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

γ vib γ elec (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) γ v / γ e 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 Я normal mode The vibrational contribution (γ v ) to the second order hyperpolarizability can be written as a function of absolute Raman intensities (I i ) and vibrational wavenumbers (ν i ) [1] : [1] C.Castiglioni et al., Solid State Commun. 82 (1992) 13 Superlinear increase of γ v ! ωЯωЯ ω IR Breaking of mutual exclusion principle N c ≡ c =3 N c ≡ c =4 N c ≡ c =5 N c ≡ c =6 N c ≡ c =8 N c ≡ c =10 N C ≡ C = 6

HOMO LUMO Fit of experimental UV-Vis spectra via Franck-Condon factors: From fit we obtained: N c = ZINDO simulations on PBEPBE/cc-pVDZ optimized geometries 1.The vibrational frequency of the excited state and its dispersion with the chain length ωЯeωЯe ωЯgωЯg 2.The displacement parameter B between the ground and excited state along the Я normal mode

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: N elec : 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