Photophysical ($,, z,, $,, t,), absorption (So * S, and TI * T,) and emis- sion (fluorescence and phosphorescence) have been evaluated for thiophene (n= 1) and oligothiophenes (n=2-5,7). In addition k,, k,, and k,,, have been evaluated for n=2-5, 7. In general 4, increases from n=2 to 5 and becomes constant (n=5-7) whereas 9, decreases overall from n=2,3 (nearly same) to constancy at n=5,7. The k,,, progressively decreases and k, remains approximately constant as n increases from n=3 to 7. The zF progressively gets longer while zT gets shorter as n increases and both become constant at n=5,7. Based on k, (or z",) and other data, it is clear that the 'B-type state is always below the 'A-type state fron n=2 to and including n=7, in contrast to other existing predictions (for n=6 and greater). Although fluo- rescence exists for all compounds, except thiophene, no phosphorescence has been observed (I$~ s 5 x except thiophene. Mechanisms rationalizing the photophysical behavior are given.

Quantum yields for triplet formation have been determined for seven common π-conjugated polymers in benzene solution using time-resolved photoacoustic calorimetry (PAC) in conjunction with fluorescence quantum yields, singlet and triplet energies. The polymers studied include three poly(thiophenes), poly(2-methoxy,5-(2^′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), a cyano derivative of MEH-PPV, a ladder type poly(p-phenylene) (MeLPPP), and a poly(fluorene). Yields of singlet oxygen formation have also been determined for these polymers in benzene by time-resolved phosphorimetry, and are in reasonable agreement with triplet yields obtained by PAC. Polythiophenes show the highest intersystem crossing yields, which are suggested to result from extensive spin-orbit coupling. Where singlet oxygen yields are less than triplet yields, it is suggested that interaction of molecular oxygen with the ground state of the polymers may be involved.