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130 results on '"Warman, J. M."'

7. Theoretical and experimental studies of the opto-electronic properties of positively charged oligo(phenylene vinylene)s: Effects of chain length and alkoxy substitution.

Author

Grozema, F. C., Candeias, L. P., Swart, M., van Duijnen, P. Th., Wildeman, J., Hadziioanou, G., Siebbeles, L. D. A., and Warman, J. M.

Subjects
POLYMERS, OPTOELECTRONICS
Abstract

In this paper a combined experimental and quantum chemical study of the geometry and opto-electronic properties of unsubstituted and dialkoxy-sustituted phenylene-vinylene oligomers (PV's) is presented. The optical absorption spectra for PV cations with different chain lengths and substitution patterns were measured using pulse radiolysis with time-resolved spectrophotometric detection from 1380 to 500 nm (0.9 to 2.5 eV). The geometries of the PV's studied were optimized using density functional theory (DFT) for both the neutral and singly charged molecule. The spectra for the PV radical cations were then calculated using singly excited configuration interaction with an intermediate neglect of differential overlap reference wave function method together with the DFT geometry. The agreement between experimental and theoretical absorption energies is excellent; most of the calculated radical cation absorption energies are within 0.15 eV of the experimental values. The pattern of dialkoxy-substitution is found to have a large effect on the optical absorption spectrum of the cation. Using the calculated charge distribution it is shown that the degree of delocalization of the charge correlates with the energy of the lowest absorption band. If alkoxy side chains are present on some of the rings the positive charge tends to localize at those sites. [ABSTRACT FROM AUTHOR]

Published
2002
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12. Two-step sequential light-induced electron transfer in a simple trichromophoric donor-donor-acceptor system

Author

Brouwer, A.M., Eijckelhoff, C., Willemse, R.J., Verhoeven, J.W., Schuddeboom, W., and Warman, J. M.

Subjects
Oxidation-reduction reaction -- Research, Porphyrins -- Spectra, Chemistry
Abstract

The photophysical behavior of several porphyrin compounds were studied to determine the sequential charge transfer processes of molecules with simple organic building blocks. Excitation at 308 nm produced an acceptor moiety with a characteristic charge-transfer emission instead of fluorescence. The porphyrin compounds exhibited variations in their fluorescence with different quantum yield values and decay times. Estimations of the energetics of charge transfer are presented.

Published
1993

14. Optoelectronic properties of quasi-linear, self-assembled platinum complexes: Pt-Pt distance dependence

Author

Debje, M. G., De Haas, M. P., Warman, J. M., Fontana, M., Stutzmann, N., Kristiansen, M., Caseri, W. R., Smith, P., Hoffmann, S., Sølling, Theis Ivan, Debje, M. G., De Haas, M. P., Warman, J. M., Fontana, M., Stutzmann, N., Kristiansen, M., Caseri, W. R., Smith, P., Hoffmann, S., and Sølling, Theis Ivan

Abstract

Charge-carrier mobilities; otopmetrics; organometallic compounds

Published
2004

42. Charge Mobilities in Organic Semiconducting Materials Determined by Pulse-Radiolysis Time-Resolved Microwave Conductivity:  π-Bond-Conjugated Polymers versus π−π-Stacked Discotics

Author

Warman, J. M., Haas, M. P. de, Dicker, G., Grozema, F. C., Piris, J., and Debije, M. G.

Abstract

A review is given of the one-dimensional, intrachain and intracolumnar, charge mobilities, Σμ1D, determined for π-bond-conjugated polymeric and for π−π-stacked columnar discotic materials using the pulse-radiolysis time-resolved microwave conductivity technique. The largest values, on the order of 10 cm2/(V s), are found for single-crystal polydiacetylenes polymerized either thermally or with low doses of radiation. Much lower values of Σμ1D, covering the range from 0.009 to 0.125 cm2/(V s), are found for solution-synthesized conjugated polymers for which six different backbone structures have been investigated. This is attributed mainly to their complex morphology and the resulting static disorder in the backbone structure. The highest mobilities for this class of material, ca. 0.1 cm2/(V s), are found for liquid crystalline derivatives of polyfluorene and poly(phenylenevinylene). Larger mobilities are found for discotic materials, with maximum values close to 1 cm2/(V s) in both the crystalline solid and liquid crystalline phases. This is attributed to their self-organizing nature and hence higher degree of structural order, which compensates for the weaker electronic coupling between monomeric units in the discotics compared with the covalently bonded conjugated polymers.

Published
2004

45. Positive Charge Carriers on Isolated Chains of MEH−PPV with Broken Conjugation:  Optical Absorption and Mobility

Author

Candeias, L. P., Grozema, F. C., Padmanaban, G., Ramakrishnan, S., Siebbeles, L. D. A., and Warman, J. M.

Abstract

Pulse radiolysis of oxygen-saturated dilute solutions of poly[2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylene vinylene]s in benzene showed charge-transfer from the solvent radical cation to the polymer, yielding positive charge carriers (radical-cations or “holes”) on the isolated chains. The charge-transfer reactions are diffusion controlled with (time-dependent) rate coefficients that are determined by the conformation adopted by the polymer chains. The radical cation of the fully conjugated polymer exhibited an absorption maximum at 1.32 eV. The absorption maximum shifted to higher energies with decreasing conjugated fraction, in a way consistent with thermal equilibration of the positive charge over the conjugated segments of different length. The mobility of the positive charge along the fully conjugated MEH−PPV chains was determined to be 0.42 cm2 V-1 s-1. The introduction of conjugated breaks decreased the charge mobility strongly, which is attributed to the disruption of the π conjugation and/or the conformational disorder caused by the flexibility of the chains at the sites of conjugation breaks.

Published
2003

46. Dipolar Excitonic States of Oligo-Anthrylenes

Author

Piet, J. J., Warman, J. M., Baumgarten, M., and Mullen, K.

Abstract

Photoexcitation of dimeric, trimeric, and tetrameric oligomers of anthracene in benzene results in the formation of relaxed S1 excitonic states with dipole moments of 6.9, 8.0, and 9.1 D respectively. Dipole reversal occurs with a relaxation time in the range of 8 ± 2 ps and is driven by a combination of fluctuations in the solvent environment and torsional motion about the central sigma bonds. The excitonic interaction at any given moment in time involves mainly only two neighboring anthracene moieties rather than delocalization over all chromophoric units.

Published
2002
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