282
T. Mutai
of the observed emission enhancement. Suppression of the radiationless decay via the
S 0 /S 1 -CI allows for emissive decay from the S 1 -IPT to the S 0 -IPT on a nanosecond
timescale.
14.3.8 Polymorph-Dependent Luminescence Color of HPIP
Crystals
From Fig. 14.5, it is apparent that the energy gap between the S 1 -IPT and S 0 -IPT
states depends greatly on the torsion angle. Unless the energy gap becomes too small
to induce efficient radiationless decay, or the torsion angle of the enol species is too
large to block the ESIPT process, an increase in the torsion angle is expected to
induce a redshift in the ESIPT luminescence. Notably, the potential energy of S 1 -
IPT has a small local minimum at around 30°, and the energy gap does not decrease
monotonically as a function of torsion angle.
As shown in Fig. 14.3, there is a slight difference in the torsion angles in the
two crystals: molecules in 1-BG have a torsion angle of 5.8° and those of the two
conformers, Y1 and Y2, in 1-Y are 1.3° and –1.0°. Because the differences in the
emission energies and torsion angles in 1-BG and 1-Y crystals are small, higher-level
ab initio calculations, MS-CASPT2(10,9)/ANO-L methods, were applied in order
to examine the effect of torsional conformation on the emission energy. Although
the results suggested a slightly lower energy for Y1 in 1-Y, there was no difference
between 1-BG and Y2. Because the estimation is based only on the conformational difference in a single molecule, it may be necessary to include the effects
of surrounding molecules, as well as the degree of conformational freedom in the
crystal [86–88].
14.4 Conclusions
Owing to the ESIPT mechanism, which generates environment-sensitive zwitterionic
excited species, HPIP 1 shows distinct PDL, i.e., blue-green (1-BG) and yellow
(1-Y). Moreover, reproducible switching of the PDL was achieved by heat-mode
control of the molecular packing, which was reasonably supported by power XRD
analysis and DSC measurements. Ab initio calculations suggested that a free S 1 -IPT
species could approach the S 0 /S 1 –CI coupled with the twisting motion of the central
C–C bond, and return to the S 0 -enol state without emitting light.
The enhanced emission of HPIP in the solid state can be explained by the following
factors: (1) blocking S 1 -IPT species (1 IPT *) from the twisting motion that reach the
CI and efficient radiationless decay, and (2) inhibition of excimer formation, because
of the unmatched energy state between 1 IPT * and surrounding 1 Enol molecules.
T. Mutai
of the observed emission enhancement. Suppression of the radiationless decay via the
S 0 /S 1 -CI allows for emissive decay from the S 1 -IPT to the S 0 -IPT on a nanosecond
timescale.
14.3.8 Polymorph-Dependent Luminescence Color of HPIP
Crystals
From Fig. 14.5, it is apparent that the energy gap between the S 1 -IPT and S 0 -IPT
states depends greatly on the torsion angle. Unless the energy gap becomes too small
to induce efficient radiationless decay, or the torsion angle of the enol species is too
large to block the ESIPT process, an increase in the torsion angle is expected to
induce a redshift in the ESIPT luminescence. Notably, the potential energy of S 1 -
IPT has a small local minimum at around 30°, and the energy gap does not decrease
monotonically as a function of torsion angle.
As shown in Fig. 14.3, there is a slight difference in the torsion angles in the
two crystals: molecules in 1-BG have a torsion angle of 5.8° and those of the two
conformers, Y1 and Y2, in 1-Y are 1.3° and –1.0°. Because the differences in the
emission energies and torsion angles in 1-BG and 1-Y crystals are small, higher-level
ab initio calculations, MS-CASPT2(10,9)/ANO-L methods, were applied in order
to examine the effect of torsional conformation on the emission energy. Although
the results suggested a slightly lower energy for Y1 in 1-Y, there was no difference
between 1-BG and Y2. Because the estimation is based only on the conformational difference in a single molecule, it may be necessary to include the effects
of surrounding molecules, as well as the degree of conformational freedom in the
crystal [86–88].
14.4 Conclusions
Owing to the ESIPT mechanism, which generates environment-sensitive zwitterionic
excited species, HPIP 1 shows distinct PDL, i.e., blue-green (1-BG) and yellow
(1-Y). Moreover, reproducible switching of the PDL was achieved by heat-mode
control of the molecular packing, which was reasonably supported by power XRD
analysis and DSC measurements. Ab initio calculations suggested that a free S 1 -IPT
species could approach the S 0 /S 1 –CI coupled with the twisting motion of the central
C–C bond, and return to the S 0 -enol state without emitting light.
The enhanced emission of HPIP in the solid state can be explained by the following
factors: (1) blocking S 1 -IPT species (1 IPT *) from the twisting motion that reach the
CI and efficient radiationless decay, and (2) inhibition of excimer formation, because
of the unmatched energy state between 1 IPT * and surrounding 1 Enol molecules.