Ultrafast Dynamics of the Transoid-cis Isomer Formed in Photochromic Reaction from 3H-Naphthopyran

Recent efforts in designing new 3H-naphthopyran derivatives have been focused on efficient coloration process with a short fading time of the colored transoid-cis TC isomer. It is desirable to avoid photoisomerization of TC leading to transoid-trans TT isomers in the photoreaction. Long lifetime of TT can hamper fast applications such as dynamic holographic materials and molecular actuators, the residual color is one of the serious issues for photochromic lenses. Herein we characterize the photophysical and photochemical channels of TC excited state deactivation competing with the unwanted TC → TT isomerization process. Transient absorption spectroscopy reveals a very short lifetime of the singlet excited TC (≈0.8 ps) and its deactivation channels as S₁→S₀ internal conversion (major), intersystem crossing S₁→T₁, pyran ring formation, photoenolization and TC → TT isomerization. Computations support the S₁→S₀ and T₁→S₀ channels as responsible for photostabilization of the TC form.     

Unravelling the Reaction Mechanism for the Fast Photocyclisation of 2‐Benzoylpyridine in Aqueous Solvent by Time‐Resolved Spectroscopy and Density Functional Theory Calculations

A combined femtosecond transient absorption (fs‐TA) and nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopic investigation of the photoreaction of 2‐benzoylpyridine (2‐BPy) in acetonitrile and neutral, basic and acidic aqueous solvents is reported. fs‐TA results showed that the nπ* triplet 2‐BPy is the precursor of the photocyclisation reaction in neutral and basic aqueous solvents. The cis triplet biradical and the cis singlet zwitterionic species produced during the photocyclisation reaction were initially characterised by ns‐TR³ spectroscopy. In addition, a new species was uniquely observed in basic aqueous solvent after the decay of the cis singlet zwitterionic species and this new species was tentatively assigned to the photocyclised radical anion. The ground‐state conformation of 2‐BPy in acidic aqueous solvent is the pyridine nitrogen‐protonated 2‐BPy cation (2‐BPy‐NH⁺) rather than the neutral form of 2‐BPy. After laser photolysis, the singlet excited state (S₁) of 2‐BPy‐NH⁺ is generated and evolves through excited‐state proton transfer (ESPT) and efficient intersystem crossing (ISC) processes to the triplet exited state (T₁) of the carbonyl oxygen‐protonated 2‐BPy cation (2‐BPy‐OH⁺) and then photocyclises with the lone pair of the nitrogen atom in the heterocyclic ring. Cyclisation reactions take place both in neutral/basic and acidic aqueous solvents, but the photocyclisation mechanisms in these different aqueous solvents are very different. This is likely due to the different conformation of the precursor and the influence of hydrogen‐bonding of the solvent on the reactions.     

Photochemische Reaktionen 49. Mitteilung [1] Spezifisch π → π*-induzierte Keton-Photoreaktionen: Die Doppelbindungsverschiebung von O-Acetyl-10α-testosteron Protonisierung der Doppelbindung seines δ5-Isomeren

The α,β-unsatured ketone 10α-testosterone has been reported previously [6] to photoisomerize in t-butanol solution to the β,γ-unsaturated ketone. The irradiation had been carried out using a high-pressure mercury lamp in a quartz vessel. For structural reasons this double bond shift cannot proceed through a photoenolization mechanism involving an intramolecular hydrogen transfer from the γ-position to the enone oxygen as has been suggested to operate in several formally analogous cases of aliphatic enone isomerizations. In the present reinvestigation, O-acetyl 10α-testosterone ( 1 ) was used, employing selectively either excitation of its n → π* (with wavelengths > 300 nm) or its π → π* absorption band (with 253,7 nm). In t-butanol solution the doublebond shift 1 → 2 could be effected with π→* excitation only. Experiments in deuterated solvent (t-BuOD) resulted in deuterium in corporation in both the δ5-ketone in the C(4)-position, cf.( 3 ) and in the conjugated ketone. These results indicate that the reactions is initiated either in the, S_π,π* state or in a high vibrational mode of the S₀ or t_ππ*state. n→ π* Excitation of 1 in t-butanol gave essentially no over-all chemical change, while in benzene solution it resulted again in a double bond isomerization ( 1 → 2 ). In analogy to results with similar enones [28] under identical conditions the deconjugation in benzene may be the consequence of an intermolecular hydrogen abstraction of the T_n,π* excited state of the enone. Another specifically π →π* induced photoreaction was observed on irradiation of the β, γ-unsaturated ketone 2 in t-BuOD with 253,7 nm. The olefinic hydrogen at C-6 of 2 was exchanged with deuterium and, to a small extent, isomerization to the conjugated ketone 1 with concomitant deuterium incorporation occurred. It is concluded that from the higher excited state of the β, γ-unsaturated ketone, but not from its S_n,π* state, an activation mode of the double bond is accessible to effect D+ addition at C-6 followed by deprotonation to 4 and to deuterated 1 , respectively.     

