of the molecular chain and the different electronic resonant structures formed nearby FC of the S2 state. Then, the formed S1 transient species, which have different geometric structures and different energy partitions, undergo different photophysical processes, such as S1 → S0 internal conversion, S1 → T1 intersystem crossing, and the S1 → S'1 photoisomerization reaction. The substitution effect on the S2(ππ*) → S1(nπ*) internal conversion dynamics and the trans-cis photoisomerization reaction is proposed in terms of the p-π conjugation interaction or the p-σ superconjugation interaction.

Resonance Raman Characterization of Tetracene Monomer and Nanocrystals: Excited State Lattice Distortions With Implications For Efficient Singlet Fission.

The characterization of specific phonon modes and exciton states that lead to efficient singlet fission (SF) may be instrumental in the design of the next generation of high-efficiency photovoltaic devices. To this end, we analyze the absolute resonance Raman (RR) cross sections for tetracene (Tc) both as a monomer in solution and as a crystalline solid in an aqueous suspension of nanocrystals. For both systems, a time-dependent wavepacket model is developed that is consistent with the absolute RR cross sections, the magnitude of the absorption cross sections, and the vibronic line shapes of the fluorescence. In the monomer, the intramolecular reorganization energy is between 1500 and 1800 cm-1 and the solvent reorganization energy is 70 cm-1. In nanocrystals, the total reorganization is diminished to less than 600 cm-1. The lowest energy exciton has an estimated intramolecular reorganization energy between 300 and 500 cm-1 while intermolecular librational phonons have a reorganization energy of about 130 cm-1. The diminished reorganization energy of the nanocrystal is interpreted in the context of the delocalization of the band-edge exciton onto about ∼7 molecules. When electron and electron-hole correlations are included within many-body perturbation theory, the polarized absorption spectra of crystalline Tc are calculated and found to be in agreement with experiment. The low-lying exciton states and optically active phonons that contribute to the polarized crystal absorption are identified. The likely role of coherent exciton phonon evolution in the SF process is discussed.

Solvent effect on the initial structural dynamics of benzaldehyde in the S3(ππ*) state—Resonance Raman spectroscopic study

AbstractThe excited‐state structural dynamics of benzaldehyde (BA) after population to the S3(πH‐1πL*) light‐absorbing state were studied using resonance Raman spectroscopy and complete active space self‐consistent field (CASSCF) calculations. The C‐band absorption cross section and the corresponding absolute resonance Raman cross sections in cyclohexane and methanol were simulated using the time‐dependent wave‐packets formalism, respectively. The results indicate that the major structural dynamics of BA and the homogeneous broadening of the absorption spectrum in cyclohexane are similar to those of BA in methanol or acetophenone (AP) in cyclohexane, whereas the inhomogeneous broadening of BA in cyclohexane is significantly different from that in methanol. It is proposed that the initial decay pathway of BA upon the C‐band absorption is mostly towards S3S2 and partly to S3,min. The roles of hydrogen bonding and polar–polar interaction in the spectral broadenings are discussed.

Decay dynamics of 6‐azauracil from the light absorbing S2(ππ*) state

AbstractThe initial decay dynamics of 6AU in the S2 state was investigated by using resonance Raman spectroscopy, the time‐dependent wave‐packet theory in a Brownian oscillator model, and complete‐active space self‐consistent field (CASSCF) protocol. The vibrational spectra and the ultraviolet 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 absorption cross section and the absolute resonance Raman cross sections were simulated simultaneously by using the time‐dependent wave‐packet theory in a Brownian oscillator model. The obtained normal mode displacements of the Franck–Condon active modes were then converted to the short‐time structural dynamics in easy‐to‐visualize internal coordinates. The roles of two pathways via the S2 → S1 internal conversion and the S2 → T3 intersystem crossing were evaluated through the comparison between the short‐time structural dynamics and the CASSCF/CASTP2 calculated structural changes between FC and S2S1 or between FC and S2T3. The results indicate that the structural dynamics in the Franck–Condon region of the S2 state is mostly toward the S2/S1 conical intersection, which supports the conclusion that the S2 → S1 internal conversion dominants the initial decay pathway of 6AU as revealed by the broadband fs TA spectroscopy, whereas the barrierless S2 → T3 intersystem‐crossing process is negligible owing to weak spin‐orbital coupling and unfavorable subsequent T3 → T2 → T1 decay. The reaction coordinates towards the planar S2(ππ*)/S1(nπ*) and severely distorted S2(ππ*)/S0 conical intersection points are proposed respectively, and the solvent effect on the initial decay mechanisms of 6AU, uracil, and thymine is clarified.

