Length-Dependent Direction-Tunable Charge-Transfer Behavior of Second-Order Optical Nonlinearity in Keggin-Type Organosilicone Derivative [PW11O39(RSi)2O]3−: A TDDFT Study

The second-order nonlinear optical (NLO) properties of Keggin-type organosilicone derivatives [PW11O39(RSi)2O]3− are investigated by time-dependent density functional theory (TDDFT). Our results indicated that the length of conjugated chain R has a crucial effect on the charge transfer (CT). The direction of CT alters with the end-cap-substituted electron donating or accepting moieties (NH2 or NO2), until the chain length reach a certain length, as the CT originates from the heteropolyanion to organosilicone moiety along the chain, further chain lengthening leaves this behavior invariant. The derivatives with long chain substitution and end-cap-substituted electron acceptor (NO2) display excellent second-order NLO responses. The system 18 with the relevant long conjugated polyphenylethynyl chain and end-cap-substituted electron acceptor (NO2) has the largest β0 value, 4538.1 × 10−30 esu. The present investigation provides important insight into the characteristic of CT and NLO properties of Keggin-type organosilicone derivatives.