Quadratic Optical Nonlinearities of N-Methyl and N-Aryl Pyridinium Salts** By Benjamin J. Coe ,* James A. Harris , Inge Asselberghs , Kurt Wostyn, Koen Clays , AndrØ Persoons , Bruce S. Brunschwig, Simon J. Coles , Thomas Gelbrich, Mark E. Light, Michael B. Hursthouse , and Keitaro Nakatani 1. Introduction The continued, intense research interest in organic molecular materials with nonlinear optical (NLO) properties stems pri- marily from the potential held by such compounds for various applications in optoelectronic and photonic devices. [1] Quadrat- ic (second order) NLO effects such as frequency doubling (sec- ond harmonic generation, SHG), and optical parametric oscil- lation allow the manipulation of laser beams in optical data processing. Although most optically nonlinear organic materi- als are neutral molecules, various charged compounds also ex- hibit NLO effects. Hemicyanine stilbazolium salts are the best studied amongst such materials and are particularly attractive for device applications. [2] For example, trans-4¢-(dimethylami- no)-N-methyl-4-stilbazolium tosylate ([5]OTs, DAST) exhibits very pronounced bulk quadratic NLO activity [3] and has re- cently been used as the basis of prototype devices for para- metric interactions and electro-optical (EO) modulation. [4] One aspect of particular interest with organic salts is the use of counterion variations to tailor crystal packing, potentially pro- ducing non-centrosymmetric bulk structures which are essen- tial for quadratic NLO effects. Furthermore, crystalline salts Adv.Funct.Mater. 2003, 13, No. 5, May DOI: 10.100/adfm200300026 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 347 Four series of dyes with dimethylamino electron donor groups and N-R-pyridinium (R=methyl Me, phenyl Ph, 2,4-dinitrophenyl 2,4-DNPh, or 2-pyrimidyl 2-Pym) elec- tron acceptors are studied as their hexafluorophosphate salts. The intramolecular charge-transfer (ICT) energies (E max ) of these compounds decrease within each of the seriesintheorderR=Me>Ph>2,4-DNPh>2-Pym,astheelectron-acceptingabilityof the pyridinium ring increases. Hyper-Rayleigh scattering with femtosecond 1300 or 800 nm lasers yields fluores- cence-free first hyperpolarizabilities b, and static first hyperpolarizabilities b 0 [H]areobtainedviathetwo-statemod- el. Dipole moment changes Dl 12 for the ICT transitions obtained from Stark spectroscopy afford b 0 [S] values by using b 0 =3Dl 12 (l 12 ) 2 /2(E max ) 2 (l 12 =transition dipole moment). The b 0 [S] data show that the combination of pyri- dyl N-arylation with conjugation extension affords large increases in b 0 . The b 0 [H] data generally agree with this conclusion, but resonance effects may explain some apparent anomalies. X-ray structural studies on various salts revealthattheuseoftosylateanionsisnotagenerallyapplicableapproachtoengineeringnoncentrosymmetricstruc- tures of pyridinium salts. However, trans-N-phenyl-4-(4-dimethylaminophenyl-4-buta-1,3-dienyl)pyridinium hexa- fluorophosphate adopts the polar space group Cc, and shows a very large powder second harmonic generation efficiency from a 1907 nm laser, which is similar to that of the well-studied material trans-4¢-(dimethylamino)-N- methyl-4-stilbazoliumtosylate(DAST). ± [*] Dr. B. J. Coe,Dr. J. A. Harris Department of Chemistry, University of Manchester Oxford Road, Manchester M13 9PL (UK) E-mail: b.coe@man.ac.uk I. Asselberghs, K. Wostyn, Prof. K. Clays, Prof. A. Persoons [+] Laboratory of Chemical and Biological Dynamics Center for Research on Molecular Electronics and Photonics University of Leuven Celestijnenlaan 200D, B-3001 Leuven (Belgium) Dr. B. S. Brunschwig Chemistry Department, Brookhaven National Laboratory, P.O. Box 5000, Upton, New York 11973±5000 (USA) Dr. S. J. Coles,Dr. T. Gelbrich, Dr. M. E. Light, Prof. M. B. Hursthouse EPSRC X-ray Crystallography Service, Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ (UK) Prof. K. Nakatani P.P.S.M., Ecole Normale SupØrieure de Cachan URA 1906, avenue du PrØsident Wilson 94235 Cachan (France) [+] Second address: Optical Sciences Center, University of Arizona, Tucson, Arizona, AZ 85721, USA. [**] This work was supported by a studentship from the EPSRC (Dr. Harris), research grants from the Fund for Scientific Research-Flanders (FWO-V, G.0261.02), from the University of Leuven (GOA/2000/03) and from the Belgian Government (IUAP P5/3) and contract DE-AC02±98CH10886 with the U.S. Department of Energy supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. Mr Wostyn is a research assistant with the Fund for Scientific Research-Flanders (FWO-V). FEATURE ARTICLE