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Reginald H Mitchell

Publications and source records attributed to Reginald H Mitchell.

17 recordsLinked to original sources
PubMed2006

Multistate pi switches: synthesis and photochemistry of a molecule containing three switchable annelated dihydropyrene units.

[structure: see text] The synthesis of the multistate photochromic switch 3 is described. This switch contains three dihydropyrene (DHP) units in the most conjugated fully closed form 3-c,c,c. The thermally stable form has the central DHP unit open and is 3-c,o,c. NMR and laser flash photolysis experiments were used to characterize the multiple states in the photoswitching of 3 by visible and UV light. Three of the possible five isomeric states of 3-c,o,c were observed. Irradiation of 3-c,o,c by visible light led to the formation of 3-o,o,o via the isomer 3-c,o,o as an intermediate, which were observed by NMR. Irradiation by UV light led to the formation of 3-c,c,c, which decays with a lifetime of 7.5 ms.

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PubMed2005

A DFT study of the thermal, orbital symmetry forbidden, cyclophanediene to dihydropyrene electrocyclic reaction. Predictions to improve the dimethyldihydropyrene photoswitches.

The orbital symmetry forbidden thermal electrocyclic equilibria between a series of cyclophanedienes and dimethyldihydropyrenes (CPD<==>DDPs) were studied using density functional theory (DFT). These reactions are important not only because of their fundamental interest but also in how they restrict the potential utility of the DDP photoswitches by limiting the thermal lifetime of the CPDs. The transition states (TSs) for these reactions could not be modeled using restricted DFT (RB3LYP) but were located using unrestricted DFT (UB3LYP). Each TS possesses significant biradical character as indicated by their spin contaminated wave functions, S2 not = 0. Specific substitution by nitrile or trifluoromethyl group(s) is predicted to strongly affect the magnitude of the activation barriers for these reactions. In particular, replacing the internal methyl groups of the CPDs/DDPs with nitrile groups is predicted to have the maximum effect and to raise the activation barriers and lifetimes of the CPDs considerably.

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PubMed2005

Measuring antiaromaticity by an analysis of ring current and coupling constant changes in a cyclopentadienone-fused dihydropyrene.

The synthesis of the chloro- and parent cyclopentadienone-fused dihydropyrenes 10 and 7 are reported. Analysis of coupling constants, and chemical shift changes between these and the nonconjugated dihydro derivatives 11 and 8, and between the benzannulene 4 and the parent annulene 12, indicates without doubt that cyclopentadienone is behaving as an antiaromatic 4n-pi system and that in its effect on the ring current of 12, cyclopentanedienone has about 80% of the effect of benzene. This is the first time that a suitable probe has been used to estimate the relative ability of a 4n-pi system to bond localize the probe in comparison to the (4n + 2)-pi system benzene.

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PubMed2005

Molecular AND and INHIBIT gates based on control of porphyrin fluorescence by photochromes.

A molecular triad consisting of a porphyrin (P) covalently linked to two photochromes-one from the dihydroindolizine family (DHI) and one from the dihydropyrene family (DHP)-has been synthesized and found to act as either a molecular AND logic gate or an INHIBIT gate, depending on the inputs and initial state of the photochromes. The basis of these functions is quenching of porphyrin fluorescence (output of the gates) by the photochromes. The spiro form of DHI does not quench porphyrin fluorescence, whereas its betaine isomer strongly quenches by photoinduced electron transfer. DHP also quenches porphyrin fluorescence, but its cyclophanediene isomer does not. The triad has been designed using suitable energetics and electronic interactions, so that although these quenching phenomena may be observed, independent isomerization of the attached photochromes still occurs. This makes it possible to switch porphyrin fluorescence on or off by isomerization of the photochromes using various combinations of inputs such as UV light, red light, and heat.

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PubMed2005

13-methyl-2,6-dithia[7]metacyclophane: a useful molecule to connect VT NMR results and structure with calculations.

Ground state energies (DFT) and 1H and 13C NMR chemical shifts are calculated for the conformers of 13-methyl-2,6-dithia[7]metacyclophane (1), and the results are compared with X-ray structural data and variable-temperature NMR data, including the determination of the activation barrier. Calculations predict the correct low energy conformer with good agreement with chemical shifts, bond distances, and angles. VT NMR data for the 10-tert-butyl-substituted derivative 2 indicate that it undergoes the same conformational equilibria as 1. This paper should enhance the confidence that organic chemists have in calculations to satisfactorily predict conformer energies.

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PubMed2004

Molecule-based photonically switched half-adder.

A molecule-based binary half-adder with optical inputs and outputs has been demonstrated. The half-adder consists of two photochromic organic molecules in solution and a third-harmonic-generating crystal. One substance acts as an AND Boolean logic gate and the other as an XOR gate. Inputs are laser pulses at 1064 or 532 nm that initiate photoisomerization reactions. Outputs are the optical absorbance of a fullerene radical anion (AND gate) and fluorescence of a porphyrin (XOR gate). The system carries out binary addition based on the laser input pulses. Half-adders in combination are capable of carrying out all mathematical operations necessary for digital computing.

