Dihydroindolizines Research Articles

Photochromism of dihydroindolizines. Part XXIV: Exploiting “Click” chemistry strategy in the synthesis of fluorenyldihydroindolizines with multiaddressable photochromic properties

A new category of photochromic dihydroindolizines (DHIs) incorporating substituted 2,7- and 4-substituted tetrazole and oxadiazole moieties in the fluorene skeleton (region A) were efficiently prepared utilizing “Click” chemistry approaches. The structure elucidation for all synthesized precursors as well as the target photochromic DHIs was carried out using some analytical and spectroscopic techniques. A highly tunable photochromic behaviors of the parent dihydroindolizine are possible by the introduction of substituents in different positions of the DHI framework. For example, the substituents in 2,7 and 4-positions of fluorene part showed the ability for extending photochromism. The photochromic behaviors of photochromic DHIs substituted in fluorene part (region A) such as the reaction kinetics and fluorescence properties and their photo-fatigue resistance were studied. It has been disclosed that the replacement of the tetrazole moieties in 2,7-position in (region A) by oxadiazole moieties has strong effect on both the spectral, kinetic characteristics, fluorescence emission and photostability. The observations of this work imply that the substituent groups in the fluorene part influenced the thermal back-reaction rates and played an imperative role in controlling all photochromic properties of the open form. The new synthesized substituted (DHIs) in the fluorene by both tetrazole moieties in 2,7-position and by oxadiazole moieties in 2,7 and 4 positions in fluorene part made these materials act as fluorophores and postulate new opportunities for the design of the next generation of photochromic materials which will make it talented materials in many applications such as electronic smart materials, photochromic glasses, photonic devices and fluorescent fabrics. Because of their noticeable fluorescence emission high photostability, these materials can be used as fluorophores, recording or storage information for numerous periods without color fading.

Photochromism of dihydroindolizines. Part XXIII: Sensitive tuning of the photophysical properties with region A and C substituents in the photochromic dihydroindolizines skeleton

Twelve new photochromic materials based on dihydroindolizine (DHI) systems were synthesized using mechanochemical reaction strategy in one-pot solid state reaction with high yield and short reaction time. The DHI skeleton in these photochromic compounds is substituted in both the fluorene part (region A) or in the pyradizine (region B) with halogen and methyl groups, respectively, in order to predict the appropriate functionality for optimal tuning of the photochromic properties of the system. Upon Irradiation of the photochromic DHIs with polychromatic light, ring opened colored betaines are formed, which underwent ring-closure reactions via thermal1,5-electrocyclization processes. Because of their relatively slower 1,5-electrocyclization, the colored betaine forms are observable in dichloromethane solution with concentration of 1 × 10−4 mol/L at 15 °C. All the absorption maxima (λmax) of the colored betaines were detected using UV/VIS/NIR spectrophotometer which were found to be in the visible region and lie between 516 nm and 660 nm. The kinetics of the bleaching process of betaines to DHIs showed that the half-lives relative to betaines were found to take place in the second and third range and lie between 9 and 416 s and fit well with the first order thermal back reaction. It has been shown that, a highly pronounced decrease in the rate of electrocyclization of betaines bearing dimethyl substituted pyridazine was noted compared with non-substituted pyridazine betaines. It has been found that inversion of the trends in the rate of electrocyclization of the betaines takes place when passing from di-methyl to tetra-methyl substituted pyradizine. Interestingly, a remarkable photo-fatigue resistance of some studied betaines is monitored. A noticeable better photo-fatigue resistance was observed in blue-green colored betaines by comparison to the red-violet colored betaines. The observation deduced from the experimental part finds explanation through DFT calculations. The energy barriers, ΔE* and ΔE*rot(CC-TC), for the 12 photochromic compounds are calculated in the range from 95.5 to 127.7 kJ/mol and 17.1 to 84.6 kJ/mol, respectively. The effect of the substituent on the second step of the mechanism, the CC bond rotation, is much more important than that of the ring-opening step. The substitution in position 4 and 5 strongly increases the energy barrier for bond rotation: these positions are therefore interesting to modulate the reactivity of the open isomer. The substitution at the position 5 by a methyl group induces a strong bathochromic shift of the absorption.

