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