COMPREHENSIVE INVESTIGATION OF THE SOLUTION PHOTOPHYSICS AND THEORETICAL ASPECTS OF OLIGOTHIOPHENES OF 1-7 RINGS

Photophysical (phi(F), tau(F), phi(T), tau(T)), absorption S(O) --> S(X) and T(I) --> T(x)) and emission (fluorescence and phosphorescence) have been evaluated for thiophene (n=1) and oligothiophenes (n=2-5, 7). In addition k(F), k(IC) and k(ISC) have been evaluated for n=2-5, 7. In general phi(F) increases from n=2 to 5 and becomes constant (n=5-7) whereas phi(T) decreases overall from n=2,3 (nearly same) to constancy at n=5, 7. The k(ISC) progressively decreases and k(F) remains approximately constant as n increases from n=3 to 7. The tau(F) progressively gets longer while tau(T) gets shorter as n increases and both become constant at n=5, 7. Based on k(F) (or tau-degrees(F) and other data, it is clear that the 1B-type state is always below the 1A-type state fron n=2 to and including n=7, in contrast to other existing predictions (for n=6 and greater). Although fluorescence exists for all compounds, except thiophene, no phosphorescence has been observed (phi(P) less-than-or-equal-to 5 x 10(-4)), except thiophene. Mechanisms rationalizing the photophysical behavior are given.