Defects Cause Sub-gap Luminescence in a Crystalline Tetracene Derivative

We use steady-state and ultrafast nonlinear spectroscopies in combination with ab initio electronic structure calculations to explain light emission below the optical gap of crystalline samples of 5,12-diphenyl tetracene (DPT). In particular, the properties of vibrational coherences imprinted on a probe pulse transmitted through a DPT single crystal indicate discrete electronic transitions below the band-gap energy of this organic semiconductor. Analysis of coherence spectra and vibrational assignments derived from calculations motivate us to conclude that electronic states due to structural defects give rise to these discrete transitions and sub-gap light emission. These results provide fundamental insights into the assignment and properties of mid-gap states in organic materials important for their application in next generation photonics and opto-electronics technologies.