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OPTICAL PROPERTIES OF ADAMANTYL END-CAPPED POLYYNES: FROM
EXPERIMENTS AND FIRST PRINCIPLES SIMULATIONS
Daniele Fazzi
Matteo Tommasini
Andrea Lucotti
Mirella Del Zoppo
Giuseppe Zerbi
W. Chalifoux
R. R. Tykwinski
γvib
γelec (DOKE)
● Linear sp carbon chains are gathering interest for the direct investigation of the physical properties of one dimensional systems.
● Adamantyl groups stabilize the highly reactive sp carbon chain.
● They are possible candidates for non linear optical applications thanks to their large second order hyperpolarizability γ.
Large γ values (among the largest for conjugated systems)
γv/γe strictly related: large e-ph coupling
Vibrational (Raman and IR) and electronic (UV-vis) spectroscopies are used to study the optical and structural properties of these systems
Dispersion of Raman and IR active normal mode, with the chain length
i i
iI
c 2224
1
15
1
Я normal mode
The vibrational contribution (γv) to the second order hyperpolarizability can be written as a function of absolute Raman intensities (Ii) and vibrational wavenumbers (νi) [1]:
[1] C.Castiglioni et al., Solid State Commun. 82 (1992) 13
Superlinear increase of γv !
ωЯ
ωIR
Breaking of mutual exclusion principle
Nc≡c=3
Nc≡c=4
Nc≡c=5
Nc≡c=6
Nc≡c=8
Nc≡c=10
NC≡C= 6
HOMO
LUMO
Fit of experimental UV-Vis spectra via Franck-Condon factors:
From fit we obtained:
Nc=20
0-00-1
0-2
0-30-4
ZINDO simulations on PBEPBE/cc-pVDZ optimized geometries
1. The vibrational frequency of the excited state and its dispersion with the chain length ωЯ
e
ωЯg
2. The displacement parameter B between the ground and excited state along the Я normal mode
2/12IEBNelec
Conclusion:1. Adamantyl capped polyynes show a relevant NLO response (γ) of both
electronic and vibrational nature
3. The coupling of the π-π* excitation with the Я coordinate is experimentally demonstrated from the detailed analysis of the UV-Vis absorption spectra. This is also supported by the theoretical analysis of HOMO and LUMO orbitals
2. The violation of the IR-Raman mutual exclusion principle points to a bent equilibrium structure of the molecules in solution (bow-like shape)
Based on the theoretical analysis of the transition dipole moment [2] and supported by the Albrecht’s theory [3], we can obtained the following relationship:
Nelec: number of electrons; I: Raman intensity;B: displacement parameter;
ΔE: energy-gap;
Experimental confirmation
[3] A. C. Albrecht, J.Chem.Phys. 34, 1476 (1961)
[2] M. G. Kuzyk, Optics Letters, 25, 1183 (2000)
ZINDO
TD-DFT