The Hammett relationship and reactions in the excited electronic state: hemithioindigo Z/E-photoisomerization

The photochemical reaction dynamics of a set of photochromic compounds based on thioindigo and stilbene molecular parts (hemithioindigos, HTI) are presented. Photochemical Z/E isomerization around the central double bond occurs with time constants of 216 ps (Z --> E) and 10 ps (E --> Z) for a 5-methyl-hemithioindigo. Chemical substitution on the stilbene moiety causes unusually strong changes in the reaction rate. Electron-donating substituents in the position para to the central double bond (e.g., para-methoxy) strongly accelerate the reaction, while the reaction is drastically slowed by electron-withdrawing groups in this position (e.g., para-nitrile). We correlate the experimental data of seven HTI-compounds in a quantitative manner using the Hammett equation and present a qualitative explanation for the application of ground-state Hammett constants to describe the photoisomerization reaction.     

Ultrafast photochemistry of dithizonatophenylmercury(II)

The initial photochromic reaction of dithizonatophenylmercury(II) in solution was investigated by femtosecond transient absorption spectroscopy. Ultrafast excitation within less than 100 fs caused a radiationless photoreaction with a time constant of 1.5 ps, which is interpreted as C=N isomerization through a conical intersection. The orthogonally twisted intermediate state was observed through its excited-state absorption. Bifurcation along pathways towards the ground states of the orange cis and blue trans configurations occurs below the funnel of the conical intersection. The photochromism of the title compound in a very polar solvent such as methanol is observed for the first time.     

Excited-state dynamics and efficient triplet formation in phenylthiophene compounds

Ultrafast transient absorption spectroscopy monitors the solution-phase dynamics of 2-phenylthiophene (PT), 2-methyl-5-phenylthiophene (MPT), and 2,4-dimethyl-5-phenylthiophene (DMPT) following excitation to the first singlet excited state. Rapid spectral evolution indicates that structural relaxation on the S(1) potential energy surface occurs within ~100 fs, whereas the picosecond-scale kinetics reveal efficient intersystem crossing to the triplet manifold of states. The rate of intersystem crossing is significantly faster for DMPT (21.6 ± 1.0 ps) than for PT (102 ± 5 ps) and MPT (132 ± 3 ps). The measurements provide new limits on the timescale for a competing isomerization reaction in which the phenyl group changes position on the thiophene ring. The role of methyl substitution in driving the intersystem crossing is discussed.     

Subnanosecond Emission Dynamics of AT DNA Oligonucleotides

UV radiation creates excited electronic states in DNA that can decay to mutagenic photoproducts. When excited states return to the electronic ground state, photochemical injury is avoided. Understanding of the available relaxation pathways has advanced rapidly during the past decade, but there has been persistent uncertainty, and even controversy, over how to compare results from transient absorption and time‐resolved emission experiments. Here, emission from single‐ and double‐stranded AT DNA compounds excited at 265 nm was studied in aqueous solution using the time‐correlated single photon counting technique. There is quantitative agreement between the emission lifetimes ranging from 50 to 200 ps and ones measured in transient absorption experiments, demonstrating that both techniques probe the same excited states. The results indicate that excitations with lifetimes of more than a few picoseconds are weakly emissive excimer and charge transfer states. Only a minute fraction of excitations persist beyond 1 ns in AT DNA strands at room temperature.     

