Jack Saltiel

Dynamics of cis-stilbene photoisomerization: the adiabatic pathway to excited trans-stilbene

Abstract Fluorescence excitation spectra of cis-stilbene in n-hexane monitored at 350 nm and 404 nm, are resolved into cis-stilbene and trans-stilbene contributions using principal component analysis with self-modeling. The results confirm that both trans and cis fluorescence originating from excitation of pure cis-stilbene solutions are due to single photon excitation of 1c to 1c*. The quantum yield for adiabatic 1c*→1t* conversion is φct*=0.0020, independent of excitation wavelength. The corrected fluorescence excitation spectrum of cis-stilbene faithfully tracks its UV absorption spectrum. The fluorescence spectrum of cis-stilbene is nearly temperature independent in n-hexane, 0–58 °C. Assuming a 1c* → 1p* → 1t* pathway for adiabatic 1t* formation where 1p* is the perpendicular phantom singlet state, leads to the conclusion that 1p* and 1t* are nearly isoenergetic.

Trans-stilbene phosphorescence

Abstract Internal and external heavy-atom effects induce phosphorescence in trans-stilbene and in indeno [2,1-α] indene, a rigid trans-stilbene analogue, in glassy media at 77 K. The origins of these emissions are found in the narrow range of 580–600 nm.

Spin-exchange between stilbene triplets and oxygen (3Σ)

Abstract Comparison of the interactions of oxygen and β-carotene with stilbene triplets suggests that electronic excitation is not transferred to oxygen. Effective interactions may proceed via a triplet encounter complex which gives oxygen (3Σ) and twisted stilbene ground state molecules.

Lifetime and geometry of 1-phenyl-2-(2-naphthyl)ethene triplets.: Evidence against the triplet mechanism for direct photoisomerization

Abstract Competitive azulene and oxygen quenching experiments demonstrate that the lifetime of 1-phenyl-2-(2-naphthyl)ethene (2-NPE) triplets, 104–150 ns, is much longer than ≈ 20 ns, the lifetime of a transient absorption previously assigned to them. The transient absorption is reassigned to the longer lived of two non-equilibrating isomeric excited transoid singlet states.

Photophysics of Diphenylacetylene: Light from the “Dark State”

A weak band at the tail of the known tolane (diphenylacetylene, DPA) fluorescence spectrum in several solvents is assigned to the forbidden 1(1)A(u) → 1(1)A(g) transition on the basis of its lifetime (∼200 ps) and its fluorescence excitation spectra. The 1(1)A(u) state, generally called the dark state, is not truly dark. We report the temperature (T) dependence of DPA fluorescence quantum yields (ϕ(f)) in methylcyclohexane (MCH) solution and the fluorescence and phosphorescence quantum yields of DPA in glassy MCH at 77 K. Significant differences between fluorescence and phosphorescence excitation spectra reveal that, in addition to the 1(1)B(1u) ← 1(1)A(g) transition, the first DPA absorption band includes a transition to another excited state, most probably the 1(1)B(2u) state, from which intersystem crossing is more efficient. The T dependence of ϕ(f) values in MCH solution is shown to be consistent with the previously reported T dependence of the lifetimes of transient DPA singlet excited state absorptions in the picosecond time scale. Transient absorption decay rate constants in hexane, methylcyclohexane and decalin as a function of T are retreated. Application of the medium enhanced barrier model shows that the medium is fully engaged with the molecular motion that is involved in the activated nonradiative decay path of the 1(1)B(1u) state. In accord with theoretical calculations and experimental observations, that process is assigned to the diabatic internal conversion of the short-lived linear fluorescent π,π* (1(1)B(1u)) state, over a low intrinsic energy barrier, to the longer lived weakly fluorescent trans-bent π,σ* (1(1)A(u)), which is the precursor of the DPA triplet state. Absorption and fluorescence measurements in several solvents show that the 1(1)B(1u)-1(1)A(g) energy gap decreases linearly with increasing medium polarizability. Our results allow a more definitive state order assignment for DPA.

Medium effects on the direct cis-trans photoisomerization of 1,4-diphenyl-1,3-butadiene in solution.

Quantum yields for the photoisomerization of trans,trans-1,4-diphenyl-1,3-butadiene (tt-DPB), determined in benzene, cyclohexane, methylcyclohexane, hexane, and perfluorohexane, confirm the low values reported earlier for benzene and cyclohexane and reveal even lower values in the last two solvents. In contrast to trans-stilbene (t-St), fluorescence and torsional relaxation leading to photoisomerization do not account exclusively for S(1)tt-DPB decay. Competing radiationless singlet excited-state decay pathways exist in tt-DPB, which do not lead to photoisomerization and may not involve large-amplitude torsional motions. Our results invalidate analyses of tt-DPB fluorescence quantum yields and lifetimes that assign all radiationless decay to the isomerization channel. Gas-phase chromatography analysis of tt-DPB photoisomerization in hexane shows the reaction to be concentration-independent and reveals, for the first time, a significant, two-bond photoisomerization pathway, φ(tt→tc) = 0.092 and φ(tt→cc) = 0.020. The dominant one-bond-twist (OBT) process is accompanied by a bicycle pedal (BP) process that accounts for almost 20% of tt-DPB photoisomerization. The OBT tt-DPB photoisomerization quantum yield is largest in benzene (Bz) and smallest in perfluorohexane (PFH). Contrary to expectations, reduction in medium friction in PFH is accompanied by a decrease in φ(tt→tc). The 1(1)B(u)/2(1)A(g) order and energy gap appear to control the contribution of torsional relaxation to radiationless decay. Lowering the 1(1)B(u) energy as in Bz favors photoisomerization. Reversal of the 1(1)B(u)/2(1)A(g) order in PFH is accompanied by short τ(f) and small φ(f) and φ(tt→tc) values that suggest the presence of competing 2(1)A(g) → 1(1)Ag relaxation paths that are unproductive with respect to photoisomerization. We conclude that the Birks extension to diphenylpolyenes of the Orlandi-Siebrand cis-trans photoisomerization mechanism is not valid. Photoisomerization appears to occur in the 1(I)B(u) state, and we argue that this applies to t-St as well.