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PubMed2004

Linked photoswitches where both photochromes open and close.

In the dithienylethene photochrome series, linking two photochromes together directly, or with an ethynyl spacer, shuts down the photoswitching of one of the photochromes. We report here, that for dihydropyrenes, two photochromes can be linked together without affecting the ability of either to photoswitch. [reaction--see text]

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PubMed2004

Photonic switching of photoinduced electron transfer in a dihydropyrene-porphyrin-fullerene molecular triad.

Photonic control of photoinduced electron transfer has been demonstrated in a dimethyldihydropyrene (DHP) porphyrin (P) fullerene (C(60)) molecular triad. In the DHP-P-C(60) form of the triad, excitation of the porphyrin moiety is followed by photoinduced electron transfer to give a DHP-P(*)(+)-C(60)(*)(-) charge-separated state, which evolves by a charge shift reaction to DHP(*)(+)-P-C(60)(*)(-). This final state has a lifetime of 2 micros and is formed in an overall yield of 94%. Visible (>or=300 nm) irradiation of the triad leads to photoisomerization of the DHP moiety to the cyclophanediene (CPD). Excitation of the porphyrin moiety of CPD-P-C(60) produces a short-lived (<10 ns) CPD-P(*)(+)-C(60)(*)(-) state, but charge shift to the CPD moiety does not occur, due to the relatively high oxidation potential of the CPD group. Long-lived charge separation is not observed. Irradiation of CPD-P-C(60) with UV (254 nm) light converts the triad back to the DHP form. Thermal interconversion of the DHP and CPD forms is very slow, photochemical cycling is facile, and in the absence of oxygen, many cycles may be performed without substantial degradation. Thus, light is used to switch long-lived photoinduced charge separation on or off. The principles demonstrated by the triad may be useful for the design of molecule-based optoelectronic systems.

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PubMed2004

An annulene-fused cyclopentadienide. A photochromic cyclopentadienodimethyldihydropyrene where the fused cyclopentadienide group resembles benzene in its effect on the dihydropyrene-metacyclophanediene valence isomerization.

The synthesis of the green cyclopentadiene-fused dimethyldihydropyrene 12 was achieved in 36% yield in 7 steps from the parent dihydropyrene 3. Reaction of 12 with KH or LiCH(2)SiMe(3) gave the [14]annulene-fused cyclopentadienide anion quantitatively. In the (1)H NMR spectra, the internal methyl protons of 12 at delta -3.9, change dramatically on formation of anion 5, becoming deshielded to delta -1.82. This is caused by the reduction in diatropicity of the [14]annulene ring on fusion to the 6pi-cyclopentadienide ring. The anion is also a photochromic switch. Irradiation of the closed form 5 with visible light opens it to the open form 5', which reverts to the closed form 5 either with UV light or thermally. The switching behavior is between that of the parent 3/3' and the benzannelated system 4/4' and suggests that in its effect on the photoswitching, cyclopentadienide is behaving chemically similar to benzene.

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PubMed2003

A photochromic, electrochromic, thermochromic Ru complexed benzannulene: an organometallic example of the dimethyldihydropyrene-metacyclophanediene valence isomerization.

The preparation of the first photochromic, organometallic derivative of the diarylethene class, the CpRu-complexed benzodimethyldihydropyrene 3, in which the organometallic is directly attached to the photochromic core, is described. The negative dark purple photochrome 3 readily bleaches to form the almost colorless cyclophanediene 3' on irradiation with visible light. The latter switches back to 3 either photochromically with UV light, electrochromically on reduction, or thermochromically on heating. Essentially quantitative conversion between the two states is possible. The open complex 3' thermally closes 2.6 times faster than the uncomplexed parent 2', but the closed form 3 opens with visible light at about 30% of the rate of uncomplexed 2. Both open forms, complexed 3' and uncomplexed 2' close equally fast with UV light.

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PubMed2003

Synthesis and photochromic properties of molecules containing [e]-annelated dihydropyrenes. Two and three way pi-switches based on the dimethyldihydropyrene-metacyclophanediene valence isomerization.

The syntheses of several new simple negative, a simple positive, and multiple negative photochromes containing the dihydropyrene-cyclophanediene photochromic system are described. The photo-openings of the negative photochromes, the [e]-annelated benzo (7), naphtho (9), anthro (11), furano (19), and triphenyleno (15) derivatives of the parent 2,7-di-tert-butyl-trans-10b,10c-dimethyl-dihydropyrene (5), as well as its 4,5-dibromo derivative (13), are described to give the corresponding cyclophanedienes, as well as their photoclosures and thermal closures back to the dihydropyrenes. These are compared to the results obtained for the positive photochrome dibenzo[e,l]dihydropyrene (21) and to the bis(dihydropyreno)chrysene (44) and the (dihydropyrenobenzo)(benzo)metacyclophanediene (47) photochromes, which have more than one photochromic switch present and thus have more than a simple "on-off" state. Thermodynamic data are obtained for the thermal closing reactions. The anthrodihydropyrene (12) has the fastest thermal closing (tau(1/2) = 20 min), while the furanodihydropyrene (19') has the slowest (tau(1/2) = 63 h) at 46 degrees C. An electrochemical readout of the state of the switch is demonstrated for the benzodihydropyrene (7).

