Fluorescence yields and molecular orientation of thin organic films: Vapor-Deposited Oligothiophenes α3T-α8T.

The fluorescence quantum yields of vapor-deposited (VD) films of α-oligothiophenes,nT, with ring numbers ofn=3-8 and layer thicknesses ofd=3-50 nm were determined at room temperature andT=77 K and compared to the yields of dilute solutions and small (5T)x clusters. The yields of highly oriented ultrathin films are of the order of ΦF=5*10(-5)-1*10(-4). The yields increase strongly with the layer thickness and also upon cooling, but do not reach the values in dilute solution. The main nonradiative deactivation step S1 → T1 in solution was quantified by(1)O2 production, the yields of which systematically decrease withn from ΦF (3T) to 0.36 (6T), in contrast to the fluorescence yields, which increase from ΦF=0.01 (2T) to 0.40 (6T). In films or clusters the S1 → T1 deactivation step must be a very unimportant side reaction: neither(1)O2 nor any signal of triplet-triplet absorption could be positively identified.