Electronic structure of benzaldehyde: A comparative study of the lowest excited singlet π* ← π and π* ← n states

VE-PPP, CNDO/2, and CNDO/s-CI methods have been used to investigate the electronic spectrum and structure of benzaldehyde. Electronic charge distributions and bond orders in the ground and lowest excited singlet π* ← π and π* ← n states of the molecule have been studied. The molecule has been shown to be nonplanar in the lowest π* ← n excited singlet state, in agreement with the conclusions drawn from the study of vibrational spectra. Dipole moments in both excited states have been shown to be larger than the ground-state value. Thus, the ambiguity in the experimental result for the π* ← π n excited singlet state dipole moment has been resolved. It has been shown that the n orbital is mainly localized on the CHO group. Furthermore, charge distributions, dipole moments, and molecular geometries are shown to be very different in the excited singlet π* ← π and π* ← n states.     

The keto→enol photoisomerization of N‐salicilydenemethylfurylamine: Nonadiabatic ab initio dynamics simulation

The photoinduced isomerization of cis‐keto and trans‐keto isomers in N‐salicilydenemethylfurylamine has been studied using the surface‐hopping approach at the CASSCF level of theory. After the cis‐keto or trans‐keto isomer is excited to S₁ state, the molecule initially moves to a excited‐state local minimum. The torsional motion around relative bonds in the chain drives the molecule to approach a keto‐form conical intersection and then nonadiabatic transition occurs. According to our full‐dimensional dynamics simulations, the trans‐keto and enol photoproducts are responsible for the photochromic effect of cis‐keto isomer excited to S₁ state, while no enol isomer was obtained in the photoisomerization of trans keto on excitation. The cis keto to enol and cis keto to trans keto isomerizations are reversible photochemical reactions. It is confirmed that this aromatic Schiff base is a potential molecular switch. Furthermore, the torsion of C N bond occurs in the radiationless decay of trans‐keto isomer, while it is completely suppressed by an intramolecular hydrogen bonding interaction in the dynamics of cis‐keto form. Moreover, the excited‐state lifetime of cis keto is longer than that of trans‐keto form due to the O···H N hydrogen bond.     

Specifically (π → π*)-Induced Cyclohexenone Reactions 6-Benzylbicyclo [4.4.0]dec-1-en-3-ones

6-Benzylbicyclo [4.4.0]dec-1-en-3-one ( 9 ) and the 2-methyl homologue ( 10 ) underwent a (γ → α )-1, 3-benzyl shift to the β,γ-unsaturated ketones 21 and 22 , respectively, when excited in the π π* absorption band. The quantum yield was ca. 0.1 at 254 nm for the formation of both products in alkane solvents. These reactions occur specifically from the S₂(π, π*) state in competition with its decay to the S₁(n, π*) and T states. The triplet reaction of 9 , initiated by n → π* irradiation and by sensitization, was a double-bond shift to 20 , whereas no identifiable product was observed from 10 under these conditions. Direct and acetone-sensitized irradiations of 21 and 22 resulted in oxadi-π-methane rearrangements to mixtures of syn- and anti- 30 and syn- and anti- 31 , respectively.     

An ab initio study of the structures and energetics of the planar ground and 90°‐twisted excited states of substituted ethylenes

Ab initio molecular orbital calculations are performed on the planar ground states (S₀), the 90°‐twisted triplet (T₁), and pyramidalized singlet (S₁) excited states of ethylene, methaniminium cation (MC), monocyano‐ (MCE), 1,1‐dicyano‐ (DCE), 1,1‐dihydroxy‐ (DHE), and 1,1‐dicyano‐2,2‐dihydroxy (DCHE) ethylenes. Equilibrium geometries are optimized at the Hartree–Fock (HF) level with the 6‐31G* basis set. Electron correlation corrections are estimated by optimizing the HF/6‐31G* geometries at the (U)MP2/6‐31G* level and then by carrying out single‐point calculations at the fourth‐order Møller–Plesset perturbation theory ((U)MP4/6‐311G**//MP2/6‐31G*). The effects of various types of perturbations on the structures, energetics, dipole moments, and state ordering of S₀, S₁, and T₁ are carefully investigated. “Positive” S₁–T₁ splittings are estimated at the HF level for all studied molecules, while “negative” S₁–T₁ splittings are obtained at the MP2 level for MC, DHE, and DCHE. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 242–254, 2001     

