He pleaded guilty in April to using an interstate facility to transmit information about a minor.
Dempsey apologized to the girl's mother during the sentencing. He said he never intended to meet the girl.
Authorities say Dempsey exchanged sexually explicit instant messages with the girl in late 2007. An Avon police officer later took the girl's place.
Dempsey said during a previous court appearance that the situation was a nightmare of his own making and wished he would wake up.
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Information from: The Hartford Courant, http://www.courant.com