Short-time dynamics and decay mechanism of 2(1H)-pyridinone upon excitation to the light-absorbing S4(21𝝅𝝅*) state

The excited-state structural dynamics and the decay mechanism of 2(1H)-pyridinone (NHP) after excitation to the S4(21ππ*) light-absorbing state were studied using resonance Raman spectroscopy and complete-active space self-consistent field (CASSCF) calculations. The B-band absorption cross-section and the corresponding absolute resonance Raman cross-sections were simulated using a simple model based on time-dependent wave-packet theory. The geometric structures of the singlet electronic excited states and their curve-crossing points were optimized at the CASSCF level of theory. The obtained short-time structural dynamics in easy-to-visualize internal coordinates were then compared with the CASSCF-predicted structural-parameter changes of S4(21ππ*)/S3(21nπ*)-MIN, S4(21ππ*)/S1(11nπ*)-MIN, and S4(21ππ*)-MIN. Our results indicate that the initial population of NHP in the S4 state bifurcates in or near the Franck-Condon region, leading to two predominant (S4S3-MIN and S4S1-MIN) internal conversion pathways. The lowest-lying S2(11ππ*) excited state is finally formed via subsequent internal conversions S3(21nπ*)/S2(11ππ*)-MIN and S1(11nπ*)/S2(11ππ*)-MIN. The enol-keto tautomeric mechanism does not seem to play a role. The decay mechanism in the singlet realm is proposed.

Resonance Raman Intensity Analysis of ClNO2 Dissolved in Methanol

Halogens such as chlorine are converted from halides, including ClNO(2), to reactive radicals by UV solar radiation. These radicals can affect ozone production and destruction in the stratosphere. Recently, it became clear that halogen radicals can also play a significant role in the chemistry of the troposphere. The photochemistry of ClNO(2) has been the subject of several studies in the gas and solid state that demonstrated a clear phase-dependent reactivity. Here, we report our initial studies of nitryl chloride in solution. Resonance Raman (RR) spectra of ClNO(2) dissolved in methanol after excitation within the 1(1)A(1)-2(1)A(1) absorption band (D band) in the region 200-240 nm are presented. RR intensity along the NO symmetric stretch coordinate (v(1)) at 1291 cm(-1) is observed at all excitation wavelengths, whereas limited intensity corresponding to the transition of the N-Cl symmetric stretch (v(3)) was only observed at 199.8 nm, whereas no intensity corresponding to the O-N-O symmetric bend (v(2)) was observed. Depolarization ratios and absolute resonance Raman cross sections for v(1) were obtained at several excitation wavelengths spanning the D band. Depolarization ratios were found to deviate significantly from 1/3, consistent with more than a single dipole-allowed electronic transition contributing to the scattering. RR intensity analysis (RRIA) reveals that two closely spaced excited electronic states contribute to the scattering, which are dissociative along the Cl-N coordinate. In this study the role the solvent environment plays in ClNO(2) state energetics and excited structural evolution along fundamental coordinates is discussed.

Resonance Raman and density functional study of the excited state structural dynamics of 3-amino-2-cyclohexen-1-one in water and acetonitrile solvents

FT-Raman and/or FT-IR spectra of 3-amino-2-cyclohexen-1-one (ACyO) in solid state and/or in solvents of water and acetonitrile were obtained. Density functional theory calculations were done to help elucidate the vibrational band assignments. The A-band resonance Raman spectra of ACyO were acquired in water and acetonitrile solvents to examine the excited state structural dynamics and the state-mixing or curve-crossing tuned by solvents. A preliminary resonance Raman intensity analysis using the time-dependent wave-packet theory and simple model was done for ACyO in acetonitrile solvent. Resonance Raman spectroscopic probing of the excited state curve-crossing or state-mixing was proposed.