Photochromism of dihydroindolizines. Part XXII: Significant effect of region B substituents on tuning the photophysical properties of photochromic dihydroindolizines: Absorption, kinetic and computational studies

Abstract Novel photochromic dihydroindolizines (DHIs) bearing different alkyl and cyclic alkyl ester group (region B) were prepared via electrophilic addition of fluorene spirocyclopropenes substituted with different alkyl ester groups in region B of the DHI skeleton to N-heterocyclic pyridazine in dry ether solution. The maxima of absorption (λmax) of the colored betaines (formed after UV-irradiation) were detected by UV/Vis absorption measurement. The studied betaines showed one visible region absorption maxima between 517 and 548 nm. The kinetics of the 1,5-electrocyclization process of the colored betaines to closed DHIs system was studied spectrophotometrically in dichloromethane solution. Interestingly, a significant decrease in the half-life time by a factor of ∼8 was observed when alkyl of the ester group changed from methyl to pentyl alkyl groups. The activation parameters indicated that the changes of ΔH≠ are counterbalanced by changes in ΔS≠ ʺcompensation effectʺ. The enthalpy and entropy of the activation energies for the thermal back-reaction were compared to investigate the generalization and the limitation of the molecular intention of the photochromic DHIs lengthways with the correlation between unwieldiness of the substituents (region B) and the rate constant of the thermal back-reaction. DFT calculations are performed to explain the 1,5-electrocyclization back-reaction mechanism, including a partial rotation prior to the final ring closure.

Photochromism of Dihydroindolizines. Part XXI: Multiaddressable Photochromic Performances based on Pyrrolo[1,2 -b ]pyridazine photochromes: Kinetics, Substituent Effect and Solvatochromism

Abstract In the present study, the thermal 1,5-electrocyclization back reaction from betaines (colored forms) to dihydroindolizines (DHIs, colorless) was studied in seven organic solvents with different polarities at various temperatures. The spectroscopic, thermodynamic and photochromic properties of such photo- and solvatochromic material based on betaines/DHIs is investigated. It is obvious that the rate of the thermal back reaction depend on solvent polarity. Exploration of the activation parameters unravel an underlying compensation between ΔH ≠ and ΔS ≠ . The 1,5-electrocylization decoloration reaction is found to follow two-step mechanism, the first step corresponds to the transoid-cisoid isomerization and the second one is the ring closure from the cisoid intermediate and formation of the carbon-carbon bond to form the initial spiro isomer. The special solvent effects were discussed based on the linear free energy relationship. The solvatochromism of the dihydroindolizines (DHIs) was studied and analyzed by means of the empirical Kamlet–Taft and Catalan solvent parameter set. The obtained results suggest that the change of polarizability within the solvents is the major factor for the observed solvent dependence.

Allosteric Inhibitory Molecular Recognition of a Photochromic Dye by a Digestive Enzyme: Dihydroindolizine makes α-chymotrypsin Photo-responsive.

The structural-functional regulation of enzymes by the administration of an external stimulus such as light could create photo-switches that exhibit unique biotechnological applications. However, molecular recognition of small ligands is a central phenomenon involved in all biological processes. We demonstrate herein that the molecular recognition of a photochromic ligand, dihydroindolizine (DHI), by serine protease α-chymotrypsin (CHT) leads to the photo-control of enzymatic activity. We synthesized and optically characterized the photochromic DHI. Light-induced reversible pyrroline ring opening and a consequent thermal back reaction via 1,5-electrocyclization are responsible for the photochromic behavior. Furthermore, DHI inhibits the enzymatic activity of CHT in a photo-controlled manner. Simultaneous binding of the well-known inhibitors 4-nitrophenyl anthranilate (NPA) or proflavin (PF) in the presence of DHI displays spectral overlap between the emission of CHT-NPA or CHT-PF with the respective absorption of cis or trans DHI. The results suggest an opportunity to explore the binding site of DHI using Förster resonance energy transfer (FRET). Moreover, to more specifically evaluate the DHI binding interactions, we employed molecular docking calculations, which suggested binding near the hydrophobic site of Cys-1-Cys-122 residues. Variations in the electrostatic interactions of the two conformers of DHI adopt unfavorable conformations, leading to the allosteric inhibition of enzymatic activity.

