F. Temps

Highly Efficient Reversible Z−E Photoisomerization of a Bridged Azobenzene with Visible Light through Resolved S1(nπ*) Absorption Bands

The reversible Z-E photoswitching properties of the (Z) and (E) isomers of the severely constrained bridged azobenzene derivative 5,6-dihydrodibenzo[c,g][1,2]diazocine (1) were investigated quantitatively by UV/vis absorption spectroscopy in solution in n-hexane. In contrast to normal azobenzene (AB), 1 has well separated S(1)(n pi*) absorption bands, peaking at lambda(Z) = 404 nm and lambda(E) = 490 nm. Using light at lambda = 385 nm, it was found that 1Z can be switched to 1E with very high efficiency, Gamma = 92 +/- 3%. Conversely, 1E can be switched back to 1Z using light at lambda = 520 nm with approximately 100% yield. The measured quantum yields are Phi(Z-->E) = 72 +/- 4% and Phi(E-->Z) = 50 +/- 10%. The thermal lifetime of the (E) isomer is 4.5 +/- 0.1 h at 28.5 degrees C. The observed photochromic and photoswitching properties of 1 are much more favorable than those for normal AB, making our title compound a promising candidate for interesting applications as a molecular photoswitch especially at low temperatures. The severe constraints by the ethylenic bridge apparently do not hinder but favor the Z-E photoisomerization reactions.

Base Sequence and Higher-Order Structure Induce the Complex Excited-State Dynamics in DNA

The high photostability of DNA is commonly attributed to efficient radiationless electronic relaxation processes. We used femtosecond time-resolved fluorescence spectroscopy to reveal that the ensuing dynamics are strongly dependent on base sequence and are also affected by higher-order structure. Excited electronic state lifetimes in dG-doped d(A)20 single-stranded DNA and dG·dC-doped d(A)20·d(T)20 double-stranded DNA decrease sharply with the substitution of only a few bases. In duplexes containing d(AGA)·d(TCT) or d(AG)·d(TC) repeats, deactivation of the fluorescing states occurs on the subpicosecond time scale, but the excited-state lifetimes increase again in extended d(G) runs. The results point at more complex and molecule-specific photodynamics in native DNA than may be evident in simpler model systems.

Photofragment velocity map imaging of H atom elimination in the first excited state of pyrrole

Photofragment velocity map imaging was used to study the H atom elimination mechanism in the first excited state of pyrrole at λ=243.1 nm. Two major channels were observed. The first one (76%) produces very fast H atoms and appears to be due to a rapid direct N–H bond breaking in the excited electronic state. The respective H atom kinetic energy distribution has a strong narrow peak at high energies, showing that ≈72% of the available energy is transferred into relative fragment translation. The observed angular recoil distribution which is described by an anisotropy parameter of β=−0.37±0.05 indicates that the excited optical transition is preferentially perpendicular with respect to the N–H dissociation coordinate. From the maximal kinetic energy release, the value of the N–H bond dissociation energy was found to be D0(N–H)=(32400±400) cm−1. The other channel (24%) leads to much slower H atoms with a very broad kinetic energy distribution, consistent with subsequent unimolecular decay reactions of the molecules in the ground electronic state after internal conversion. This conclusion was supported by similar experiments for N-methylpyrrole which showed only H atoms from the second channel and no fast component. The results corroborate the conclusion that the lowest electronic state of pyrrole has πσ* anti-bonding character and is repulsive with respect to the stretching of the N–H bond.

Unimolecular dissociation dynamics of highly vibrationally excited DCO(X̃ 2A). II. Calculation of resonance energies and widths and comparison with high-resolution spectroscopic data

We present a theoretical study of the unimolecular dissociation of DCO in the electronic ground state, X 1A, using a new ab initio potential energy surface. Altogether we have analyzed about 140 resonances up to an energy of ≈1.4 eV above the D+CO threshold, corresponding to the ninth overtone in the CO stretching mode (v2=9). The agreement of the resonance positions and widths with recent stimulated emission pumping measurements of Stock et al. [J. Chem. Phys. 106, 5333 (1997), the preceding article] is pleasing. The root-mean-square deviation from the experimental energies is only 16 cm−1 over a range of about 16 500 cm−1 and all trends of the resonance widths observed in the experiment are satisfactorily reproduced by the calculations. A strong 1:1:2 stretch–stretch–bend resonance prohibits a unique assignment for the majority of vibrational states.

Rotation–vibration state resolved unimolecular dynamics of highly vibrationally excited CH3O (X̃ 2E). I. Observed stimulated emission pumping spectra

Using the technique of stimulated emission pumping (SEP) spectroscopy, highly excited vibration–rotation states of the CH3O (X 2E) molecule were probed up to energies of E≤10 000 cm−1. The highest excitation energies exceed the asymptotic H–H2CO dissociation limit of the molecule [ΔrH00(H–H2CO)≊6900 cm−1]. Work was carried out at different experimental resolutions. First, low resolution survey SEP spectra were found to exhibit persistent vibrational structure up to energies far above the dissociation limit. The observed main features were found to be assignable, in a zero‐order picture that leaves aside possible mode‐to‐mode couplings, to the progression of the excited C–O stretch vibration states (ν3). The widths of the respective features correspond to localized short‐time vibrational motion for times of ≥0.3 ps (≥10 C–O vibrational periods). Second, in high resolution scans over the coarse vibrational features, characteristic clumps of individual vibration–rotation eigenstates were revealed. These clu...

