280
T. Mutai
Based on the above findings, we examined the repetitive-switching ability of
the PDL of 1. [Yellow → blue-green] Powdered crystals of 1-Y were kept at 135
°C for 1 min, in order to convert them to the blue-green-emitting state. [Bluegreen → yellow] Next, they were heated at 150 °C for 3 min and rapidly cooled, to
generate the yellow-emitting state. Several color-switching cycles were successfully
carried out, demonstrating that the color of the ESIPT luminescence of 1 could be
interconverted via the heat-mode dry process.
14.3.5 Quantum Chemical Analysis
The luminescence property of 1 was further analyzed quantum chemically [78]. The
ground-state geometries (S 0 ) of 1 were optimized using density functional theory
(DFT) with the 6-31G(d,p) basis set in conjunction with the B3LYP functional. For
the optimized S 0 geometries, the vertical excitation energies were evaluated with
time-dependent (TD) DFT at the B3LYP/6-31+G(d,p) level of theory [79]. As a
reference, the equation-of-motion with coupled-cluster singles and doubles (EOMCCSD) method was performed to estimate the transition energies using the cc-pVDZ
basis set [79]. The viability of the DFT calculations was verified by several higherlevel methods. The quantitative vertical excitation energies including dynamical electron correlations were estimated by multi-state complete active-space second-order
perturbation theory (MS-CASPT2) [80]. The singlet excited-state (S 1 ) geometries
and energy profile as a function of torsion angle (θ ) were evaluated by the complete
active-space self-consistent field method (CASSCF) using the atomic natural orbital
small basis set (CASSCF(6,6)/ANO-S) [80]. The fully relaxed S 1 potential energy
surface (PES) was examined at the CASSCF(6,6)/ANO-S-MB (minimal base) level
of theory.
14.3.6 S 0 and S 1 PES Scan Along the Central C-C Bond
Torsion
In the ground state, the enol form (S 0 -enol) was energetically stable than the IPT form
(S 0 -IPT) at all torsional angles (0–90°). The lowest-energy geometry in both states
was the coplanar conformation, which could be due to the intramolecular hydrogen
bond. The coplanar S 1 -IPT form was more stable (32.5 kJ mol
−1 ) than the S 1 -enol
form, indicating that ESIPT could undergo from the enol form. The observed decay
times of the ESIPT luminescence were 0.52 and 5.26 ns in fluid and frozen THF
solutions, respectively. Because ESIPT is an extremely fast process (<1 ps) [81–83],
the conversion from S 1 -enol to S 1 -IPT proceeds instantaneously.
The effects of torsion angle on the energy level of the S 1 - and S 0 -IPT state were
evaluated. As shown in Fig. 14.5, an increase in the torsion angle from 0 to 45°
T. Mutai
Based on the above findings, we examined the repetitive-switching ability of
the PDL of 1. [Yellow → blue-green] Powdered crystals of 1-Y were kept at 135
°C for 1 min, in order to convert them to the blue-green-emitting state. [Bluegreen → yellow] Next, they were heated at 150 °C for 3 min and rapidly cooled, to
generate the yellow-emitting state. Several color-switching cycles were successfully
carried out, demonstrating that the color of the ESIPT luminescence of 1 could be
interconverted via the heat-mode dry process.
14.3.5 Quantum Chemical Analysis
The luminescence property of 1 was further analyzed quantum chemically [78]. The
ground-state geometries (S 0 ) of 1 were optimized using density functional theory
(DFT) with the 6-31G(d,p) basis set in conjunction with the B3LYP functional. For
the optimized S 0 geometries, the vertical excitation energies were evaluated with
time-dependent (TD) DFT at the B3LYP/6-31+G(d,p) level of theory [79]. As a
reference, the equation-of-motion with coupled-cluster singles and doubles (EOMCCSD) method was performed to estimate the transition energies using the cc-pVDZ
basis set [79]. The viability of the DFT calculations was verified by several higherlevel methods. The quantitative vertical excitation energies including dynamical electron correlations were estimated by multi-state complete active-space second-order
perturbation theory (MS-CASPT2) [80]. The singlet excited-state (S 1 ) geometries
and energy profile as a function of torsion angle (θ ) were evaluated by the complete
active-space self-consistent field method (CASSCF) using the atomic natural orbital
small basis set (CASSCF(6,6)/ANO-S) [80]. The fully relaxed S 1 potential energy
surface (PES) was examined at the CASSCF(6,6)/ANO-S-MB (minimal base) level
of theory.
14.3.6 S 0 and S 1 PES Scan Along the Central C-C Bond
Torsion
In the ground state, the enol form (S 0 -enol) was energetically stable than the IPT form
(S 0 -IPT) at all torsional angles (0–90°). The lowest-energy geometry in both states
was the coplanar conformation, which could be due to the intramolecular hydrogen
bond. The coplanar S 1 -IPT form was more stable (32.5 kJ mol
−1 ) than the S 1 -enol
form, indicating that ESIPT could undergo from the enol form. The observed decay
times of the ESIPT luminescence were 0.52 and 5.26 ns in fluid and frozen THF
solutions, respectively. Because ESIPT is an extremely fast process (<1 ps) [81–83],
the conversion from S 1 -enol to S 1 -IPT proceeds instantaneously.
The effects of torsion angle on the energy level of the S 1 - and S 0 -IPT state were
evaluated. As shown in Fig. 14.5, an increase in the torsion angle from 0 to 45°