Photochromism of dihydroindolizines. Part XX: Synthesis and photophysical behavior of fluorenyldihydroindolizines photochromes based “Click” chemistry strategy

Abstract Novel type of photochromic dihydroindolizines (DHIs) bearing substituted 2,7- disubstituted triazolofluorene (region A) with different acetylenic linker-conjugates and substituted pyridazines as heterocyclic base (region B) have been productively synthesized through “Click” chemistry strategies by different chemical and photochemical pathways. The chemical structures of the new photochromic triazolo-dihydroindolizines were established on the bases of analytical and spectroscopic tools. Polychromatic light irradiation of the photochromic DHIs led to ring opened colored betaines, which underwent reversible thermal 1,5-electrocyclization. The red to reddish-violet betaines returned back through 1,5-electrocyclization to the corresponding DHIs with diverse rate constants depending on the subsitutents in fluorene terminal triazole moieties. The unique photochromic properties of newly synthesized compounds maybe useful for a plethora of new applications.

Substituent parameters impacting isomer composition and optical properties of dihydroindolizine molecular switches.

In an attempt to understand which factors influence constitutional isomer control of 6'- and 8'-substituted dihydroindolizines (DHIs), a series of asymmetric pyridines was condensed with dimethyl spiro[cycloprop[2]ene-1,9'-fluorene]-2,3-dicarboxylate. The substituents on the pyridial derivatives ranged from donating to withdrawing and demonstrated control over the isomeric ratios for all DHIs. Substituent control proved to be selective for the highly donating amino, which exclusively formed the 8' isomer. The same ratios were reproduced via photolytic experiments, which suggested that the condensation reaction is dominated by the product's thermodynamic stability. The electronic influences of the substituents extends beyond isomer control, as it impacts the DHIs' optical properties and electrocyclization (switching) rates to the spiro conformers. Our results allow us to predict the syntheses and properties of future 6'- or 8'-substituted DHIs, molecules that will be applied in understanding the role of the dipole vector orientation to work function switching.

Photochromism of dihydroindolizines. Part 16: Tuning of the photophysical behavior of photochromic dihydroindolizines in solution and in polymeric thin film

In this work, photochromic materials based on the dihydroindolizine (DHI) system were synthesized in multistep reactions using chemical and photochemical methods. Some of the synthesized photochromic dihydroindolizine derivatives were substituted on the fluorene (region A) and pyridazine (region C) moieties in order to provide the appropriate functionality for optimal tuning of the photochromic properties of the system. Irradiation of the photochromic DHIs with polychromatic light led to ring opened colored betaines, which underwent thermal 1,5-electrocyclization. The red to green colored betaines produced after UV irradiation returned back through 1,5-electrocyclization to the corresponding DHIs with different rate constants depending on the substituents in both fluorene and pyridazine regions. The kinetic measurements of the thermal 1,5-electrocyclization under different temperatures that ranged from −10 to 25 °C showed that the half-lives of the colored betaines fall in the second to hours domain. Interestingly, these materials showed a very good photochromic behavior not only in solution but also in the PMMA matrix. Irradiation of a slide prepared by the deep-coating method led to the formation of the colored betaine and the kinetics of the thermally reversible 1,5-electrocyclization and the AFM image of the film has been recorded. Indeed, the chemical and thermal stability of the investigated betaines in polymer (PMMA) will render such species useful for a plethora of new of applications.

