Spectroscopic Studies on Dyes. II. The Structure of N,N'-Dimethylindigo<sup>1</sup>

Weinstein, Julius; Wyman, George M.

doi:10.1021/ja01597a038

Cited by 62 publications

(35 citation statements)

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“…5,6 For indigo derivatives in which the intramolecular hydrogen bonds are missing, on the other hand, the cis isomer is readily generated by photoexcitation. [5][6][7][8][9][10][11][12][13][14] For these indigo derivatives, it has been proposed that triplet states are involved as intermediates in the trans → cis photoisomerization process. [15][16][17][18][19][20][21][22] Indigo itself, however, exhibits a very low quantum yield of intersystem crossing.…”

Section: Introductionmentioning

confidence: 99%

Molecular mechanisms of the photostability of indigo

Yamazaki

Sobolewski

Domcke

2011

Phys. Chem. Chem. Phys.

135

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The photophysics of indigo as well as of bispyrroleindigo, the basic chromophore of indigo, has been investigated with ab initio electronic-structure calculations. Vertical electronic excitation energies and excited-state potential-energy profiles have been calculated with the CASSCF, CASPT2 and CC2 methods. The calculations reveal that indigo and bispyrroleindigo undergo intramolecular single-proton transfer between adjacent N-H and C=O groups in the (1)ππ* excited state. The nearly barrierless proton transfer provides the pathway for a very efficient deactivation of the (1)ππ* state via a conical intersection with the ground state. While a low-lying S(1)-S(0) conical intersection exists also after double-proton transfer, the latter reaction path exhibits a much higher barrier. The reaction path for trans→cis photoisomerization via the twisting of the central C=C bond has been investigated for bispyrroleindigo. It has been found that the twisting of the central C=C bond is unlikely to play a role in the photochemistry of indigo, because of a large potential-energy barrier and a rather high energy of the S(1)-S(0) conical intersection of the twisted structure. These findings indicate that the exceptional photostability of indigo is the result of rapid internal conversion via intramolecular single-proton transfer, combined with the absence of a low-barrier reaction path for the generation of the cis isomer via trans→cis photoisomerization.

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Section: Introductionmentioning

confidence: 99%

Molecular mechanisms of the photostability of indigo

Yamazaki

Sobolewski

Domcke

2011

Phys. Chem. Chem. Phys.

135

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“…[14][15][16] In the context of the molecules studied in this work, it is also important to mention that the obvious molecule to preclude ESIPT between the N-H and CdO groups would seem to be a dialkylindigo such as N,N′-dimethylindigo. [17][18][19][20][21][22] However, in this case the deviation from coplanarity of the dimethyl derivative leads to a strongly twisted structure about the central CdC bond, in contrast with the virtually planar structure of indigo, and it appears that cis-trans photoisomerization becomes a favorable and highly competitive pathway. 23,24 This presumably occurs together with fluorescence, although this latter process does not appear to have been studied in detail.…”

Section: Introductionmentioning

confidence: 99%

Photophysics of an Indigo Derivative (Keto and Leuco Structures) with Singular Properties