Excited-State Geometry Method for Calculation of the Absolute Resonance Raman Cross Sections of the Aromatic Amino Acids

The time correlator formalism was used to calculate the absolute resonance Raman cross sections for the aromatic amino acids based on density functional theory calculations of the ground-state potential energy surfaces combined with projection along normal mode eigenvectors in the excited state. The geometric difference between the minima of the ground and excited states along each normal mode was calculated to provide inputs for the time correlator in the linear approximation. The calculated dimensionless nuclear displacements, Delta(i), provide the electron-phonon coupling constants, S(i) = Delta(i)(2)/2, for the corresponding Raman active mode of frequency omega(t). The method is generally applicable to molecules that are Franck-Condon active. As an example we have chosen to calculate the absolute resonance Raman cross sections of models of the aromatic amino acids phenylalanine, tyrosine, and tryptophan. We discuss the role played by substituents on the aromatic ring that decrease vibronic activity to a level that permits application of the time correlator. While the method may have limitations for molecules of high symmetry, the current study of excited-state displacements and electronic structure indicates that the L(a),(b) states are Franck-Condon active in the aromatic molecules studied.

Study of resonance Raman cross section of aqueous β -carotene at low concentrations

Applying Teflon-AF waveguide cell, we determined the absolute resonance Raman cross sections (RCS) for C=C and C-C stretching modes of β-carotene in aqueous solution at low concentrations (10-6–10-10 M) by measuring Raman intensity. It was unexpected that the RCS increased rapidly with the decrease of concentration at extremely low concentration range (10-8–10-10 M), and it reached the value of 1.22×10-21 cm2 molecule-1Sr-1 at 2.5×10-10 M larger than general RCS (10-30 cm2 molecule-1Sr-1) for 109 times. Additionally, we give the tentative theory exploration for the experimental results that the large RCSs at low concentrations attributed to both resonance Raman effect and dipole-induced dipole interactions between solute and solvent molecules.

Multistate Photochemical Reaction Dynamics of ClNO in Solution: An Absolute Resonance Raman Intensity Analysis Study

The excited-state reaction dynamics of nitrosyl chloride (ClNO) are studied using absolute resonance Raman intensity analysis. The absolute resonance Raman cross sections for ClNO dissolved in cyclohexane and acetonitrile are measured at several excitation wavelengths spanning the absorption band commonly referred to as the “A band” (λmax ≈ 200 nm). The resonance Raman and absorption cross sections are modeled using the time-dependent formalism. Resonance Raman depolarization ratios are also measured and are found to be consistent with at least two electronic transitions participating in the scattering process. Therefore, the standard time-dependent formalism approach was modified by incorporating two excited states into the analysis, with state contributions deconvolved through modeling of the depolarization ratios in addition to the absolute resonance Raman and absorption cross sections. The spectroscopic observables are well reproduced using this two-state model. The analysis presented here demonstrates that the photoexcitation of solution-phase ClNO results in a substantial evolution of the N−Cl stretch coordinate consistent with the dissociation of the N−Cl bond. Significant structural evolution is also observed along the bend, with minimal excited-state structural evolution observed along the NO stretch. The structural evolution along the dissociative N−Cl stretch coordinate is found to be solvent-dependent, and the origin of this dependence is related to changes in the ground-state equilibrium geometry as a function of solvent environment. Finally, the homogeneous line width undergoes a significant increase in acetonitrile relative to cyclohexane, and this increase is proposed to reflect the modification of the excited-state interactions and nonadiabatic relaxation dynamics.

