14 Photosynergetic Enhancement of Photosensitivity …
243
Scheme 14.2 Examples of terarylenes with different p-conjugation length
lene structure has thus been suggested to have additional flexibility in controlling
various photofunctionality because of expanded π-conjugation.
The additional aromatic unit in the terarylene structure was expected to stabilize
its ring-open form isomer and enhance the enthalpy increase upon ring cyclization.
Therefore, spontaneous thermal cycloreversion reaction from the ring-closed form
was supposed to be firster than the diarylethene. Since the decay lifetime of the
colored state of diarylethene is enough long (can be 10
6 years) [10, 11], diarylethenes
are recognized as “P-type” one. We have then expected tunability in the fading
lifetime of terarylenes. It was practically modulated in the range of several years
to several seconds, in which the decay kinetics was especially enhanced with fourphenylacetyl units (Scheme 14.3) [14]. Their specifically short lifetime of closed form
was attributed mostly not to the activation energy but to the apparent frequency factor
in the Arrhenius-type kinetics. The spontaneous reverse kinetics seems beneficial for
molecular-based switching systems.
The photochromic ring-cyclization of terarylenes is considered to proceed in the
conrotatory motion among the hexatriene structure as like as diarylethene. In this
sense, their ground-state structure of quasi-C2 symmetry is expected to be photoactive, which is typically called as “anti-parallel” structure for diarylethene. Indeed,
some crystalline diarylethenes which are fixed in the “anti-parallel” conformation
exhibit almost 100% of photochemical quantum yield of ring-cyclization [10, 11].
Conformational equilibrium of terarylene should be more complex than diarylethene
as its non-symmetric central hetero-aromatic units. However, those with higher
stability and larger population ratio in the quasi-C2 symmetric structure should
promote relatively high photochemical quantum yield. In the typical examples shown
in Scheme 14.4, specific intramolecular tethering interaction between specific atoms,
Scheme 14.3 Photochromic terarylenes with fast-fading dynamics
243
Scheme 14.2 Examples of terarylenes with different p-conjugation length
lene structure has thus been suggested to have additional flexibility in controlling
various photofunctionality because of expanded π-conjugation.
The additional aromatic unit in the terarylene structure was expected to stabilize
its ring-open form isomer and enhance the enthalpy increase upon ring cyclization.
Therefore, spontaneous thermal cycloreversion reaction from the ring-closed form
was supposed to be firster than the diarylethene. Since the decay lifetime of the
colored state of diarylethene is enough long (can be 10
6 years) [10, 11], diarylethenes
are recognized as “P-type” one. We have then expected tunability in the fading
lifetime of terarylenes. It was practically modulated in the range of several years
to several seconds, in which the decay kinetics was especially enhanced with fourphenylacetyl units (Scheme 14.3) [14]. Their specifically short lifetime of closed form
was attributed mostly not to the activation energy but to the apparent frequency factor
in the Arrhenius-type kinetics. The spontaneous reverse kinetics seems beneficial for
molecular-based switching systems.
The photochromic ring-cyclization of terarylenes is considered to proceed in the
conrotatory motion among the hexatriene structure as like as diarylethene. In this
sense, their ground-state structure of quasi-C2 symmetry is expected to be photoactive, which is typically called as “anti-parallel” structure for diarylethene. Indeed,
some crystalline diarylethenes which are fixed in the “anti-parallel” conformation
exhibit almost 100% of photochemical quantum yield of ring-cyclization [10, 11].
Conformational equilibrium of terarylene should be more complex than diarylethene
as its non-symmetric central hetero-aromatic units. However, those with higher
stability and larger population ratio in the quasi-C2 symmetric structure should
promote relatively high photochemical quantum yield. In the typical examples shown
in Scheme 14.4, specific intramolecular tethering interaction between specific atoms,
Scheme 14.3 Photochromic terarylenes with fast-fading dynamics