Through-Space Charge Transfer and Nonlinear Optical Properties of Substituted Paracyclophane

Abstract

The introduction, within a π-conjugated donor−acceptor molecule, of an intermediate barrier to electron tunneling and its size scaling and influence on electronic polarization properties have remained so far elusive issues of great potential interest toward the fine-tuning of the linear and nonlinear optical properties of molecular materials. Paracyclophane (pCP) provides a most relevant cornerstone for more elaborate compounds where donor and acceptor substituents are made to interact through a sterically constrained π−π stack. A first attempt in this direction is reported here with the synthesis of a model dipolar 4-(4-dihexylaminostyryl)-16-(4-nitrostyryl)[2.2]paracyclophane and the subsequent experimental and theoretical study of its quadratic nonlinear optical properties. A major outcome of this investigation is the evidence of a significant “through-space” charge transfer as unambiguously designated by the strong departure of the β quadratic hyperpolarizability tensor of the full doubly substituted molecule (60 × 10-30 esu) from the additive β value (18 × 10-30 esu) expected for strictly noninteracting singly substituted pCP moieties. This desired increase of nonlinear efficiency upon substitution is not offset by the usual red-shift of the absorption spectrum which generally curtails application perspectives in more common uninterrupted conjugated chains. The collective nonlinear polarization behavior involving the full end-to-end molecular structure is confirmed by theoretical calculations using the Collective Electron Oscillator (CEO) approach which furthermore indicates a significantly enhanced role of electron−hole pair delocalization in the higher order nonlinear response, compared to the linear polarizability or the static dipole moment.

Probing π-coupling in molecular junctions

“…Although previous spectroscopic measurements on pCp containing chromophores have demonstrated strong through-space coupling (11)(12)(13)(14), the results reported here enable the electronic conduction across the pCp linkage to be compared to its fully conjugated analog. The junction I-V characteristics demonstrate that the Au͞1͞Au junction is approximately a factor of two more conductive than the Au͞2͞Au junction.…”

“…Although it can be argued that the HOMO-1 orbital is probably more important to charge transport because it has a larger electron density on the terminal Au atoms, and hence better coupling to the metal electrodes, both orbitals undoubtedly contribute to the measured charge transport. The calculated electronic structure for 2 bound to two Au atoms, demonstrates that the low-lying through-space delocalized state (11)(12)(13)(14) can effectively facilitate charge transport in a molecular junction.…”

“…1). It is known that the optical properties of well-defined, linked conjugated oligomers such as 2 are often perturbed by transannular extension of conjugation (11)(12)(13)(14). Although these previous optical measurements have been used to determine the extent of through-space delocalization, nothing is known about how this arrangement influences the conductance of such molecules.…”

Amplification of the linear and nonlinear optical response of a chiral molecular crystal

“…Two important studies of substituted paracyclophane compounds, 10,11 using the collective electron oscillation (CEO) approach, 12 also indicated a significant role of through-space charge transfer in the second-order nonlinear response of these compounds. The crystal engineering strategy of co-crystallizing anionic-cationic moieties has also been used to enhance the quadratic nonlinear optical properties.…”

Theoretical Study of Effect of the Number of N,N-dimethyl-4-nitroaniline Units in Novel “Parallel Connection” Chromophores on Its Nonlinear Optical Properties

“…Generally, an increase in the β value is accompanied by a bathochromic shift due to a larger π-conjugated length and/or stronger donor and acceptor ability [9][10][11][12][13]. Therefore, there is always a tradeoff between nonlinearity and transparency [14][15][16][17][18][19][20][21][22][23]. To resolve this problem, we had designed and synthesized a series of H-typed chromophores with two parallel and nonconjugated D-π-A units [24][25][26][27].…”