Polychromatic Photoluminescence of Polymorph Boron Dipyrromethene Crystals and Heterostructures

Micrometer-sized boron dipyrromethene crystalline rods were grown from solution. Fluorescence microscopy images reveal that each rod displays characteristic visible light emission of a different color. In a particular case, optical heterostructures with discrete, differently colored sections are observed within a single microrod. Microphotoluminescence (μ-PL) spectra of green and red rods at room temperature show multiple contributions, indicating the presence of microdomains. Temperature-dependent μ-PL measurements further confirm this, as red emission decreases and green emission increases at lower temperatures. These observations are discussed as a result of crystalline polymorphism, leading to a local variation of the highest occupied molecular orbital–lowest unoccupied molecular orbital energy difference. An Arrhenius plot quantifies the hopping barrier for the charge carriers to reach the low emission energy (red) regions. A line scan of a single rod further supports that microdomains of green- and orange–red-emitting crystal phases are present in a single rod. Time-resolved microwave conductivity studies clarify that microdomain-free green rods display 2 orders of magnitude longer photocarrier lifetime and 5-fold higher photoconductivity than the red rods with many small band-gap regions.