Synthesis of dihydroindolizines for potential photoinduced work function alteration

Seeking to immobilize photochromophores on metallic surfaces, we have synthesized four molecules which contain both a photoresponsive dihydroindolizine (DHI) core and a sulfur containing moiety, which allow for their assembly onto gold substrates. Sonogashira, Suzuki, or Ullmann couplings are employed to generate pyridines with pendant thioacetates (or disulfides). The pyridines are condensed with spiro[2-cyclopropene-1,9′-[9 H]fluorene]-2,3-dimethyl ester affording the targeted DHIs.

Photochromism of dihydroindolizines: part XIV. Synthesis and photophysical behavior of photochromic dihydroindolizine-tripodal linkers toward anchoring sensitizers to semiconductor nanoparticles

Photochromic dihydroindolizines (DHIs) 4a,5-dihydropyrrolo[1,2-b]pyridazine based tripodal-linker systems with adamantane core and ethyl benzoate tripods as anchoring groups have been successfully synthesized. In addition, new spirocyclopropene precursors have been prepared through both chemical and photochemical processes. The photochromic properties of the newly synthesized DHIs derivatives have been optimized and fine-tuned by the incorporation of various substituents on the fluorene (region A) and pyridazine (region C) moieties. Several alternative routes for the synthesis of the DHIs under investigation have been established. The Sonogashira crosscoupling reaction was utilized for fragment coupling between DHIs and the phenylacetylene tether of the adamantane core. Several reaction conditions of this key reaction were surveyed to obtain optimal yields of a new series of coupling products targeted for anchoring to semiconductor nanoparticles. The chemical structures of the newly synthesized materials were elucidated by both analytical and spectroscopic tools. Irradiation of the photochromic DHIs with polychromatic light resulted in ring opened colored betaines which underwent cycloreversion reactions via thermal 1,5-electrocyclization processes. The kinetic of the thermal 1,5-electrocyclization was studied by using a UV/VIS/NIR spectrophotometer. The kinetic measurements showed the half-lives of the colored betaines to be in the second domain. A pronounced increase in the half-lives of betaines bearing dimethyl-substituted pyridazine was noted compared with non-substituted pyridazine betaines. A strong effect of solvent polarity on the λmax and half-lives of the betaines was observed. The further adjustment of the absorption maxima and the kinetic properties via the manipulation of substituents on the fluorene (region A) and pyridazine moieties (region C) should yield more refined systems for application as supports onto metal-oxide surfaces which remains an active area of our ongoing research. Copyright © 2010 John Wiley & Sons, Ltd.

Photochromism of dihydroindolizines. Part XV: Synthesis and photophysical properties of dihydroindolizine photoswitches bearing a conjugated aryleneethynylene tripodal linker system

Different photochromic dihydroindolizines (DHIs) bearing conjugated aryleneethynylene tripodal linker systems were synthesized using several Sonogashira coupling reactions. The spirocyclopropene precursors incorporating different acetylenic bridge moieties at the 2-position of the fluorene moiety were synthesized via chemical and photochemical routes. Multiaddressable photochromic properties of the DHI derivatives substituted in the fluorene (region A) and pyridazine (region C) parts were studied. Optimization of the formation of the DHIs was also done by applying different palladium-mediated Sonogashira coupling reactions. Irradiation of the photochromic DHIs with polychromatic light led to colored betaines which undergo thermal 1,5-electrocyclization. The kinetics of the thermal 1,5-electrocyclization were studied by using a multichannel FT–UV–Vis spectrophotometer. A pronounced effect on the kinetic behavior of the 1,5-electrocyclization process of the betaines was observed by changing substitution from non-substituted to dimethyl-substituted pyridazines. Photodegradation experiments and the bleaching and fading cycles revealed high photostability of the betaines under investigation. These properties of betaines of tripodal linker conjugates will help these materials to find applications.