Sequential merocyanine product isomerization following femtosecond UV excitation of a spiropyran

The ring-opening dynamics of the photochromic switch 1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-(2H)-indole] in tetrachloroethene is studied with both femtosecond time-resolved ultraviolet (UV)/visible and UV/mid-infrared (IR) pump-probe spectroscopy. During the first picosecond we identify two new transient features in the UV/vis experiments, the first of which we assign to spiropyran S1 --> S(n) absorption (lifetime < or = 0.2 ps). The second feature (lifetime 0.5 +/- 0.2 ps) we tentatively assign to the merocyanine T2 state. After 1 ps both probing methods show biexponential merocyanine formation kinetics, with average time constants of 17 +/- 3 and 350 +/- 20 ps. In the UV/IR experiments, the initial dynamics show more dispersion in formation times than in the UV/vis measurements, whereas the slower time constant is the same in both. A weak transient IR signal at approximately 1360 cm(-1) demonstrates that this biexponentiality is caused by a sequential isomerization between two merocyanine species. Lifetimes provide evidence that the merocyanine S1 state is not involved in the photochemical reaction.     

Polyimide Dendrimers Containing Multiple Electron Donor–Acceptor Units and Their Unique Photophysical Properties

A high‐yielding synthesis of a series of polyimide dendrimers, including decacyclene‐ and perylene‐containing dendrimer D6 , in which two types of polyimide dyes are present, is reported. In these constructs, the branching unit is represented by trisphenylamine, and the solubilizing chains by N‐9‐heptadecanyl‐substituted perylene diimides. The photophysical properties of the dendrimers have been studied by absorption, steady‐state, and time‐resolved emission spectroscopy and pump–probe transient absorption spectroscopy. Photoinduced charge‐separated (CS) states are formed on the femtosecond timescale upon visible excitation. In particular, in D6 , two different CS states can be formed, involving different subunits that decays independently with different lifetimes (ca. 10–100 ps).     

Isomerization dynamics of the 2-phenylazo-1,3-dimethylimidazolium cation photoexcited to the S2 (π, π*) state as studied by transient absorption spectroscopy in the time domain of 10(-13) to 10(3) seconds

Photoinduced isomerization of a novel photochromic cation, [2PA-Mmim](+) (2-phenylazo-1,3-dimethylimidazolium cation), was studied by optical spectroscopic methods. The UV-Vis absorption spectra of the [2PA-Mmim](+) cation show two prominent bands starting around 410 and 520 nm, corresponding to the S(0)-S(2) (π, π*) and S(0)-S(1) (n, π*) transitions, respectively. The photoisomerization mechanism is studied by femtosecond time-resolved transient absorption experiments performed after S(0)-S(2) (π, π*) excitation in several solvents with different viscosity, including ionic liquids. The transient absorption signals at two representative wavelengths were fitted by bi-exponential functions, which yield four decay components. The photoisomerization mechanism is discussed in light of the relaxation schemes available for azobenzene. Only one of the components depends on the solvent viscosity and it changes from 1.2 ps (dichloromethane, 0.4 cP) to 5.6 ps ([Bmim][BF(4)], 93 cP). This component is assigned to a molecule at the S(1) state, which is responsible for the "rotational" isomerization. The weak dependence on the solvent viscosity of this component is explained in terms of local change in the viscosity as a result of local heating due to excess energy released at S(2)-S(1) internal conversion. The other three components of ～0.4, 1.0 and 10 ps are attributed to relaxation processes of the molecule at S(2), S(1) and S(0) states, respectively. The quantum yields for the forward E-Z photoisomerization are ～0.15 after S(2) excitation. The backward Z-E isomerization is slow with a lifetime of 1 hour and an activation energy of 91 kJ mol(-1) through an "inversion" mechanism.     

Excited State Intramolecular Proton Transfer of 1-Hydroxyanthraquinone

The excited state intra-molecular proton transfer dynamics of 1-hydroxyanthraquinone in solution are investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations. Two characteristic bands of excited state absorption and stimu-lated emission are observed in transient absorption spectra with the excitation by the pump wavelength of 400 nm. From the delayed stimulated emission signal, the time scale of the intra-molecular proton transfer is determined to be about 32 fs. The quantum chemistry calculations show that the molecular orbits and the order of the S₂ and S₁ states are rever-sal and a conical intersection is demonstrated to exist along the proton transfer coordinate. After proton transfer, the second excited state of tautomer populated via the conical intersection undergoes the internal conversion with ～200 fs and the following intermolecular energy relaxation with ～16 ps. The longer component 300 ps can be explained in terms of the relaxation from excited-state tautomer to its ground state. From our observations, two proton transfer pathways via a conical intersection are proposed and the dominated one preserves the molecular orbits.     

