Ultrafast Z → E photoisomerisation of structurally modified furylfulgides.

Femtosecond broadband transient absorption spectroscopy has been used in a comparative study of the ultrafast photo-induced Z → E isomerisation reactions of four photochromic furylfulgides with selected structural motifs in n-hexane as solvent. The results show that all studied Z-fulgides exhibit fast and direct processes along barrierless excited-state pathways involving a conical intersection (CI) between the S1 and S0 electronic states. The excited-state lifetimes range from τ1 = 0.18 ps for the methyl derivative to τ1 = 0.32 ps for the benzofurylfulgide. The impulsive rise of the absorption by vibrationally hot Z- and E-isomers back in the electronic ground state following electronic deactivation and isomerisation indicates that the initially prepared wave packet persists even after passage of the CI. Furthermore, the results provide qualitative evidence for a quickly dephasing vibrational coherence in the electronic ground state. In contrast to the significant changes observed for the corresponding E- and C-isomers [Renth et al., Int. Rev. Phys. Chem., 2013, 32, 1-38], the excited-state dynamics of the Z-isomers is not affected by varied sterical hindrance from methyl and isopropyl substituents at the central hexatriene unit, or by intramolecular bridging, and remains unaltered upon extension of the π-electron system in a benzannulated furyl fulgide.

Electronic and steric effects on the photo-induced C→E ring-opening of structurally modified furylfulgides.

The ultrafast C→E ring-opening reactions of four selectively modified furylfulgides have been studied by means of ultrafast broadband transient absorption spectroscopy after femtosecond laser excitation at λ = 500 nm. A large difference in the dynamics was found in the case of benzannulation at the furyl moiety as an example for an electronic effect by extension of the conjugated π-electron system compared to furylfulgides carrying sterically different alkyl substituents at the central cyclohexadiene (CHD) ring. The measured very similar spectro-temporal absorption maps for the furylfulgides with a methyl or isopropyl group at the CHD ring or an intramolecular alkyl bridge from the CHD to the furyl moiety showed two distinctive excited-state absorptions with slightly different decay times. The first time constant (τ(1) = 0.39-0.57 ps) was assigned to the rapid departure of the excited wavepacket from the Franck-Condon region. The slightly longer second decay time of τ(2) = 0.66-0.92 ps, depending on the compound, was attributed to the electronic deactivation and ring-opening through a conical intersection to the S(0) state. In contrast, the benzannulation at the furyl moiety was found to lead to a bi-phasic excited-state decay with τ(2) = 4.7 ps and a much slower additional contribution of τ(3) = 17.4 ps, ≈25 times longer compared to the normal furylfulgides. The drastic change is attributed to a trapping of excited molecules in a local potential energy minimum en route to the conical intersection.

Tuning of switching properties and excited-state dynamics of fulgides by structural modifications.

The ultrafast photo-induced dynamics of the E-isomers of four selected photochromic fulgides with distinct structural motifs have been elucidated by femtosecond broadband transient absorption spectroscopy in n-hexane as solvent. E→C and E→Z isomerisations, respectively, with time constants of ∼0.12 ± 0.02 ps and ∼0.34 ± 0.03 ps taking place in parallel were found for derivatives with a methyl substituent at the central hexatriene (HT) unit. In contrast, fulgides with increased steric constraints by an iso-propyl substituent or by intramolecular bridging displayed virtually zero E→Z isomerisation, but instead a desired accelerated and more efficient ring closure in a reaction time of only ∼50 ± 10 fs. Both photoisomerisations appear to follow excited-state pathways with distinctive conical intersections. For the ring closure, direct barrierless pathways with steep downhill gradients are likely. Furthermore, the results indicate conformer-specific reactions, with ring closure exclusively by the E(α) conformer and E→Z isomerisation predominantly by the E(ß) conformer, because the E(α)→Z channel is unfavoured by the faster and kinetically more competitive E(α)→C reaction. DFT calculations of the equilibrium structures showed that the sterically demanding groups at the HT unit shift the conformer equilibria towards the E(α) conformers. At the same time, they appear to cause a favourable pre-orientation of the furyl unit that accelerates the conrotatory ring closure in the E(α)→C reaction. Benzo-annulation of the furyl unit has little effect on the observed dynamics. Overall, the results demonstrate how the excited-state dynamics and thereby the photoswitching properties of fulgides can be successfully tuned and improved by structural modifications at the chromophores.

