Caffeine boiling and melting points
Anhydrous caffeine occurs in two different polymorphic crystal forms, with a transition between the two forms occurring at $\pu{136 °C}.$ The high temperature form has a melting point of $\pu{236 °C}.$
The vapor pressure of the high temperature solid form is described by the equation:
$$\log P[\pu{Pa}] = -\frac{5477}{T[\pu{K}]} + 14.395,\tag{1}$$
and for the liquid
$$\log P[\pu{Pa}] = -\frac{3918}{T[\pu{K}]} + 11.14.\tag{2}$$
So, $\pu{173 °C}$ is incorrect; the boiling point is more like $\pu{365 °C}.$
References

Griesser, U. J.; Szelagiewicz, M.; Hofmeier, U. Ch.; Pitt, C.; Cianferani, S. Vapor Pressure and Heat of Sublimation of Crystal Polymorphs. Journal of Thermal Analysis and Calorimetry 1999, 57 (1), 45–60. DOI: 10.1023/A:1010188923713.
Bothe, H.; Cammenga, H. K. Phase Transitions and Thermodynamic Properties of Anhydrous Caffeine. Journal of Thermal Analysis 1979, 16 (2), 267–275. DOI: 10.1007/BF01910688.