Conjugated Oligomers with Stable Radical Substituents: Synthesis, Single Crystal Structures, Electronic Structure, and Excited State Dynamics

Understanding and controlling spin dynamics in organic semiconductors is of significant technological interest. We present a comprehensive joint experimental and computational study elucidating excited-state dynamics and kinetics of oligothiophenes covalently linked to two radicals. The synthesis, steady-state, and ultrafast photophysics, magnetic properties, computational modeling, and single crystal X-ray diffraction of a series of oligothiophenes with appended nitronyl nitroxide (NN) diradicals (RAx and RBx) are presented. We show that incorporation of the diradicals results in an intriguing molecular packing that is reminiscent of organic cages, unusual excited-state dynamics, and interesting photophysical and magnetic properties. We find an increase in the distance and dihedral angle between the diradical rings and the oligothiophene core result in weak antiferromagnetic interactions. Single crystal X-ray diffraction and computational modeling suggest that efficient conjugation along the backbone leads to an efficient spin-polarization transfer. Insertion of p-phenylene linkers that separate the oligothiophene core from the NN radical component by an average of only 4.3 Å results in a decrease in orbital overlap between the chromophore and singly occupied molecular orbital of the two NN radicals and a weak spin polarization along the thiophene core. Computations also predict a biradical ground state with a small singlet–triplet energy gap (ΔEST) of 0.6 kcal/mol or less, where the triplet lies above the singlet, suggesting that in some of these molecules both the singlet and triplet states are thermally populated. Together, the steady-state optical absorption, computational study, and ultrafast transient absorption suggest enhanced internal conversion is the dominant pathway for rapid decay in RAx and RBx diradical series due to two major factors: (i) incorporation of the radicals results in new low-lying singlet and triplet states (S1/T1) that act as “trap states”; and (ii) generation of multiple singlet and triplet states that are essentially degenerate in energy. Since incorporation of NN diradicals leads to more than 20 low-lying near degenerate singlet, triplet and quintet states, both intersystem crossing and internal conversion become viable decay mechanisms for the decay of the Sn and Tn states back to the S0 and T0. These results establish and correlate structural and electronic parameters that impact spin coupling, spin delocalization, and determine general trends in predicting energy levels of excited states.