Femtosecond spectroscopic studies of the one- and two-photon excited-state dynamics of 2,2,17,17-tetramethyloctadeca-5,9,13-trien-3,7,11,15-tetrayne: a trimeric oligodiacetylene

The excited-state dynamics of an oligomer of polydiacetylene, 2,2,17,17-tetramethyloctadeca-5,9,13-trien-3,7,11,15-tetrayne, dissolved in n-hexane have been studied by femtosecond fluorescence upconversion and polarized transient absorption experiments under one- and two-photon excitation conditions. Spectroscopically monitoring the population relaxation in the excited states in real time results in a distinct time separation of the dynamics. It has been concluded that the observed dynamics can be fully accounted for on the basis of the two lower excited states of the target molecule. The S1 (2(1)Ag) state, which cannot be excited from the ground state with one-photon absorption, is verified to be populated via internal conversion in 200+/-40 fs from the strong dipole-allowed S2 (1(1)Bu) state. The population in the "hot" S1 state subsequently cools with a time constant of 6+/-1 ps and decays back to the ground state with a lifetime of 790+/-12 ps.     

Ring-closure and isomerization capabilities of spiropyran-derived merocyanine isomers

We report the photochemistry of two ring-open isomers, namely TTC and TTT, of a bidirectional photoswitchable spiropyran, 6,8-dinitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6,8-dinitro BIPS). Both isomers are capable of ring closure after excitation with visible fs laser pulses, as disclosed by pump-wavelength-dependent transient absorption experiments in the visible spectral range. The main isomer TTC has its maximum absorption at 560 nm, whereas the minor isomer TTT is red-shifted (600 nm). The excited-state lifetimes differ strongly (τ ≈ 900 ps for TTT and τ ≈ 95 ps for TTC), nevertheless the quantum efficiencies for ring closure (40% for TTC and 35% for TTT) and isomerization (1-2% for TTC and 1-2% for TTT) are comparable. With regard to the bidirectional photoswitching capabilities, 6,8-dinitro BIPS is the first molecular switch based on a 6π-electrocyclic reaction where both ring-open isomers are capable of ring closure.     

Non‐Bonded Interactions Drive the Sub‐Picosecond Bilin Photoisomerization in the Pfr State of Phytochrome Cph1

Phytochromes are protein‐based photoreceptors harboring a bilin‐based photoswitch in the active site. The timescale of photosignaling via C₁₅=C₁₆ E‐to‐Z photoisomerization has been ambiguous in the far‐red‐absorbing P_fr state. Here we present a unified view of the structural events in phytochrome Cph1 post excitation with femtosecond precision, obtained via stimulated Raman and polarization‐resolved transient IR spectroscopy. We demonstrate that photoproduct formation occurs within 700 fs, determined by a two‐step partitioning process initiated by a planarization on the electronic excited state with a 300 fs time scale. The ultrafast isomerization timescale for P_fr‐to‐P_r conversion highlights the active role of the nonbonding methyl–methyl clash initiating the reaction in the excited state. We envision that our results will motivate the synthesis of new artificial photoswitches with precisely tuned non‐bonded interactions for ultrafast response.     

Study of acyl group migration by femtosecond transient absorption spectroscopy and computational chemistry

The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1a at 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet npi* excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 +/- 90 fs. The 1npi* excited state undergoes very fast intersystem crossing (ISC, 11 +/- 1 ps) to form the lowest triplet pipi* excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 +/- 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 +/- 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV-vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed.     

