  The control of spins and spin to charge conversion in organics requires
understanding the molecular spin-orbit coupling (SOC), and a means to tune its
strength. However, quantifying SOC strengths indirectly through spin relaxation
effects has proven diffi- cult due to competing relaxation mechanisms. Here we
present a systematic study of the g-tensor shift in molecular semiconductors
and link it directly to the SOC strength in a series of high mobility molecular
semiconductors with strong potential for future devices. The results
demonstrate a rich variability of the molecular g-shifts with the effective
SOC, depending on subtle aspects of molecular composition and structure. We
correlate the above g -shifts to spin-lattice relaxation times over four orders
of magnitude, from 200 {\mu}s to 0.15 {\mu}s, for isolated molecules in
solution and relate our findings for isolated molecules in solution to the spin
relaxation mechanisms that are likely to be relevant in solid state systems.
