Writing expression syntax for Calculate Field in ArcPy?

I'm stuck on my CalculateField function with how to correctly type the syntax.

I'm not particularly tied to the variables I assigned up front.

The issue is syntax for the expression.

I am currently using ArcGIS 10.3 and Python 2.7.8.

This is the line that is getting me:

arcpy.CalculateField_management(fc, "Cohort", expression="""openYear + " to " + closeYear""")

How should I be writing the expression?

expression="'str(openYear)' + " to " + 'str(closeYear)'"

The rest is a sample of my loop statement.

#################################
for fc in fcList:
    openYear=fc[12:16]
    closeYear=fc[20:24]
    # Add "Cohort" field to *USA feature classes in scratch.mdb
    print fc
    arcpy.AddField_management(fc, "Cohort", "TEXT")
    print "Cohort added."
    # Calculate "Cohort" field to match feature class name in scratch.mdb  
    arcpy.CalculateField_management(fc, "Cohort", expression="""openYear + " to " + closeYear""")