Theoretical study of the absorption and nonradiative deactivation of 1-nitronaphthalene in the low-lying singlet and triplet excited states including methanol and ethanol solvent effects

The photophysics of the 1-nitronaphthalene molecular system, after the absorption transition to the first singlet excited state, is theoretically studied for investigating the ultrafast multiplicity change to the triplet manifold. The consecutive transient absorption spectra experimentally observed in this molecular system are also studied. To identify the electronic states involved in the nonradiative decay, the minimum energy path of the first singlet excited state is obtained using the complete active space self-consistent field∕∕configurational second-order perturbation approach. A near degeneracy region was found between the first singlet and the second triplet excited states with large spin-orbit coupling between them. The intersystem crossing rate was also evaluated. To support the proposed deactivation model the transient absorption spectra observed in the experiments were also considered. For this, computer simulations using sequential quantum mechanic-molecular mechanic methodology was used to consider the solvent effect in the ground and excited states for proper comparison with the experimental results. The absorption transitions from the second triplet excited state in the relaxed geometry permit to describe the transient absorption band experimentally observed around 200 fs after the absorption transition. This indicates that the T(2) electronic state is populated through the intersystem crossing presented here. The two transient absorption bands experimentally observed between 2 and 45 ps after the absorption transition are described here as the T(1)→T(3) and T(1)→T(5) transitions, supporting that the intermediate triplet state (T(2)) decays by internal conversion to T(1).     

Ultrafast UV-mid-IR investigation of the ring opening reaction of a photochromic spiropyran

We present a femtosecond UV-mid-IR pump-probe study of the photochemical ring-opening reaction of the spiropyran 1',3',3',-trimethylspiro-[-2H-1-benzopyran-2,2'-indoline] (also known as BIPS) in tetrachloroethene, using 70 fs UV excitation pulses and probing with 100 fs mid-IR pulses. The time evolution of the transient IR absorption spectrum was monitored over the first 100 ps after UV excitation. We conclude that the merocyanine product is formed with a 28 ps time constant, contrasting with a 0.9 ps time constant obtained in previous investigations where the rise of absorption bands at visible wavelengths were associated with product formation. We deduce from the observed strong recovery of the spiropyran IR absorption bleaches that, in tetrachloroethene, the main decay channel for the S(1) excited state of the spiropyran BIPS, is internal conversion to the spiropyran S(0) state with a quantum yield of > or = 0.9. This puts an upper limit of 0.1 to the quantum yield of the photochemical ring-opening reaction.     

Photoproperties of isolated cis and trans stilbene molecules

A detailed account is given of the experimental approach to measuring transient spectra of dilute gases using picosecond pulses. The picosecond continuum generated by Nd:glass laser pulses is used to probe gaseous samples and spectra are recorded in a double beam arrangement. The pump and probe pulses interact with the sample over a few centimeters by means of a dielectric waveguide. Picosecond time resolved spectra, relative fluorescence quantum yield measurements, and fluorescence spectra are reported for trans-stilbene under collision free conditions. The lifetime of the optically prepared states at 265 nm and 287 nm are 15 ps and 55 ps respectively, measured by the decay of the transient absorption. The deuteration effect is less than 20%. The variation of the fluorescence yield with vibrational energy excess in the excited state of trans is fitted to these lifetime measurements to yield the variation of nonradiative decay due to twisting of trans-stilbene. Cis-stilbene is suggested to twist in less than 1 ps. Consideration of the spectral results yields new information about the isomerization of stilbene, in particular that there exists a barrier to twisting in the isolated molecule and that vibrational energy redistribution at the trans configuration is probably not complete on the time scale of our experiments. A pictorial model for discussing constant energy relaxation phenomena is introduced.     