Spezifisch π→π*-induzierte Reaktionen von γ-Dimethoxymethylcyclohexen-2-onen: 1,3-Umlagerung und Wasserstoffabstraktion durch das α-Kohlenstoffatom

When α,β-unsaturated γ-dimethoxymethyl cyclohexenones are excited to the S₂(π,π*) state, certain unimolecular reactions can be observed to compete with S₂ → S₁ internal conversion. These reactions do not occur from the S₁(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8 , 9 → 32 + 33 ), γ → α formylacetal migration (cf. 6 → 27 , 8 → 30 , 9 → 34 , 12 → 37 ), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 → 28 , 8 → 31 , 12 → 38 , 20 → 40 + 41 ). The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ ( 40a + 41a ) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*-induced transformations is placed energetically higher than the S₁(n,π*) state, and it is either identical with the thermally equilibrated S₂(n,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar π→π*-induced cyclization (→ 42 ) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross-conjugated cyclohexadienones.     

Solid‐state NMR studies of cis‐1,4‐polyisoprene crosslinked with dicumyl peroxide in the presence of triallyl cyanurate

The network of dicumyl peroxide (DCP)/triallyl cyanurate (TAC) crosslinked cis‐1,4‐polyisoprene was studied by solid‐state NMR techniques such as direct‐polarization (DP), cross‐polarization (CP), and proton T₂ experiments. Line broadening and cis/trans isomerization of mobile carbons were observed in the DP experiments. The information on rigid carbons of network structures was observed with the CP technique. Motional heterogeneity was examined by proton T₂ relaxation experiments. Decreases in long T₂ (T_2L) values from the mobile non‐network structures and short T₂ (T_2S) values from the rigid network structures were observed with an increase in peroxide or coagent concentration. The percentage of T_2S in T₂ relaxation, which is related to network density, was observed to increase with peroxide and coagent addition. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1417–1423, 2000     

Tailoring Excited State Properties and Energy Levels Arrangement via Subtle Structural Design on D‐π‐A Materials

The donor‐π‐conjugated‐acceptor (D‐π‐A) structure is an important design for the luminescent materials because of its diversity in the selections of donor, π‐bridge and acceptor groups. Herein, we demonstrate two examples of D‐π‐A structures capable to finely modulate the excited state properties and arrangement of energy levels, TPA‐AN‐BP and CZP‐AN‐BP , which possess the same acceptor and π‐bridge but different donor. The investigation of their photophysical properties and DFT calculation revealed that the D‐π‐A structure with proper donor, π‐bridge and acceptor can result in separation of frontier molecular orbitals on the corresponding donor and acceptor with an obvious overlap on the π‐bridge, resulting in a hybridized local and charge‐transfer (HLCT ) excited state with high photoluminescent (PL ) efficiencies. Meanwhile, their singlet and triplet states are arranged on corresponding moieties with large energy gap between T₂ and T₁ , and a small energy gap between S₁ and T₂ , which favor the reverse intersystem crossing (RISC ) from high‐lying triplet levels to singlet levels. As a result, the sky‐blue emission non‐doped OLED based on the TPA‐AN‐BP reached maximum external quantum efficiency (EQE ) of 4.39% and a high exciton utilization efficiency (EUE ) of 77%. This study demonstrates a new strategy to construct highly efficient OLED materials.     

Zur Photochemie des Caryophyllens und des iso-Caryophyllens Umlagerungen ihrer 1,5-Diensysteme nach direkter π,π*-Anregung in flüssiger Phase

