S1 S0 Internal Conversion Research Articles

One-Dimensionally Conjugated Carbocyanine Dyes Isolated under Cold Gas-Phase Conditions: Electronic Spectra and Photochemistry.

One-dimensionally conjugated carbocyanine dyes are of significant research interest, particularly for their electronic photoexcitation, owing to a wide variety of characteristics, including a good analogy to "free electrons in a one-dimensional box" model and trans-cis photoisomerization along the conjugated chain. Despite these important aspects, their electronic spectra remain ambiguous in terms of their assignment owing to the significant effects of their surrounding environment. In this study, we present the electronic spectra of two cyanine dyes, 1,1'-diethyl-2,2'-carbocyanine (pinacyanol, 1) and 1,1'-diethyl-4,4'-carbocyanine (cryptocyanine, 2), measured under cold (∼10 K) gas-phase conditions, to determine the intrinsic electronic transition energy and provide clear assignments for the spectra. The obtained visible photodissociation spectra demonstrate (1) spectral shifts in response to both solvent and temperature, (2) the contribution from the vibrational excitation in the excited state (Franck-Condon (FC) activity), and (3) the coexistence of conformers caused by the orientation of the side ethyl groups. These factors affect the electronic transition energy up to ∼1000 cm-1 in total for both 1 and 2, which corresponds to an effective length of 0.5 Å in terms of the "one-dimensional box" model. Furthermore, a difference was observed in the effective bandwidth of the spectra between 1 and 2 based on a comparison with the simulated FC patterns around the origin band; the bandwidth was substantially larger for 2 than that of 1, implying the shorter lifetime of 2 in the photoexcited S1 state. With the aid of density functional theory (DFT) calculations of the relaxed potential energy curves, we partly ascribed this to the fast trans-cis photoisomerization via C═C bond twisting on the S1 surface, followed by S1-S0 internal conversion.

Quantum-state resolved lifetime of triplet ([formula omitted]) formaldehyde

We report state-resolved lifetimes of formaldehyde in the first excited triplet ã3A2 electronic state. Pump-laser excitation within 2800 cm−1 of the triplet origin is followed by variably delayed F2-laser ionization. Lifetimes fluctuate from >100 μs to ∼100 ns and a large isotope effect is observed. The average decay rate increases exponentially with increasing vibrational energy above the electronic origin, consistent with a mechanism involving tunneling through the barrier on the electronic ground state to the CO+H2/D2 continuum. By accessing the triplet state we are able to investigate the molecular elimination reaction down to ∼25 kJ/mol below the zero-point-energy corrected activation barrier. A model based on empirical RRKM rates allows the intersystem crossing matrix elements to be estimated. The T1–S0 intersystem crossing matrix elements are comparable to the S1–S0 internal conversion matrix elements observed at similar vibrational excitation.

Spectroscopic and Photophysical Characterization of Acetylenic Fluorophores: The Role of the Proximity Effect on Increasing Internal Conversion.

The spectroscopic and photophysical behavior of a series of six extended arylacetylenes and one 1,3-diyne derivative was studied in solvents of different polarity to gain insight into the relationships between molecular architecture and optical/photophysical properties. The radiative decay channel was revealed to be the most important one for all the compounds, particularly in nonpolar solvents. A notable fluoro-solvatochromism was observed for the 1,3-diyne derivative in line with the large increase of its dipole moment under excitation. A peculiar behavior was observed for nitro-substituted aryl acetylenes on increasing solvent polarity, which was explained by the presence of an upper forbidden singlet state (S2 ) that is more polar than the allowed S1 state; this was confirmed by quantum mechanical calculations. The large decrease of the S2 -S1 energy gap in a moderately polar solvent causes a strong increase of S1 -S0 internal conversion to the detriment of fluorescence, which is in agreement with Lim's proximity effect model.

Theoretical Prediction of the S1−S0 Internal Conversion of 6-Cyanoazulene

Ab initio complete active-space self-consistent field (CASSCF) and second-order Multireference Möller-Plesset perturbation (MRMP2) calculations were performed to examine the S1-S0 internal conversion of 6-cyanoazulene (6CNAZ). The azulene skeletons of 6CNAZ in S0 and S1 have features that resemble those of azulene. The stable geometry in S0 is characterized by (i) a C2v structure, (ii) an aromatic bond-equalized structure in which all the peripheral skeletal bond distances resemble an aromatic CC bond distance, and (iii) a single bond character of the transannular bond. The stable geometry in S1 is characterized by a nonaromatic C2v structure. Contrary to similarities of the stable geometries in S0 and S1 between 6CNAZ and azulene, the conical intersection (S1/S0-CIX) of 6CNAZ is different from that of azulene. The S1/S0-CIX of 6CNAZ takes a planar structure, whereas that of azulene takes a nonplanar structure in the seven-membered ring (Amatatsu, Y.; Komura, K. J. Chem. Phys. 2006, 125, 174311/1-8). On the basis of those computational findings, we predict the photochemical behavior of 6CNAZ in the S1-S0 internal conversion.