Rotationally-hindered furyl fulgides.

Fulgides are a representative class of photochromic organic molecules which exhibit several interesting properties for diverse applications in fields such as data storage or high-resolution spectroscopy. The crystal structures of three furyl fulgides with different steric constraints were determined and for two of the compounds both the E and Z isomer structures were defined. The compounds are 3-[(E)-1,3-dimethyl-4,5,6,7-tetrahydro-2-benzofuran-4-ylidene]-4-isopropylidenetetrahydrofuran-2,5-dione, C(17)H(18)O(4), (I-E), 3-[(E)-1,3-dimethyl-5,6,7,8-tetrahydro-4H-cyclohepta[c]furan-4-ylidene]-4-isopropylidenetetrahydrofuran-2,5-dione, C(18)H(20)O(4), (II-E), and the Z isomer, (II-Z), and 3-isopropylidene-4-[(E)-1-(5-methoxy-2-methyl-1-benzofuran-3-yl)ethylidene]tetrahydrofuran-2,5-dione, C(19)H(18)O(5), (III-E), with two molecules in the asymmetric unit, and the Z isomer, (III-Z). The structures of the E and Z isomers show only little differences in the bond lengths and angles inside the hexatriene unit. Because of the strained geometry there are deviations in the torsion angles. Furthermore, small differences in the distances between the bond-forming C atoms in the electrocyclization process give no explanation for the unequal photochromic behaviour.

Photocyclizable resorcin[4]arene dimers.

The synthesis of covalently linked dimers, containing two resorcin[4]arene moieties connected over two 9,10-functionalized anthracene units, is reported. Besides the synthetic route, which involves for example the formation of anthracene-9,10-dialkanols ( and ), and characterization of the compounds, the photochemical properties, introduced through the anthracene groups, were investigated by means of UV/VIS spectroscopy. Both resorcin[4]arene dimers ( and ) were able to undergo an intramolecular [4+4] cycloaddition, therefore changing the size of the inner cavity. Unfortunately, the back reaction, which was expected to take place on irradiation below 300 nm or upon heating, was not observed yet and will be the focus of our future work.

Regulation of nitric oxide (NO) production by plant nitrate reductase in vivo and in vitro.

NO (nitric oxide) production from sunflower plants (Helianthus annuus L.), detached spinach leaves (Spinacia oleracea L.), desalted spinach leaf extracts or commercial maize (Zea mays L.) leaf nitrate reductase (NR, EC 1.6.6.1) was continuously followed as NO emission into the gas phase by chemiluminescence detection, and its response to post-translational NR modulation was examined in vitro and in vivo. NR (purified or in crude extracts) in vitro produced NO at saturating NADH and nitrite concentrations at about 1% of its nitrate reduction capacity. The K(m) for nitrite was relatively high (100 microM) compared to nitrite concentrations in illuminated leaves (10 microM). NO production was competitively inhibited by physiological nitrate concentrations (K(i)=50 microM). Importantly, inactivation of NR in crude extracts by protein phosphorylation with MgATP in the presence of a protein phosphatase inhibitor also inhibited NO production. Nitrate-fertilized plants or leaves emitted NO into purified air. The NO emission was lower in the dark than in the light, but was generally only a small fraction of the total NR activity in the tissue (about 0.01-0.1%). In order to check for a modulation of NO production in vivo, NR was artificially activated by treatments such as anoxia, feeding uncouplers or AICAR (a cell permeant 5'-AMP analogue). Under all these conditions, leaves were accumulating nitrite to concentrations exceeding those in normal illuminated leaves up to 100-fold, and NO production was drastically increased especially in the dark. NO production by leaf extracts or intact leaves was unaffected by nitric oxide synthase inhibitors. It is concluded that in non-elicited leaves NO is produced in variable quantities by NR depending on the total NR activity, the NR activation state and the cytosolic nitrite and nitrate concentration.