When caryophyllene ( 1 ) and isocaryophyllene ( 2 ) are irradiated at their π,π*- transitions (λ ≤ 253.7 nm) they rearrange relatively rapidly and in high yield to mixtures of isomeric sesquiterpene hydrocarbons. Compounds 3–11 , the structural determinations of which are described, represent 90% of the photoisomers. All the rearrangements involve the 1, 5-diene systems, and, with the exception of cis-trans isomerizations, fragmentation of the doubly allylic σ-bond and disappearance of the trisubstituted double bond always occur. The cyclopropane, acetylene, allene, and 1, 5-diene compounds resulting are the first examples of this type of photo-isomerization products obtained from unconjugated monomeric 1, 5-dienes by direct excitation with light. Their formation appears to be stereospecific, and is dependent on the stereochemistry of the double bond system in 1 and 2 . In addition to the cis-isomer 2 caryophyllene yields only compounds 3, 4 , and 5 , while from isocaryophyllene, compounds 8, 9, 10 and 11 are formed in addition to 1 . The only isomerization products common to both caryophyllenes ( 1 and 2 ) are 6 and 7 . This behaviour and the lack of positive identification of a biradical (triplet) intermediate lead to the probable conclusion that the greater part of the photoisomerization occurs directly from the excited singlet states. Possible mechanisms for the formation of the photoisomerization products are discussed.     

Solvent and temperature effects on the fluorescence and competitive photoreactions of cis-9-styrylanthracene

A fluorimetric and photochemical study of cis-9-styrylanthracene as a function of temperature has been carried out in different solvents with the aim of answering some open questions about the behavior of its first excited singlet state, S₁. In non-polar solvents, a parallel photoreactive pathway, leading to a cyclization adduct, was found to compete with fluorescence and isomerization already at 200 K, its contribution increasing markedly with temperature. The cis » trans photoisomerization occurs prevalently by a triplet mechanism, a detectable contribution of diabatic and adiabatic isomerization in S₁ being operative in these solvents from room temperature upwards. In polar solvents, the main deactivation pathway competitive with fluorescence is isomerization to trans, which occurs prevalently through a mixed singlet mechanism with a major diabatic and a minor adiabatic components.     

Modeling the structure,absorption spectra,and cis-trans isomerization of thiacarbocyanine dyes

The relative stability of the trans-and cis-isomers of 3,3′-diethylthiacarbocyanine (Dye1) and 3,3′-diethyl-9-methylthiacarbocyanine (Dye2)¹, as well as sections of the potential energy surfaces along the internal coordinate of the isomerization reaction, were studied using the density functional theory. Calculation of the minimum energy pathway for the isomerization reaction showed that the barrier for rotation about the C₈–C₉ bond is higher for Dye1 than for Dye2. Local minimums were found for the singlet excited state of the 8,9-cis-and trans-isomers of the dyes. In the case of the trans-isomers, substantial changes in the dye structure do not occur and the local minimum of the excited state corresponds to the geometry of the starting trans-isomers, which favors efficient fluorescence. A search for the nearest local minimum of the singlet excited state of the 8,9-cis-isomers leads to structures, which differ significantly from the starting structures, and the intensity of the S₁ → S₀ transition in those structures appears to be practically zero. The results are in agreement with experimental data on the absorption, fluorescence, and fluorescence excitation spectra of the dyes.     

1‐Aminoindan‐2‐ol,a Suitable Ligand for the Synthesis of Chiral,Intramolecularly Stabilized Compounds of Aluminum,Gallium, and Indium

The reactions of enantiomerically pure (1R, 2S)‐(+)‐cis‐1‐aminoindan‐2‐ol, (1S, 2R)‐(‐)‐cis‐1‐aminoindan‐2‐ol, and racemic trans‐1‐aminoindan‐2‐ol with trimethylaluminum, ‐gallium, and ‐indium produce the intramolecularly stabilized, enantiomerically pure dimethylmetal‐1‐amino‐2‐indanolates (1R, 2S)‐(+)‐cis‐Me₂AlO‐2‐C*HC₇H₆‐1‐C*HNH₂ ( 1 ), (1S, 2R)‐(‐)‐cis‐Me₂AlO‐2C*HC₇H₆‐1‐C*HNH₂ ( 2 ), (1R, 2S)‐(+)‐cis‐Me₂GaO‐2‐C*HC₇H₆‐1‐C*HNH₂ ( 3 ), (1R, 2S)‐(+)‐cis‐Me₂InO‐2‐C*HC₇H₆‐1‐C*HNH₂ ( 4 ), (1S, 2R)‐(‐)‐cis‐Me₂InO‐2‐C*HC₇H₆‐1‐C*HNH₂ ( 5 ), and racemic (+/‐)‐trans‐Me₂InO‐2‐C*HC₇H₆‐1‐C*HNH₂ ( 6 ). The compounds were characterized by ¹H NMR, ¹³C NMR, ²⁷Al NMR and mass spectra as well as 1 and 3 to 6 by determination of their crystal and molecular structures. The dynamic dissociation/association behavior of the coordinative metal‐nitrogen bond was studied by low temperature ¹H NMR spectroscopy.     