et al. 2006

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Spectral and photophysical properties of the indigo derivative Cibalackrot in keto and reduced (leuco) forms were studied by absorption spectra, fluorescence and pulse radiolysis and compared with the structurally similar indigo. With the keto form of this dye, fluorescence (φ F ) 0.76) and intersystem crossing (φ T ) 0.11) are dominant, whereas with indigo, efficient internal conversion (φ IC ) 0.99) is observed, probably involving proton transfer through intramolecular hydrogen bonds. With Cibalackrot, this pathway is blocked, supporting the above model for indigo. With the reduced form of Cibalackrot, more than 98% of the absorbed quanta are dissipated through S 1 ∼∼f S 0 internal conversion, which contrasts with leuco-indigo, where fluorescence (φ F ) 0.35), internal conversion (φ IC ) 0.53) and intersystem crossing (φ T ) 0.12 5 ) are found to be competitive. In addition, a synthetic precursor of Cibalackrot (preCiba) was also investigated. This has a rigid molecular structure (with a moiety identical to Cibalackrot and the other to indigo), but intra-or intermolecular proton transfer is allowed between adjacent carbonyl and N-H groups. With this precursor in its keto structure the photophysical parameters are generally very close to those of the keto form of indigo, and different from those of Cibalackrot. A more detailed investigation of the time-decay profiles of preCiba in dioxane (and with added water and D 2 O) has shown that these follow biexponential laws with a shorter component of 14-25 ps, which appears associated with a risetime at longer wavelength emissions (and to a positive preexponential at shorter emission wavelengths) and a longer lived (decay) component of 104-130 ps. In the steady-state spectra of preCiba, the variation with temperature reveals a blue shift of the emission maxima, which is interpreted as the presence (simultaneous emission) of two species (keto and enol) in the excited state. Indigo and deuterated indigo are also found to present a similar behavior. The overall data are interpreted as to be due to an excited-state process involving the proton transfer between keto and enol forms. Rate constants with values of 7 × 10 10 s -1 for preCiba and 1.6 × 10 11 s -1 for deuterated indigo were obtained. This inverse isotope effect is justified on the basis of the proposed model for proton-transfer excited-state deactivation.

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“…Thioindigo and substituted indigo dyes in cis + trans isomerization (1)(2)(3)(4) show interesting photochemical behaviour, which has been proposed for solar energy conversion and storage (5). A triplet mechanism, postulated by Wyman and co-workers (1,6), is now generally accepted for the trans + cis photoisomerization.…”

Section: Introductionmentioning

confidence: 99%

Trans to cis photoisomerization of N,N′-disubstituted indigo dyes via excited singlet states; a laser flash photolysis and steady state irradiation study

Görner¹,

Pouliquen²,

Kossanyi³

1987

Can. J. Chem.

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The relaxation pathways of excited trans-N,N′-diacylindigo dyes (trans-1–trans-4) and of the rigid trans (5) and cis (6) indigo analogues have been studied by nanosecond flash photolysis and steady state measurements. In fluid solution the trans to cis photoisomerization of 1–4 occurs via a singlet excited state mechanism on direct excitation, and the triplet state appears to be reached only by sensitized reaction. The lower quantum yield for trans → cis isomerization of 2–4 at room temperature, as compared to 1, is due to a thermally activated internal conversion step leading to the trans ground state. This pathway competes with fluorescence and twisting of the molecule around the central double bond in the excited singlet state. Increasing the viscosity of the medium decreases the twisting of the flexible molecules and, as a consequence, increases the population of the lowest triplet state. A triplet transient, the intensity of which increases with the viscosity of the medium, could be characterized at low temperature under direct excitation. A transient species, which can be assigned to a triplet state on the basis of quenching experiments and on the fact that the same transient could be found under sensitized conditions, also has been observed at room temperature for the rigid 5 and 6 molecules. Quenching measurements of triplet sensitizers by 5, trans-1, and trans-2 enables the localization of the triplet energy at around 40 kcal/mol.

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Spectroscopic Studies on Dyes. II. The Structure of N,N'-Dimethylindigo¹

Cited by 62 publications

References 0 publications

Molecular mechanisms of the photostability of indigo

Molecular mechanisms of the photostability of indigo

Photophysics of an Indigo Derivative (Keto and Leuco Structures) with Singular Properties

Trans to cis photoisomerization of N,N′-disubstituted indigo dyes via excited singlet states; a laser flash photolysis and steady state irradiation study

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Spectroscopic Studies on Dyes. II. The Structure of N,N'-Dimethylindigo1

Cited by 62 publications

References 0 publications

Molecular mechanisms of the photostability of indigo

Molecular mechanisms of the photostability of indigo

Photophysics of an Indigo Derivative (Keto and Leuco Structures) with Singular Properties

Trans to cis photoisomerization of N,N′-disubstituted indigo dyes via excited singlet states; a laser flash photolysis and steady state irradiation study

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Spectroscopic Studies on Dyes. II. The Structure of N,N'-Dimethylindigo¹