Cyclometallated iridium(III) complexes with substituted 1,10-phenanthrolines: a new class of highly active organometallic second order NLO-phores with excellent transparency with respect to second harmonic emission

[Ir( ppy)(2)( 5- R- 1,10- phen)][ PF6] ( ppy = cyclometallated 2- phenylpyridine, phen = phenanthroline, R = H, Me, NMe2, NO2) and [ Ir( ppy)(2)( 4- R',7- R'- 1,10- phen)][ PF6] ( R' = Me, Ph) complexes are characterized by one of the highest second order NLO response ( measured by the EFISH technique) reported for a metal complex, mainly due ( as suggested by a theoretical SOS- TDDFT investigation) to MLCT processes from the ppy Ir based moiety acting as donor push system to pi* orbitals of phen, acting as an acceptor pull system; the good transparency to the second harmonic emission renders these NLO- phores appealing as building blocks for molecular materials with second harmonic generation.