Photochromism of Dihydroindolizines Part XI: Synthesis of Novel Carbon-Rich Photochromic Dihydroindolizines-Based Potential Electronic Devices

Novel carbon-rich photochromic dihydroindolizine (DHI) derivatives substituted in the fluorene part (region A) in addition to the new spirocyclopropene 6 have been synthesized. The synthesis of dimethyl 2′,7′-diethynylspiro[cycloprop[2]ene-1,9′-fluorene]-2,3-dicarboxylate precursor 6 was accomplished in five steps, starting with the literature known conversion of fluorene to 2,7-dibromo-9H-fluoren-9-one in 56% yield over three steps. The chemical structures of the new synthesized materials have been elucidated by both analytical and spectroscopic tools. Three alterative synthetic pathways for the synthesis of DHI 9 have been established.

Photochromism of dihydroindolizines Part IX: First attempts towards efficient self-assembling organogelators based on photochromic dihydroindolizines and N-acyl-1,ω-amino acid units

Six new photochromic materials 9– 11, 13– 15 based on photochromic dihydroindolizine (DHI) system covalently linked to ester, acid or sodium N-acyl-11 aminoundecanoate through amidic or urethane linkage have been synthesized. These new compounds have been synthesized via nucleophilic addition of 11-methyl 4-pyridaizine aminoundecanoate 3 and methyl 11-methoxycarbonyl 4-pryradizine aminoundecanoate 6 to spirocyclopropene 1. Developing and tuning of the photophysical properties of the synthesized compounds by amide and urethane substitutes in the 4-position of the pyridazine base part have been achieved. The absorption maxima ( λ max) and the half-lives ( t 1/2) of the colored betaines were detected in all cases using multichannel UV/vis spectrophotometric measurements. Irradiation of DHI 9, 10, 13 and 14 in CH 2Cl 2 solution at ambient temperature leads to the formation of red to red-violet colored betaines 8, 10′ 12 and 14′, respectively. The kinetics of the bleaching process of betaines back to DHIs were found to take place in the second range in dichloromethane solution (53–96 s) and fit well the first order thermal back reaction. Some of these DHIs showed a higher photostability than the standard one. Interestingly, they are shown to act as efficient gelators for polar organic fluids, and obviously they exhibiting a thermosensitive response as low molecular mass organogelators. In these fluids, the aggregative properties are totally suppressed upon conversion to neutral carboxylic species. The gels of these carboxylate sodium salts are shown to be markedly affected by light irradiation. Supramolecular gelating assemblies can be disrupted by the photoinduced ring opening of the chromene subunit, so that the macroscopic flowing property is recovered. Upon a further thermal treatment, the system is reversibly converted back to the supramolecular network. Controlled gelation could be achieved using temperature, light or acidity as external stimuli. These new synthesized photochromic gels with their multi-addressable properties will find their applications as photoresponsive materials and nanotechnology.

Photochromism of dihydroindolizines: Part VIII. First holographic image recording based on di- and tetrahydroindolizines photochromes

The photochromic compounds dihydroindolizine (DHI) and tetrahydroindolizines (THIs) containing dicyano and diesters were prepared via nucleophilic addition of the corresponding heterocyclic bases to spirocyclopropenes. These compounds showed good photochromic reactions both in solution and in polymethylmethacrylate (PMMA)-matrix by photo-irradiation. Holographic recording was measured by employing the thin films of PMMA-DHI and PMMA-THI as recording media. The PMMA-films of DHIs 1– 3 were studied with regard to their photochemical switching potential of holograms. In the PMMA-film, these coherent beams were interfering to create a holographic pattern, showing equidistant bright and dark lines. Laser light at 532 nm induced the bleaching processes 1a → 1 and 2a, 3a → 2 , 3 thus writing the pattern as sinusoidal modulated interference intensity in the photochromic material. The photochemical burning of the interference pattern by the processes 2 → 1a and 2a, 3a → 2, 3 is generating the negative holographic pattern in the form of 1 or 3. The positive recording technique showed that 1-PMMA produce a hologram grating employing a λ > 400 nm laser beam with only 1 mW/cm 2 intensity. This was sufficient to generate a holographic grating due to the high-forward quantum yield for 1 → 1a of 0.8. The present results indicate that, the UV-laser generated holograms are produced with lower light intensities compared to the vis-laser process in the negative technique. The recording of volume holograms was achieved on the basis of data and information recording system.

Photochromism of dihydroindolizines part VII: multiaddressable photophysical properties of new photochromic dihydroindolizines bearing substituted benzo[i]phenanthridine as a fluorescing moiety

AbstractSixteen benzo[i]phenanthridine derivatives 8a‐p were prepared via photocyclization of cis‐trans substituted 4‐styrylquinolines in low‐to‐moderate yields. The chemical structure of the photocyclized benzo[i]phenanthridine derivatives was unambiguously elucidated by means of both spectral and analytical tools. The photochromic (PC) dihydroindolizines (DHIs) 8a‐p based on benzo[i]phenanthridines were prepared in 19–57% yields via nucleophilic addition of benzo[i]phenanthridines 4a‐p to spirocyclopropenes 5. The 1D, 2D, NOESY NMR spectra, mass spectrometry, and elemental analysis were used for characterization of the chemical structures of the newly synthesized DHIs 8a‐p. Developing and tuning of the photophysical properties of the synthesized compounds by substituents in the base part have been achieved. The absorption maxima (λmax) and the half‐lives (t1/2) of the colored zwitterionic forms 7a‐p were detected in all cases by flash‐photolysis measurements due to the fast 1,5‐electrocyclization back to the DHI system. Irradiation of DHI 8a‐p in'CH2Cl2 solution with polychromatic light leads to the formation of green to green–blue colored betaines 7a‐p after cooling with liquid nitrogen. The kinetics of the fast bleaching process of betaines 7a‐p to DHIs 8a‐p, studied by flash photolysis as well as low temperature FT‐UV/VIS, were found to take place in the millisecond range (432–2675 ms) in dichloromethane solution and fitted well a first‐order thermal back reaction. The fluorescence spectra as well as the fluorescence quantum yield were studied. Noticeable bathochromic and hypsochromic shifts in the emission spectra by changing the substituents in the base part were monitored. Interestingly, the photo‐fatigue resistance of some studied betaines 7a‐p showed a higher t30‐value than the standard one (dicyanopyridazine DHI). Large solvatochromic effects on the absorption maxima (λmax) as well as a substantial increase in the half‐lives (t1/2) with solvent polarity of betaines 7a‐p were also observed. The multiaddressable PC properties of DHIs 8a‐p will help these compounds to find applications. Copyright © 2007 John Wiley & Sons, Ltd.

Photochromism of dihydroindolizines Part VI: synthesis and photochromic behavior of a novel type of IR‐absorbing photochromic compounds based on highly conjugated dihydroindolizines

AbstractNovel photochromic bis‐styryl dihydroindolizines (DHIs) 5a‐β bearing different cycloalkanone ring sizes were prepared in 11–42% yield via nucleophilic addition of bis‐styryl cycloalkanones 2a–k to substituted spirocyclopropenes 1a–g. The absorption maxima (λmax) of the colored betaines 4a‐β (formed after UV‐irradiation) were detected by millisecond flash photolysis, laser flash photolysis as well as FT‐UV/VIS measurements. All betaines 4a‐β showed two absorption maxima, one in the visible region (500–575 nm) and another one in the IR‐region (800–878 nm). The kinetics of the fast cyclizing process of betaines 4a‐β to DHIs 5a‐β were studied in dichloromethane solution by millisecond flash photolysis and were found to take place in the millisecond range (76–450 msec). The strong colorability of betaines 4a‐β may be attributed to the highly conjugated betaine structures as well as to the presence of the carbonyl chromophore. Interestingly, a remarkable photo‐fatigue resistance of some studied betaines 4 showed t30‐values higher than the standard one. Large solvatochromic effects on the absorption maxima (λmax) as well as a substantial increase of the half‐lives (t1/2) with solvent polarity of betaines 3a‐β were also observed. Copyright © 2006 John Wiley & Sons, Ltd.

