Abstract
The vibrational properties of a series of push-pull polyynes have been studied by infrared and Raman spectroscopy. The simultaneous activation of a strong infrared and Raman mode, i.e. the collective vibration of CC bonds of the sp carbon chain, highlights the effectiveness of a polyyne bridge in promoting charge transfer between the electron donor (D) and acceptor (A) ends, thus suggesting that ad-hoc functionalized polyynes are worth being explored as building blocks of organic materials with attractive first-order optical nonlinearity. The optical, electronic, and vibrational properties of these molecules have been investigated with the support of density functional theory calculations, as well as the electronic and vibrational first hyperpolarizabilities ($\beta$). The mid-low region of the IR spectra (800-1600 cm$^{-1}$) has been analyzed in detail, searching for marker bands of the specific terminations of the different sp carbon chains thus achieving a complete vibrational characterization of sp hybridized push-pull systems.
Highlights
Polyynes are π-conjugated systems formed by sp-hybridized carbon chains featuring alternated triple and single carbon bonds[1,2,3,4,5]
We focus on the A[n] series, taken as representative of all the ynamines samples investigated in this work
Their polar donor and acceptor endgroups connected through polyyne bridges generate a strong dipole moment along the direction of the sp carbon chain, like other push-pull systems
Summary
Polyynes are π-conjugated systems formed by sp-hybridized carbon chains featuring alternated triple and single carbon bonds[1,2,3,4,5]. Carbon atomic wires in condensed phases tend to rearrange into more stable sp systems, through crosslinking or oxidation reactions[28]. It has been proven that the presence of bulky endgroups hinders crosslinking, increasing the stability of the molecules up to the remarkable length of 48 carbon atoms[26,29,30]. It was demonstrated that the encapsulation of polyynes in rotaxanes or carbon nanotubes allowed to synthesize and stabilize long linear carbon chains [31,32,33,34]
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