Energetic insights on two dye key molecules: N -methylphenothiazine and N -methylphenoxazine

•Determination of the temperature-vapour pressure dependence by mass-loss Knudsen effusion method of N-methylphenothiazine.•Measurement of the massic energy of combustion, in oxygen, of the crystalline N-methylphenothiazine.•Gas-phase enthalpies of formation for N-methylphenothiazine and N-methylphenoxazine.•Correlation for assessment of gas-phase enthalpy of formation of cyclic amino derivatives.•Gas-phase enthalpies of formation estimated from G3(MP2)//B3LYP approach.