Absolute Resonance Raman Intensity Analysis of Isopropyl Nitrate in the Condensed Phase

The first resonance Raman spectra of isopropyl nitrate (IPN) are reported. Absolute resonance Raman scattering cross-sections for IPN dissolved in cyclohexane and acetonitrile are measured at excitation wavelengths spanning the strong π−π* absorption located at ∼200 nm. Resonance enhancement is observed for seven vibrational coordinates, all involving the −ONO2 chromophore, demonstrating that the photoinduced excited-state structural evolution is dominated by structural changes localized to the −ONO2 group. The absorption and absolute resonance Raman cross-sections are modeled using the time-dependent formalism for absorption and Raman scattering. This analysis demonstrates that the photoinitiated excited-state structural-relaxation dynamics are multidimensional, and consistent with N−O bond cleavage. This observation is consistent with N−O bond dissociation being the dominant photodissociation pathway for larger (e.g., three or more carbon atoms) alkyl nitrates. Comparison of the results in cyclohexane a...

Resonance Raman intensity analysis of chlorine dioxide dissolved in chloroform: The role of nonpolar solvation

Absolute resonance Raman cross sections for chlorine dioxide (OClO) dissolved in chloroform are obtained at several excitation wavelengths spanning the photochemically relevant B12–A22 optical transition. The absolute scattering cross sections of OClO are determined by reference to the 666 cm−1 transition of chloroform whose absolute scattering cross sections are reported here. The time-dependent theory for Raman and absorption are used to develop a mode-specific description of the A22 excited state surface. This description demonstrates that photoexcitation of OClO leads to significant structural evolution along the symmetric stretch and bend coordinates, with only limited evolution occurring along the asymmetric stretch. This description is similar to that determined for OClO dissolved in cyclohexane and water demonstrating that the excited-state structural evolution of OClO is similar in these solvents. Analysis of the OClO absolute scattering cross sections establishes that the homogeneous linewidth is 95±15 cm−1 in chloroform, essentially identical to the linewidths in cyclohexane and water. To establish the origin of this linewidth, the fluorescence cross section for OClO dissolved in cyclohexane is measured and found to be consistent with an excited-state lifetime of ∼200 fs. Comparison of this lifetime to the homogeneous linewidth establishes that the homogeneous broadening is dominated by solvent-induced pure dephasing. It is proposed that the apparent solvent independence of the homogeneous linewidth reflects the mechanical response of the solvent to the photoinitiated change in solute geometry. In support of this hypothesis, the homogeneous linewidth is reproduced using the viscoelastic continuum model of nonpolar solvation. Finally, it is argued that the restricted evolution along the asymmetric-stretch coordinate is due to dielectric solvent–solute interactions consistent with the increase in inhomogeneous linewidth with an increase in solvent polarity.

Excited-State Dynamics of Chlorine Dioxide in the Condensed Phase from Resonance Raman Intensities

Resonance Raman spectra of chlorine dioxide (OClO) dissolved in cyclohexane obtained with excitation throughout the 2B1−2A2 electronic transition are presented. Resonance Raman intensity corresponding to all vibrational degrees of freedom (the symmetric stretch, bend, and asymmetric stretch) is observed, demonstrating that excited-state structural evolution along all three coordinates occurs upon photoexcitation. The electronic absorption and absolute resonance Raman cross sections are reproduced employing the time-dependent formalism for Raman scattering using an anharmonic description of the 2A2, excited-state potential-energy surface. Analysis of the resonance Raman cross-sections demonstrates that both homogeneous and inhomogeneous broadening mechanisms are operative in cyclohexane. Comparison of the experimentally determined, gas-phase 2A2 surface to that in solution defined by the analysis presented here shows that although displacements along the symmetric stretch and bend are similar in both phases, evolution along the asymmetric stretch is dramatically altered in solution. Specifically, employing the gas-phase potential along this coordinate, the predicted intensity of the overtone transition is an order of magnitude larger than that observed. The analysis presented here demonstrates that the asymmetric stretch overtone intensity is consistent with a reduction in excited-state frequency along this coordinate from 1100 to 750 ± 100 cm-1. This comparison suggests that differences in evolution along the asymmetric stretch may be responsible for the phase-dependent reactivity of OClO. In particular, the absence of substantial evolution along the asymmetric stretch in solution results in the ground-state symmetry of OClO being maintained in the 2A2 excited state. The role of symmetry in defining the reaction coordinate and the nature of the solvent interaction responsible for modulation of the excited-state potential energy surface are discussed.

