An investigation on the second-order NLO properties of novel cationic cyclometallated Ir(III) complexes of the type [Ir(2-phenylpyridine) 2 (9-R-4,5-diazafluorene)] + (R = H, fulleridene) and the related neutral complex with the new 9-fulleriden-4-monoazafluorene ligand Claudia Dragonetti a,b,⇑ , Adriana Valore b,c , Alessia Colombo a,b , Stefania Righetto a,b , Giovanni Rampinini d , Francesca Colombo e , Luca Rocchigiani f , Alceo Macchioni f a Dip. di Chimica Inorganica, Metallorganica e Analitica ‘‘Lamberto Malatesta’’, dell’Università degli Studi di Milano, Italy b UdR di Milano dell’INSTM, via Venezian 21, I-20133 Milano, Italy c Istituto di Scienze e Tecnologie Molecolari del CNR, Italy d Dip. di Chimica Organica e Industriale, dell’Università degli Studi di Milano, via Venezian 21, I-20133 Milano, Italy e Dip. di Scienze Chimiche ed Ambientali, dell’Università dell’Insubria, Via Valleggio 11, I-22100 Como, Italy f Dip. di Chimica, Università di Perugia, via Elce di Sotto 8, I-06123 Perugia, Italy article info Article history: Received 28 June 2011 Received in revised form 29 September 2011 Accepted 5 October 2011 Available online 14 October 2011 Keywords: Cyclometallated Iridium(III) complexes Diazafluorene Monoazafluorene Fullerene Nonlinear optical properties Diffusion NMR abstract Novel cationic cyclometallated Ir(III) complexes bearing diaza- or monoazafluorene substituted or not with a C60-fullerene moiety have been synthesized together with a novel interesting ligand 9-fuller- iden-4-monoazafluorene and the related Ir(III) neutral complex [Ir(ppy) 2 (9-fulleriden-4-monoazafluo- rene)]. All complexes show a negative large value of lb 1.907 (600 to 2190  10 48 esu), as determined by the Electric Field Induced Second Harmonic generation (EFISH) technique working at 1.907 lm in 10 3 M CH 2 Cl 2 solution. The NLO response increases upon introduction of the fullerene moi- ety, due to a decrease of the interaction forces between the anion and cation within the ion pair which leads to an increase of the percentage of free ions, as evidenced by diffusion NMR experiments. Besides, it appeared that for the neutral complex [Ir(ppy) 2 (9-fulleriden-4-monoazafluorene)] the EFISH lb 1.907 value is lower than that of cationic Ir(III) complexes, but comparable or slightly lower to the values of other neutral Ir(III) complexes such as [Ir(ppy) 2 (RCOCR 0 COR)] (ppy = cyclometallated 2-phenylpyridine; R = Me, Ph; R 0 = H, 2,4-dinitrophenyl), confirming that the second-order NLO response of these neutral chromophores is dominated by ILCT processes concerning cyclometallated 2-phenylpyridines. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction In the last two decades, organometallic and coordination com- plexes have stood out as new intriguing molecular chromophores with second-order nonlinear optical (NLO) properties, since they can offer, compared to traditional organic second-order NLO chro- mophores, additional electronic properties acting on the NLO response such as charge-transfer transitions between the metal and the ligands, usually at low energy and of relatively high inten- sity, tunable by virtue of the nature, oxidation state and coordina- tion sphere of the metal center [1]. For instance, coordination to a metal of second order push–pull NLO chromophores such as stilbazoles, bipyridines,phenanthrolines and terpyridines bearing a NR 2 donor group produces a significant increase of their NLO response, due to a red-shift of the intraligand charge-transfer transition (ILCT) induced by the metal acting as a Lewis acceptor. On the contrary when the NLO chromophores bear a strong electron acceptor group, the NLO response is mainly con- trolled by metal to ligand charge-transfer transitions (MLCT) [1]. Few years ago, various cationic cyclometallated Ir(III) com- plexes with a phenanthroline ligand [2] have shown interesting photoemissive properties. Density Functional Theory (DFT) investi- gations on this class of luminescent Ir(III) complexes [2] have shown that the HOMO is mainly an antibonding combination of Ir (t 2g ) orbitals and p orbitals of the cyclometallated ligand while the LUMO is a p / antibonding orbital of the chelated phenanthro- line. Therefore transitions between HOMO and LUMO orbitals have a significant charge-transfer character and these Ir(III) complexes could behave as second order NLO chromophores with the cyclometallated ligands acting as donors towards the acceptor p / system of he phenanthroline ligand. Some of us have confirmed 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.10.018 ⇑ Corresponding author at: Dip. di Chimica Inorganica, Metallorganica e Analitica ‘‘Lamberto Malatesta’’, dell’Università degli Studi di Milano, Italy. Tel.: +39 02 50314358; fax: +39 02 50314405. E-mail address: claudia.dragonetti@unimi.it (C. Dragonetti). Inorganica Chimica Acta 382 (2012) 72–78 Contents lists available at SciVerse ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica