In this article we present a short review on low temperature scanning tunneling microscopy (LT-STM) investigations of specially designed molecules, performed with the help of manipulation techniques. The possibilities offered by LT-STM to build structures at the atomic scale are here extended to the manipulation of complex molecules. Single porphyrin-based molecules deposited on copper substrates are investigated and manipulated by means of their interaction with the STM tip. In a constant height mode of manipulation, the tunneling current recorded during the process contains detailed information on the internal mechanics of the molecule when guided by the STM tip. Moreover, recording images in constant height mode allows to investigate the interaction between tip and molecule with high spatial resolution. Finally we show that the manipulation of the molecules on Cu(211) offers the possibility to selective modify their internal conformation realizing the principle of a conformational molecular switch.