Photochromic Control of Photoinduced Electron Transfer. Molecular Double-Throw Switch

A molecular double-throw switch that employs a photochromic moiety to direct photoinduced electron transfer from an excited state donor down either of two pathways has been prepared. The molecular triad consists of a free base porphyrin (P) linked to both a C(60) electron acceptor and a dihydroindolizine (DHI) photochrome. Excitation of the porphyrin moiety of DHI-P-C(60) results in photoinduced electron transfer with a time constant of 2.3 ns to give the DHI-P(*)(+)-C(60)(*)(-) charge-separated state with a quantum yield of 82%. UV (366 nm) light photoisomerizes the DHI moiety to the betaine (BT) form, which has a higher reduction potential than DHI. Excitation of the porphyrin of BT-P-C(60) is followed by photoinduced electron transfer with a time constant of 56 ps to produce BT(*)(-)-P(*)(+)-C(60) in 99% yield. Isomerization of BT-P-C(60) back to DHI-P-C(60) may be achieved with visible light, or thermally. Thus, photoinduced charge separation originating from the porphyrin is reversibly directed down either of two different pathways by photoisomerization of the dihydroindolizine. The switch may be cycled many times.

Photochromism of Dihydroindolizines. Part III [1]. Synthesis and Photochromic Behavior of Novel Photochromic Dihydroindolizines Incorporating a Cholesteryl Moiety

Ten novel photochromic dihydroindolizines (DHIs) based on 1,5-electrocyclization and bearing a cholesteryl moiety at 7′-position of the DHI skeleton were synthesized. 1D, 2D, NOESY 1H NMR spectra, mass spectrometry, and elemental analysis were used for their characterization. Irradiation of the DHIs in CH2Cl2 solution with polychromatic light leads to the formation of red to red-violet colored betaines. Most colored betaine forms are notable in CH2Cl2 solution at room temperature because of their slow 1,5-electrocyclization except in one case, where the colored betaine could be observed only after cooling with liquid nitrogen due to the fast electrocyclization back reaction. The kinetics of the reverse 1,5-electrocyclization of the colored betaines into the corresponding DHIs were detected using both UV/VIS and flash photolysis measurements.

Photochromic nucleic base units suitable for nucleic acid labelling

New molecules incorporating a uracil nucleic base and a dihydroindolizine (DHI) unit linked via spacer arms, i.e., uracil-spyrodihydroindolizine ( 4), were synthesised as models for light sensitive systems for nucleic acid labelling. The uracil-DHI ( 4) undergoes easy photocoloration to the uracil-betaine ( 5) generating the UV-detectable species. Preliminary results show rather weak binding of uracil-DHI ( 4) to calf thymus-DNA.

A New Photochromic System Based on a Pyridazinopyrrolo[1,2-b]pyridazine with Ultrafast Thermal Decoloration

The nucleophilic addition of the substituted pyridazines 2a−e to diacetylspirocyclopropene (1) in dry ether solution afforded the dihydroindolizines 3a−e, which undergo ring opening to the betaine 3′a−e after irradation with UV light. Condensation of the diacetyl groups of the dihydroindolizines (DHIs) 3a−e with hydrazine hydrate in a diethyl ether/ethanol mixture gave the new pyridazinopyrrolo[1,2-b]pyridazines 4a−e. Compounds 4a−e showed no photochromism at room temperature or after cooling with liquid nitrogen. However laser flash spectroscopy was successfully used for the determination of both the half-life and absorption maxima of the betaines 4′a−e.