Second-order nonlinear optical responses of carboranyl-substituted indole/indoline derivatives: impact of different substituents.

•The βtot value can be tuned by introducing substituent groups.•Substituent effect on the NLO response is different for closed/open-ring forms.•The difference on NLO response is due to the change in charge transfer pattern.•Carborane has little contribution to the NLO responses for closed-ring forms.