Understanding the ultrafast excited state dynamics in organic semiconductors after optical
excitation is a key requisite on the road towards efficient organic solar cells. Additionally,
the creation of functional interfaces built from organic molecular switches
and the read-out of the photochromic state are essential for molecular electronics. In
this thesis, static second harmonic generation (SHG) measurements were utilized to
investigate the photochromism of different indolylfulgimide derivatives immobilized on
silicon. During this, the influence of chemical modifications on the switching efficiencies
(cross-sections) and the non-linear optical contrast between the switching states
were investigated. In the second part of this thesis, femtosecond time-resolved second
harmonic generation measurements were used to investigate the ultrafast decay mechanism
of optically induced electronically excited states in organic semiconductors and
donor/acceptor systems. These led to observations of relaxation into dimer induced
states, charge trapping at native silicon oxide and ultrafast vibronic relaxation. For the
donor/acceptor configurations, depending on the molecular orientation at the interface
and the excitation energy, the creation of charge transfer states were investigated