Unimolecular dissociation dynamics of highly vibrationally excited DCO (X̃ 2A). I. Investigation of dissociative resonance states by stimulated emission pumping spectroscopy

The vibrational level structure and unimolecular dissociation dynamics of highly vibrationally excited X 2A DCO were investigated using the method of stimulated emission pumping spectroscopy (SEP). Single vibration-rotation states were probed with excitation energies up to E(X)=18 200 cm−1, ≈12 700 cm−1 above the asymptotic D-CO dissociation limit. The vibrational level structure of the molecule was found to be determined by distinctive polyads arising from a 1:1:2 resonance between the CD stretching, CO stretching, and DCO bending vibrations. Anharmonic coupling mechanisms give rise to considerable level mixings, especially regarding the CD and CO stretching motion. Thus, only a minority of vibrational states can be unambiguously assigned. The spectral line shape profiles of ≈100 highly excited “resonance states” in the continuum above the D-CO dissociation limit were measured at high resolution. The profiles are homogeneously broadened. The unimolecular decay rates, obtained from the observed line wid...

Superior Z→E and E→Z photoswitching dynamics of dihydrodibenzodiazocine, a bridged azobenzene, by S1(nπ*) excitation at λ = 387 and 490 nm.

The ultrafast Z→E and E→Z photoisomerisation dynamics of 5,6-dihydrodibenzo[c,g][1,2]diazocine (1), the parent compound of a class of bridged azobenzene-based photochromic molecular switches with a severely constrained eight-membered heterocyclic ring as central unit, have been studied by femtosecond time-resolved spectroscopy in n-hexane as solvent and by quantum chemical calculations. The diazocine contrasts with azobenzene (AB) in that its Z rather than E isomer is the energetically more stable form. Moreover, it stands out compared to AB for the spectrally well separated S(1)(nπ*) absorption bands of its two isomers. The Z isomer absorbs at around λ = 404 nm, the E form has its absorption maximum around λ = 490 nm. The observed transient spectra following S(1)(nπ*) photoexcitation show ultrafast excited-state decays with time constants τ(1) = 70 fs for the Z and <50 fs for the E isomer reflecting very fast departures of the excited wave packets from the S(1) Franck-Condon regions and τ(2) = 270 fs (320 fs) related to the Z→E (resp. E→Z) isomerisations. Slower transient absorption changes on the time scale of τ(3) = 5 ps are due to vibrational cooling of the reaction products. The results show that the unique steric constraints in the diazocine do not hinder, but accelerate the molecular isomerisation dynamics and increase the photoswitching efficiencies, contrary to chemical intuition. The observed isomerisation times and quantum yields are rationalised on the basis of CASPT2//CASSCF calculations by a S(1)/S(0) conical intersection seam at a CNNC dihedral angle of ≈96° involving twisting and torsion of the central CNNC moiety. With improved photochromism, high quantum yields, short reaction times and good photostability, diazocine 1 and its derivatives constitute outstanding candidates for photoswitchable molecular tweezers and other applications.

Rotation–vibration state-resolved unimolecular dynamics of highly excited CH3O (X 2E). Part 3.—State-specific dissociation rates from spectroscopic line profiles and time-resolved measurements

Vibration–rotation quantum-state resolved measurements of the unimolecular dissociation rates of highly vibrationally excited CH3O (X 2E) have been performed over a wide range of excitation energies (7000 ⩽E/cm–1⩽ 10 000). Single excited CH3O (X) quantum states were prepared using the method of stimulated emission pumping (SEP). State-specific decay constants were determined from direct time-resolved measurements using laser-induced fluorescence detection (LIF) of the excited states and from SEP line profiles measured at higher resolution. In very narrow energy windows, the measured decay constants were found to vary statistically by up to two orders of magnitude. These state-specific fluctuations are in contrast with the traditional picture from unimolecular rate theory (e.g. RRKM theory). The fluctuations were analysed statistically. The average decay rates were found to increase with increasing molecular excitation energy. This general trend could be nicely described by an RRKM model on average. Indications for small deviations were observed at high energies. Viewed in connection with related data on the kinetics of intramolecular vibrational energy randomization (IVR) processes, these deviations may reflect the inherent limitations of statistical theory at high energies where dissociation and IVR compete.

Collision‐induced intersystem crossing of CH2 from ã 1A1 to X̃ 3B1: A case study of the mixed‐state model

The rate coefficients for collision‐induced intersystem crossing (CIISC) of methylene from the a 1A1 first excited to the X 3B1 ground electronic state, CH2 (a 1A1)+M→CH2 (X 3B1)+M, were investigated within the framework of the mixed‐state mechanism [see, e.g., K. F. Freed, in Potential Energy Surfaces, edited by K. P. Law (Wiley, New York, 1980)]. Accordingly, the overall electronic relaxation was assumed to proceed via a sequence of rotational transitions within the a manifold and allowed transitions from the a to the X manifold originating via ‘‘gate’’ states of a which are states that contain some triplet character due to spin–orbit coupling with nearby X rovibrational states. The perturbed a and perturbing X levels and relevant interaction matrix elements were identified from the available spectroscopic data. Rate coefficients for rotational relaxation processes were obtained from collision broadening measurements of CH2 (X) far‐infrared laser magnetic resonance (FIR‐LMR) transitions. Tak...

Direct measurements of state specific unimolecular dissociation rate constants of highly excited single rotation vibration quantum states of CH3O (X̃ 2E)

Vibration rotation quantum state resolved unimolecular dissociation lifetimes of highly excited CH3O (X 2E) molecules have been measured using the method of stimulated emission pumping (SEP) in connection with transient laser induced fluorescence (TLIF) excitation spectroscopy for preparing selected single target states and for their time resolved detection. The measured decay rate constants for six closely spaced levels with precisely known total excitation energies around E(X)≊7450 cm−1 and J=0.5 (or 1.5) were found to vary erratically between 9⋅105 s−1≤k≤3⋅107 s−1. The results are compared to predictions by unimolecular rate theory and discussed with respect to the question of mode specific vs statistical reactivity.