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PubMed2002

Dimethyldihydropyrene-dehydrobenzoannulene hybrids: studies in aromaticity and photoisomerization.

The synthesis and study of dehydrobenzoannulene (DBA)-dimethyldihydropyrene (DDP) hybrids as models for the investigation of aromaticity in weakly diatropic systems is reported. Three new monofused DBA-DDP hybrids have been synthesized, and their NMR spectra are discussed with regard to quantifying the aromaticity remaining in multibenzene-fused DBAs. Nucleus-independent chemical shifts, determined at a series of locations for each compound, bond lengths, and (1)H and (13)C NMR chemical shifts were calculated and used to probe the aromaticity of these hybrids. Systems where more than one annulene/DBA is fused to the DDP core have also been obtained, and their potential use in photoinduced isomerization applications is discussed.

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PubMed2002

A theoretical and experimental scale of aromaticity. The first nucleus-independent chemical shifts (NICS) study of the dimethyldihydropyrene nucleus.

Nucleus-independent chemical shift (NICS) values were calculated at several locations for a series of dimethyldihydropyrenes (DDPs). These NICS values were used to assess the relative aromaticities of the dimethyldihydropyrene nucleus (DDPN) of these DDPs and to construct a NICS scale of aromaticity. The NICS and experimentally determined relative aromaticities of these DDPNs are in complete agreement, verifying that NICS can be used not only to classify a compound as aromatic but also to determine the degrees of aromaticity of structurally related systems.

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PubMed2002

Effective aromaticity of tricarbonylchromiumbenzene, about 25 enhanced over that of benzene: structural evidence from a complexed benzannulene.

The tricarbonylchromium complex of 2,7-di-tert-butyl-trans-12c,12d-dimethylbenzo[e]dihydropyrene is prepared in 65% yield from the parent annulene. Analysis of the (1)H NMR spectrum leads to coupling constant data and Günther's "Q" value data which unequivocally indicate that the tricarbonylchromiumbenzene moiety behaves like an aromatic system of greater bond fixing ability, aromaticity, than benzene itself; this analysis is supported by X-ray structural data and by the rotational position of the tricarbonyl podal. Analysis of chemical shift data indicates that the tricarbonylchromium benzene moiety has about 25% enhanced "aromaticity" over that of benzene.

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PubMed2002

Mechanistic insights into the photochromism of trans-10b,10c-dimethyl-10b,10c-dihydropyrene derivatives.

A series of dimethyldihydropyrene derivatives was studied to elucidate the photochemical mechanism associated with the switching between the dimethyldihydropyrene (DHP, closed) and metacyclophanediene (CPD, open) forms of the molecule. Quantum yields of ring opening and closure, fluorescence quantum yields and lifetimes, as well as laser flash photolysis studies were performed to establish the effect of substituents on the switching efficiency. Ring opening of the DHPs occurs from the first singlet excited state. The low quantum yields for the ring opening reaction observed (< or =0.042) are a consequence of the low rate constant (< or =1.7 x 10(7) s(-1)) for this process. The quantum yields for ring closure of the CPD were determined for select compounds and were of the order of 0.1-0.4. These results show that the efficiency for ring opening of this class of compounds is intrinsically low, but can be modulated to some extent by the introduction of substituents. These properties should be taken into account when considering what type of photoswitching devices DHPs might be useful for.

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PubMed2002

Conformational changes of 2,11-dithia[3.3]metacyclophane. A new look using VT NMR and calculation.

The conformational changes of 2,11-dithia[3.3]metacyclophane are reexamined utilizing VT NMR data and calculations (ab initio, semiempirical, density functional, and molecular mechanics) to show that the syn to syn' inversion that occurs with exchange of the benzylic hydrogens proceeds most easily through bridge wobbles of the anti isomers and that the critical barrier is the conversion of the syn - chair - chair isomer to the anti-chair-chair isomer, barrier (found) = 9.3-9.6 kcal/mol, barrier (calc) = 10.3 kcal/mol, such that when this conversion is slow on the NMR time scale, the benzylic hydrogens no longer exchange, and the syn-chair-chair isomer becomes frozen. The syn-boat-chair isomer, however, can continue to invert to the anti-boat-chair isomer until lower temperatures, and thus, the benzylic hydrogens continue to exchange for this isomer. Thus, while bridge wobbles of the syn isomers have the largest barriers, bridge wobbles of the anti isomers have the smallest barriers, and so the barriers of the syn-to-anti conversions play a much greater role than previously determined.

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