# Create a Pandas Excel writer using XlsxWriter as the engine.
with pd.ExcelWriter('pandas_multiple.xlsx', engine='xlsxwriter') as writer:    
    # Write each dataframe to a different worksheet.
    final_df.to_excel(writer, sheet_name='Sheet1')
    df_unigrams.to_excel(writer, sheet_name='Sheet2')
    df_bigrams.to_excel(writer, sheet_name='Sheet3')