Study of <Emphasis Type="Italic">cis</Emphasis>–<Emphasis Type="Italic">trans</Emphasis> isomerization mechanism of 3,3′-azobenzene disulphonate in the lowest singlet and triplet electronic states by density functional theory

Abstract  

The cis–trans isomerization pathways of 3,3′-azobenzene disulphonate in the S₀ and T₁ states are studied by DFT method at the B3LYP/6-31G(d,p) level. In the S₀ state, the cis–trans isomerization concerns the complex pathway that is characterized by the inversion of one NNC angle combined with rotation around the NC bond, and the three sequential transition states are also found on the potential energy profile. Therefore, the cis–trans isomerization of 3,3′-azobenzene disulphonate can be understood in terms of a pathway involving successive rotation, inversion, and rotation processes. The energy barrier of the S₀ state is 22.79 kcal mol⁻¹. In the T₁ state, the isomerization mainly concerns the rotational pathway around the NN double bond, and the two isomers are connected through only one transition state. The isomerization of the T₁ state is related to a lower energy barrier, 5.02 kcal mol⁻¹, but requires a change in spin-multiplicity.     

Specifically (π → π*)-Induced Cyclohexenone Reactions 4a-(Z-1-Propenyl)-bicyclo[4.4.0]dec-1 (8a)-en-2-one and 4a-Z-1-Propenyl-bicyclo[4.4.0]deca-1(8a), 7-dien-2-one

4a-(Z-1-Propenyl)-bicyclo[4.4.0]dec-1(8a)-en-2-one ( 6 ) and 4a-(Z-1-propenyl)-bicyclo[4.4.0]deca-1(8a), 7-dien-2-one ( 17 ) undergo an intramolecular hydrogen transfer from the methyl group of the propenyl substitutent to the α-carbon atom of the enone group, and cyclization to the [4.4.3]propellane derivatives 9 and 30 , respectively, when excited in the π → π* wavelength region. The quantum yield for (Z)- 6 → 9 under optimum conditions is 0.29 at 254 nm. These reactions occur specifically from the S₂ (π,π*) state, competing with the S₂ → T decay. The triplet reactions of 6 are E–Z double-bond isomerization, double-bond shift to (E,Z)- 8 , and rearrangement to (E)- 10 . Further investigations concern some structural limitations in the scope of the reaction type 6 → 9 and enone S₂ reactivity in general.     

Photochemische Umwandlungen, 41 [1] 3ω → 3 π-Isomerisierungen von Monohetero-trishomobenzol-derivaten. Vorläufige mitteilung

The all-cis-oxa- and azatrishomobenzene diesters 4a and 4b resp. undergo thermally a very clean 3ω → 3π isomerization reaction yielding the heterocyclonona-2, 5, 8-triene derivatives 6a and 6b resp. (E_a = 27.4 and 26.5 kcal/mole). In contrast, the cis, cis, trans-oxatrishomobenzene diester 9 is stable up to 170°. Some applications and limitations of this 3ω → 3π-route to iso- and heterocyclononatriene derivatives are discussed.     

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

The dynamics of the excited states of 1‐aminofluoren‐9‐one (1AF) and 1‐(N,N‐dimethylamino)‐fluoren‐9‐one (1DMAF) are investigated by using steady‐state absorption and fluorescence as well as subpicosecond time‐resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen‐bonded form in aprotic solvents, the excited‐state intramolecular proton‐transfer reaction is the only relaxation process observed in the excited singlet (S₁) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen‐bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S₁(LE), or S₁(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge‐transfer, S₁(TICT), state. A crossing between the excited‐state and ground‐state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S₁(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen‐bond‐donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen‐bonded complex formed between the S₁(TICT) state and the solvent is possibly avoided and the hydrogen‐bonded complex is weakly emissive.     

A study of the excited state structure and vibrations of hydroquinone by ab initio calculations and resonant two-photon ionization spectroscopy

Hydroquinone (HYQ) in the lowest electronically excited state has been studied by ab initio quantum chemical calculations and resonant two-photon ionization (R2PI) spectroscopy. Calculations at the MP2/6-31G* and CIS/6-31G* levels yield satisfactory results on structures and vibrational frequencies of the cis-HYQ and trans-HYQ in both the S₀ and S₁ states. Only transitions involving in-plane modes are observed in the R2PI spectrum of HYQ. All spectral bands including some newly observed ones have been successfully assigned with the help of our computed results and analogy with the reported spectra for similar molecules.