278
J Fluoresc (2012) 22:269–279
Acknowledgment One of the authors Dr. J. Jayabharathi, Associate
professor, Department of Chemistry, Annamalai University is thankful
to Department of Science and Technology [No. SR/S1/IC-07/2007]
and University Grants commission (F. No. 36-21/2008 (SR)) for
providing funding to this research work.
It is well known that the flexibility/rigidity of the picrate
derivatives can control the mechanics of internal conversion
and it can be analyzed in terms of the planarity of the π-
electron system [25]. This loss of planarity in the excited
state implies a less rigid structure and the excitation energy
is more easily converted to vibrational energy and dissipat-
ed as heat, enhancing the internal conversion processes.
Drexhage et al., [26] pointed out that the structure loosens
up upon excitation and enhancing the non-radiative
deactivation processes. In these picrate derivatives (1–4),
the presence of increased electrostatic interaction between
the N-protonated piperidone ring and the picryl anion ring
so that the picryl anion ring lies perpendicular to the plane
of the piperidone ring i.e., non-coplanarity. Such a
distortion from planarity leads to the non-radiative deacti-
vation. Pavlopoulos et al., [3] also referred to the non-
planarity of the chromophore provided by bulky substitu-
ents to explain the low fluorescence quantum yield.
The present photophysical characteristics of these picrate
derivatives (1–4) would suggest a lower optical efficiency
because of its lower fluorescence quantum yield. However,
high optical densities are required to produce signals and
under these conditions, the losses at the resonator cavity by
the reabsorption and reemission effects can be important.
These picrate derivatives are characterized by a higher
Stokes shift which would reduce the losses at the resonator
cavity by reabsorption and reemission effects, governed by
the overlap between the absorption and emission spectra.
Therefore, the lower fluorescence quantum yield to some
extent, be compensated by its higher Stokes shift and high
optic efficiencies for these picrate derivatives [27]. Because
the higher Stokes shift and highest fluorescence quantum
yield of the picrate derivatives are recommended to achieve
the highest optic efficiencies in liquid media.
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Conclusions
The presence of increased electrostatic interaction between
the N-protonated piperidone ring and picryl anion ring,
makes the picrate anion ring lie perpendicular to the plane
of the piperidone ring i.e., non-coplanarity. The picrate
chromophore core originates a distortion from planarity in
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an increase in the rate constant of non-radiative deactivation
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