The present invention provides compounds of Formula I:wherein: R1 is a label s; L is present or absent and when present is a linking group; and x represents an integer from 1 to 10; or a pharmaceutically acceptable salt thereof. Four-terminal three-layer BTRANs provide substantially identical operation in either direction with forward voltages of less than a diode drop. Adjacent ring segments are linked by differently long connectors with U-shaped compensating sections.