A computational study of ultrafast acid dissociation and acid-base neutralization reactions. II. The relationship between the coordination state of solvent molecules and concerted versus sequential acid dissociation

We investigate the role played by the coordination state of pre-existing water wires during the dissociation of moderately strong acids by means of first-principles molecular dynamics calculations. By preparing 2,4,6-tricyanophenol (calc. pKa～0.5) in two different initial states, we are able to observe sequential as well as concerted trajectories of dissociation: On one hand, equilibrium dissociation takes place on a ～50 ps timescale; proton conduction occurs through three-coordinated water wires in this case, by means of sequential Grotthus hopping. On the other hand, by preparing 2,4,6-tricyanophenol in a hydration state inherited from that of equilibrated phenol (calc. pKa=7.6), the moderately strong acid finds itself in a presolvated state from which dissociation can take place on a ～1 ps timescale. In this case, concerted dissociation trajectories are observed, which consist of proton translocation through two intervening, four-coordinated, water molecules in 0.1-1.0 ps. The present results suggest that, in general, the mechanism of proton translocation depends on how the excess proton is injected into a hydrogen bond network. In particular, if the initial conditions favour proton release to a fourfold H-bonded water molecule, proton translocation by as much as 6-8 A? can take place on a sub-picosecond timescale.     

Picosecond isomerization in photochromic ruthenium-dimethyl sulfoxide complexes

The complexes [Ru(tpy)(bpy)(dmso)](OSO(2)CF(3))(2) and trans-[Ru(tpy)(pic)(dmso)](PF(6)) (tpy is 2,2':6',2' '-terpyridine, bpy is 2,2'-bipyridine, pic is 2-pyridinecarboxylate, and dmso is dimethyl sulfoxide) were investigated by picosecond transient absorption spectroscopy in order to monitor excited-state intramolecular S-->O isomerization of the bound dmso ligand. For [Ru(tpy)(bpy)(dmso)](2+), global analysis of the spectra reveals changes that are fit by a biexponential decay with time constants of 2.4 +/- 0.2 and 36 +/- 0.2 ps. The first time constant is assigned to relaxation of the S-bonded (3)MLCT excited state. The second time constant represents both excited-state relaxation to ground state and excited-state isomerization to form O-[Ru(tpy)(bpy)(dmso)](2+). In conjunction with the S-->O isomerization quantum yield (Phi(S)(-->)(O) = 0.024), isomerization of [Ru(tpy)(bpy)(dmso)](2+) occurs with a time constant of 1.5 ns. For trans-[Ru(tpy)(pic)(dmso)](+), global analysis of the transient spectra reveals time constants of 3.6 +/- 0.2 and 118 +/- 2 ps associated with these two processes. In conjunction with the S-->O isomerization quantum yield (Phi(S)(-->)(O) = 0.25), isomerization of trans-[Ru(tpy)(pic)(dmso)](+) occurs with a time constant of 480 ps. In both cases, the thermally relaxed excited states are assigned as terpyridine-localized (3)MLCT states. Electronic state diagrams are compiled employing these data as well as electrochemical, absorption, and emission data to describe the reactivity of these complexes. The data illustrate that rapid bond-breaking and bond-making reactions can occur from (3)MLCT excited states formed from visible light irradiation.     

Primary reaction dynamics of halorhodopsin,observed by sub-picosecond IR – vibrational spectroscopy

The primary all-trans to 13-cis chromophore isomerization of the light driven chloride pump halorhodopsin has been studied by means of transient absorption spectroscopy in the visible and mid-infrared regime at a time resolution of better than 100 and 220 fs, respectively. The picosecond vibrational dynamics are dominated by two time constants, i.e., 2 and 7.7 ps in accordance with the biphasic decay of the retinal excited electronic state and electronic ground state formation with 1.5 and 6.6 ps. The transient vibrational spectra of the participating electronic states strongly suggest the existence of two distinct S₁ populations as a result of an early branching reaction. It is shown that the 13-cis product is formed with the fast time constant, whereas the all-trans educt state is repopulated via both time constants. Concomitant protein dynamics are indicated by spectral changes on a similar time scale in the amide region.