Photoisomerization of Pre- and Provitamin D3 in EPA at 77 K: One-Bond-Twist, Not Hula-Twist.

The photoisomerizations of previtamin D3 (Pre) and provitamin D3 (Pro) in EPA at 77 K were monitored using fluorescence spectroscopy. In the glassy EPA medium equilibrated tachysterol (Tachy) exists as a mixture of three conformers, the major of which we assign to s-trans,s-cis- and s-cis,s-cis-conformers (tEc- and cEc-Tachy). By contrast, Pre exists exclusively as the s-cis,s-cis-conformer (cZc-Pre) and undergoes cis → trans photoisomerization to cEc-Tachy. Light-induced ring-opening of Pro gives three Pre conformers, the major of which is tZc-Pre instead of the expected cZc-Pre. Curve resolution based on singular value decomposition yields the fluorescence spectra of the conformers. Structural assignments are based on experimental and theoretical evidence showing that the s-cis,s-cis-conformers of Pre and Tachy absorb UV light to the red of s-trans-s-cis-conformers. The photoisomerizations of tZc- and cZc-Pre in EPA proceed by the one bond twist (OBT) mechanism to give tEc- and cEc-Tachy, respectively.

Supramolecular photochemical synthesis of an unsymmetrical cyclobutane

2-styrylbenzothiazole (1) and cinnamic acid (2) derivatives containing 15-crown-5 ether moieties form a supramolecular assembly in the presence of Ba(2+) cations in acetonitrile. The assembly is stabilized by hydrogen bonding between the heterocyclic N atom of 1 and the proton of the carboxylic group of 2, by sandwich Ba(2+) complex formation between the crown ether moieties of 1 and 2, and by pi-pi stacking interactions. Irradiation of solutions containing these supramolecular complexes leads to highly specific formation of an unsymmetrical cycloadduct. This investigation provides an interesting example of supramolecular control of [2 + 2]-photocyclization in solution.

Fluorescence, fluorescence-excitation and absorption spectra of trans-1-(2-anthryl)-2-phenylethene conformers

Fluorescence spectra of trans-1-(2-anthryl)-2-phenylethene (APE) obtained under varying conditions of excitation wavelength and oxygen concentration in toluene are resolved into two distinct components by application of principal component analysis with self-modeling (PCA-SM). Self-modeling is guided by the constraint that Stem-Volmer quenching plots for the individual conformers be independent of excitation wavelength. The same process applied on a matrix of fluorescence-excitation spectra leads to resolved conformer-specific fluorescence-excitation spectra. Consistency between the fluorescence and the fluorescence-excitation spectrum of each conformer is established. The fluorescence-excitation spectr and literature fluorescence quantum yields are used to resolve the UV absorption spectrum of APE. The pure conformer spectra obtained in this work are compared with those from earlier PCA-SM treatments in which self-modeling procedures were based on the Lawton and Sylvestre nonnegativity constraint and on a maximal spectral dissimilarity constraint.

Photoisomerization of All‐cis‐1,6‐diphenyl‐1,3,5‐hexatriene in the Solid State and in Solution: A Simultaneous Three‐Bond Twist Process

The photophysical and photochemical behaviors of alltrans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) have attracted a great deal of attention because, as the first member of the family of vinylogous a,w-diphenylpolyenes with a 2Ag lowest excited state, ttt-DPH serves as a polyene model. Mediumimposed constraints enhance the torsional barriers of olefins in the lowest excited singlet state and inhibit cis–trans photoisomerization. Two mechanisms involving concerted rotation about two bonds in the S1 state, which were initially postulated to explain retinyl photoisomerization within the protein environments in rhodopsin and bacteriorhodopsin, have been applied generally to account for photoisomerization in volume-confining media: The bicycle-pedal mechanism (BP) involves simultaneous rotation in the S1 state about two polyene double bonds, and the hula–twist mechanism (HT) involves simultaneous rotation about a double bond and an adjacent essential single bond (equivalent to a 1808 translocation of one CH unit). Our recent work on the cis,cis-1,4-diphenyland cis,cis-1,4-di(o-tolyl)-1,3-butadienes (cc-DPB and cc-DTB) revealed the BP process as the only photoisomerization pathway in the solid state at room temperature and as a significant pathway competing with the usual one-bond-twist pathway in the soft isopentane glass at 77 K. Recognizing that the BP process accounts for the interconversion of ctt-DPH and tct-DPH isomers in solution and for the one-way ctc-DPH!ttt-DPH photoisomerization of symmetrical p,p’-disubstituted cis,trans,cis-1,6-diphenyl1,3,5-hexatriene derivatives in the solid state, we reasoned that the analogous extended BP process in the hitherto unknown all-cis DPH might give tct-DPH in the solid state. Observations reported herein show that instead, irradiation of ccc-DPH in the solid state gives ttt-DPH directly in a crystalto-crystal reaction. To our knowledge this is the first observation of simultaneous three-bond cis–trans photoisomerization. cis-1,6-Diphenylhex-3-en-1,5-diyne was prepared from phenylethyne and cis-1,2-dichloroethene as previously described and hydrogenated over the Lindlar palladium catalyst to afford ccc-DPH as an oil (Scheme 1). Purification