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S. Kilina, E. Badaeva, S. Tretiak, A. Piryatinski, A. Saxena, and A.R. Bishop, ”Bright and Dark Excitons in Semiconductor Carbon Nanotubes: Insights from electronic structure calculations”, Phys. Chem. Chem. Phys. (journal cover page)21, 4113 - 4123 (2009).
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Journal cover page: PDF file [200K]
F. Terenziani, C. Katan, M. Blanchard-Desce, E. Badaeva, and S. Tretiak, ”Enhanced two-photon absorption of organic chromophores: theoretical and experimental assessments”, Adv. Mat. (Review Article, journal cover page) 20 1-38 (2008).
PDF file [1.7Mb]
Journal cover page: PDF file [500K]
G.D. Scholes, S. Tretiak, T.J. McDonald, W.K. Metzger, C. Engtrakul, G. Rumbles, and M.J. Heben, ”Low-lying exciton states determine the photophysics of semiconducting single wall carbon nanotube”, J. Phys. Chem. C (journal cover page), 111, 11139-11149 (2007).
PDF file [620K]
Journal cover page: PDF file [810K]
S.
Tretiak, ”Triplet absorption in
carbon nanotubes: a TD-DFT study,” Nano Letters (journal
cover page), 7, 2201-2206 (2007).
PDF file [500K]
Journal cover page: PDF file [130K]
S.
Kilina and S. Tretiak, ”Excitonic
and vibrational properties of single-walled semiconducting carbon
nanotubes”, Adv. Func.
Mat. (journal cover
page) 17, 3405-3420 (2007). [FEATURE ARTICLE]
PDF file [2.5Mb]
Journal cover page: PDF file [500K]
S. Tretiak, V. Chernyak, and
PDF file [430K]
Journal cover page: PDF file [210K]
Cool movies
Photoexcited
breathers in conjugated polyenes
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Schematic representation of excited state molecular dynamics propagation. The excited state energy Ee(q) as a function of nuclear coordinates q, displacements D, vibrational reorganization energy Ev, vertical absorption WA, and fluorescence WF frequencies. |
To analyze excited-state dynamics of cis-polyacetylene with chain length N=60 repeat units, both electronic and vibrational degrees of freedom need to be followed. The bond-length alternation parameter, BLA (which reflects an uneven distribution of the p electrons over the bonds, Peierls distortion, and is therefore strongly coupled to the electronic system) is used to monitor the vibrational dynamics (top right). A real-space two-dimensional representation of the transition density matrices representing the electronic transition between the ground state and an electronically excited state, is convenient to follow electronic dynamics (top left). Photoexcitation creates an electron-hole pair or an exciton by moving an electron from an occupied orbital to an unoccupied orbital. Each element of the transition density reflects the dynamics of this exciton projected on a pair of atomic orbitals given by its indices, therefore, 2D plot depicts probabilities of an electron moving from one molecular position (horizontal axis) to another (vertical axis) upon electronic excitation, and axes labels represent the number of repeat units.
Download movie [file
size 13 Mb]
The movie shows that upon the photoexcitation, the exciton created is initially delocalized along the entire chain (t=0). Due to strong electron-phonon coupling an exciton rapidly distorts the lattice in the middle of the chain (t=16 fs) and localizes itself in this region on the timescale of ~20 fs. When an exciton distorts the lattice, dynamical vibrational excitations (phonons) are created, appearing as waves in the bond-length alternation on the edges of the exciton potential well (t=44 fs). The subsequent dynamics can be qualitatively described as following: the phonon "waves" propagate in opposite derections, reflect from the chain ends, and meet in the middle of the chain pulling an exciton out of its well and delocalizing the excitation (e.g. t=100 fs). The exciton, in turn, attempts to localize again, creating more phonons, i.e. the energy is exchanging between electronic and vibrational degrees of freedom. This variation of diagonal delocalization is a characteristic "breathing" pattern with period of 34 fs which is not associated with any of the vibrational normal modes (which may be observed in Raman of IR spectra).