Excited-state structure and photochemical ring-opening dynamics of 1,3,5-cyclo-octatriene from absolute resonance Raman intensities

Absolute resonance Raman cross sections are measured for 1,3,5-cyclo-octatriene (COT) in cyclohexane with excitation from 325 to 200 nm. These intensities and the absorption spectrum are modeled using a fully thermalized time-correlator theory to quantitate the excited-state equilibrium geometry displacements along 19 Raman-active normal modes. The resonance Raman spectra show significant intensity in low-frequency modes corresponding to planarization of the eight-membered ring. The 140 cm−1 twist-boat planarization (Δ=4.6) and the 339 cm−1 ring deformation (Δ=1.6) are particularly strong. However, no intensity is observed in modes which project onto the predicted disrotatory ring-opening motion, such as the nontotally symmetric CH2 twist fundamental or its overtone. Analysis of the fluorescence quantum yield (φF=2×10−6) gives an excited state lifetime on the order of ∼30 fs. These results show that ring planarization is the first step in the disrotatory ring opening of COT followed by rapid depopulation of the initially prepared state to a lower-lying excited electronic state upon which the actual ring opening occurs. Comparison of these results with the excited-state dynamics of other pericyclic systems suggests that pericyclic rearrangements occur only once a planar structure is established and that the bond rearrangement occurs predominantly on a low-lying, optically forbidden excited state.

Excited-state structure and electronic dephasing time of Nile blue from absolute resonance Raman intensities

Absolute resonance Raman cross sections are measured for Nile blue 690 perchlorate dissolved in ethylene glycol with excitation at 514, 531, and 568 nm. These values and the absorption spectrum are modeled using a time-dependent wave packet formalism. The excited-state equilibrium geometry changes are quantitated for 40 resonance Raman active modes, seven of which (590, 1141, 1351, 1429, 1492, 1544, and 1640 cm−1 ) carry 70% of the total resonance Raman intensity. This demonstrates that in addition to the prominent 590 and 1640 cm−1 modes, a large number of vibrational degrees of freedom are Franck–Condon coupled to the electronic transition. After exposure of the explicit vibrational progressions, the residual absorption linewidth is separated into its homogeneous [350 cm−1 half-width at half-maximum (HWHM)] and inhomogeneous (313 cm−1 HWHM) components through an analysis of the absolute Raman cross sections. The value of the electronic dephasing time derived from this study (25 fs) compares well to previously published results. These data should be valuable in multimode modeling of femtosecond experiments on Nile blue.

Absolute resonance Raman intensities demonstrate that the spectral broadening induced by the β-ionone ring in retinal is homogeneous

The sensitivity of absolute resonance Raman cross sections to the excited state homogeneous width is used to examine whether the electronic spectral breadth induced by the β-ionone ring of retinal is primarily a homogeneous or an inhomogeneous effect. Resonance Raman intensities have been measured for both all-trans retinal and its 5,6-dihydro analog. The dihydro analog has a structured absorption spectrum because the broadening effect of the C6–C7 bond is lost when the π system does not extend into the ionone ring. While the two molecules have similar relative Raman intensities, the absolute cross sections are about a factor of two lower in all-trans retinal, indicating that the ‘‘effective homogeneous linewidth’’ is significantly larger in all-trans retinal than in the dihydro analog. Calculations carried out to model both the absorption spectra and the resonance Raman intensities require an effective homogeneous linewidth that is more than twice as large for all-trans retinal (∼1200 cm−1) as for the 5,6-dihydro analog (∼500 cm−1). The additional width is most simply attributed to Franck–Condon activity in the C6–C7 torsional degree of freedom; however, no intense resolved lines are observed in the Raman spectrum between 20 and 800 cm−1. The torsional modes involved must have very low ground-state frequencies and may be observable only through their contribution to the apparent Rayleigh and Raman linewidths.