The stilbazolium-based organic salt, DAST, is probably the most well-known commercial organic non-linear optical (NLO) material. Chemical derivatives of DAST have therefore been designed to engender even better performing NLO materials; the DAST-derivative, BP3, is one success in this regard. This paper provides a proof-of-principle in the use of structure factors, F, to determine the effects of the crystalline environment on intramolecular charge transfer (ICT), and thus the second-order NLO response of such molecules. A concerted experimental and computational approach is adopted. In particular, the application of the relatively new x-ray wave function refinement method, which tempers theoretical calculations with experimentally derived structure factors to yield an isolated molecule influenced by crystal-field forces, enables the impact of those crystal-field forces on ICT to be established. This study employs high-resolution x-ray diffraction data for its experimental component, this resolution being necessary given the marked challenges that are associated with the complicated interionic interactions within an organic salt. The results presented pinpoint the molecular-scale features that afford BP3 better second-order NLO prospects than DAST, laying the foundations for the molecular engineering of DAST derivatives that are better tailored to optimize NLO function.
J.M.C. is grateful for the BASF/Royal Academy of Engineering Research Chair in Data-Driven Molecular Engineering of Functional Materials, which is partly supported by the STFC via the ISIS Neutron and Muon Facility. J.M.C. also thanks the 1851 Royal Commission of the Great Exhibition for the 2014 Design Fellowship, hosted by Argonne National Laboratory where DOE Office of Science, Office of Basic Energy Sciences, supported work done under Contract No. DE-AC02-06CH11357. T.-C.L. acknowledges the Taiwanese Government for a Studying Abroad Scholarship. C.M.A. is indebted to the EPSRC, UK, for a DTA Ph.D. studentship (Grants No. EP/J500380/1 and No. EP/L504920/1). The authors thank Prof. Dr. S. Grabowsky, Institute of Inorganic Chemistry and Crystallography, University of Bremen, Germany for access to computational facilities and supervisorial support for L.A.M. O.-P.K. is grateful for the support by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning, Korea (No. 2014R1A5A1009799 and No. 2016R1A2B4011050). All authors thank the EPSRC UK National Service for Computational Chemistry Software (NSCCS), EP/J003921/1, and acknowledge contributions from its staff in supporting this work.
