  We report a detailed ab-initio study of the of the microscopic degradation
mechanism of FIrpic, a popular blue emitter in OLED devices. We simulate the
\emph{operando} conditions of FIrpic by adding an electron-hole pair (exciton)
to the system. We perform both static calculations with the TDDFT framework and
we also simulate the evolution of the system at finite temperature via
Car-Parrinello molecular dynamics. We found triplet excitons are very effective
in reducing the Ir-N bond breaking barrier of the picolinate moiety. After the
first bond breaking, the two oxygen of picolinate swap their position and
FIrpic can either remain stable in an "open" configuration, or loose a
picolinate fragment, which at a later stage can evolve a CO$_2$ molecule. Our
method can be applied to other light emitting Ir-complexes in order to quickly
estimate their stability in OLED devices. In Paper~II we complement our
theoretical study with a parallel experimental investigation of the key
degradation steps of FIrpic in an aged device.