Ultrafast Excited-State Isomerization Dynamics of 1,1‘-Diethyl-2,2‘-Cyanine Studied by Four-Wave Mixing Spectroscopy

Excited-state dynamics and solvent-solute interactions of 1,1'-diethyl-2,2'-cyanine iodine (1122C) in alcoholic solutions are investigated using time-integrated three-pulse photon-echo spectroscopy. 1122C serves as a model compound for ultrafast photoinduced isomerization-a key process in the light reception of plants, bacteria, and human vision. The photoreaction in 1122C is interrogated in dependence on solvent and excitation wavelength. The wavelength-dependent three-pulse photon-echo peak shift indicates strong alterations of the reaction pathways and points to the existence of a direct internal conversion channel in close proximity to the Franck-Condon point of absorption. The solvent-dependent S1-S0 internal conversion time does not follow conventional sheared viscosity dependence, suggesting that the solvent local friction has to be considered to account for the observed isomerization kinetics. The concerted discussion of transient grating and three-pulse photon-echo peak-shift data allows us to derive a complete picture of the solvent-solute interaction-controlled photoreaction. The results obtained are related to other work on reactive systems and are discussed in the framework of multilevel response functions.

Reaction coordinate analysis of the S1-S internal conversion of azulene

The reaction coordinate of the S(1)-S(0) internal conversion of azulene has been analyzed using ab initio complete active space self-consistent field method. The stable geometry in S(0) (S(0) geometry) takes a bond-equalized structure where all the peripheral skeletal bond distances are similar to an aromatic CC bond distance. The transannular bond is similar to a normal C-C single bond. The first event upon electronic excitation into S(1) at S(0) geometry is characterized by the following two simultaneous changes in the skeletal bonds; the transannular bond in S(1) increases its double bond character and the aromaticity of the peripheral bonds disappears. In consequence, the most stable azulene in S(1) (S(1) geometry) has a biradical character. To reach the conical intersection between S(1) and S(0) (S(1)S(0)-CIX) where radiationless relaxation takes place, the seven-membered ring greatly deviates from a planar structure. After a transition into S(0) at S(1)S(0)-CIX, the bond-equalized structure is recovered immediately and then the nonplanarity decreases so that azulene again takes the stable planar S(0) geometry. In order to deepen the understanding of the S(1)-S(0) internal conversion, the dipole moments along the reaction coordinate have been analyzed.

Femtosecond photoelectron imaging of pyridazine: S1 lifetime and (3s(n−1),3p(n−1)) Rydberg state energetics

The first real-time study on pyridazine in the S1(n,π*) state is presented. The S1 state is found to dephase with a time constant of 323±17 ps at its origin, and the electronic dephasing mechanism is attributed to the S1–S0 internal conversion. The S1 lifetime is found to decrease rather sharply as the internal energy increases. The 3s (n−1) and 3p (n−1) Rydberg states of pyridazine are clearly identified in angle- and energy-resolved photoelectron images obtained in the (1+2′) photoionization scheme, providing their respective term values of 5.68±0.03 and 6.28±0.04 eV.

The role of specific normal modes during non‐Born–Oppenheimer dynamics: the S1–S0 internal conversion of β‐carotene interrogated on a femtosecond time‐scale with coherent anti‐Stokes Raman scattering

AbstractThe population flow from the first excited singlet state (S1) to the electronic ground state (S0), facilitated through the S1–S0 internal conversion, and the subsequent internal vibrational energy redistribution and vibrational cooling processes are monitored selectively with respect to the different forms of nuclear motion in all‐trans‐β‐carotene. This is realized by exciting the molecule into the second excited singlet state (S2) with an auxiliary pump laser pulse and interrogating the population recovery into the electronic ground state by means of a time‐delayed, coherent anti‐Stokes Raman (CARS) process. This spectroscopic scheme is referred to as a pump‐CARS scheme in analogy to the classical pump–probe scheme in time‐resolved spectroscopy. Here, the profound enhancement of the CARS signal intensity in the case of a Raman resonance to a specific vibrational mode is utilized as an intensity filter that amplifies the signal from the vibrational modes of interest, making the contributions from other vibrational modes negligible. This filter allows for the population flow in a specific vibrational mode to be monitored as the radiationless electronic transition between the S1 and S0 state takes place. This spectroscopic scheme opens up the possibility of identifying the vibrational motion with a large‐amplitude motion in the CC double bond symmetric stretch as the primary acceptor of population from the S1 state. A mechanism with which the other normal modes are populated is postulated. Copyright © 2002 John Wiley & Sons, Ltd.

Intersecting conical intersection seams in tetra-atomic molecules: the S1–S0 internal conversion in HNCO

Symmetry-allowed intersections of two states of different symmetry, different-symmetry intersections, are the class of conical intersection most commonly studied in small molecules. A same-symmetry seam of conical interaction, originating from the same two states, may intersect this different-symmetry seam. These intersections of intersections are themselves continuously connected, forming a multidimensional subspace in the symmetry-allowed seam. A confluence of seams requires changes in the conical topography which can affect nuclear dynamics significantly. The indirect photodissociation of HNCO to H + NCO results from an internal conversion attributable to the portion of the S0-S1 seam with HNCO in a trans configuration. A previously introduced perturbative analysis is used to show that a manifold of confluences of conical intersections exists in this region. These confluences can influence the internal conversion dynamics and should be incorporated into a detailed description of this process. This represents the first time a dynamically relevant manifold of confluences has been demonstrated in a tetra-atomic molecule.