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J Fluoresc
All electronic structure computations were carried out using
the Gaussian 09 program [39] running on GridChem [40].
excited state. The change in bond distance and bond angle in
DMF leading to the formation of excited state keto which
contributes to the major red shifted emission peak is represent-
ed in Table 2. Mulliken charges on N31, H34, and O33 of the
ground and excited state optimized geometries of the enol
form as well as the excited state geometry of keto form are
given in Table S1 (supporting data) respectively. Molecular
planarity of R1 rotamer plays a significant role in ESIPT fa-
cilitating the proton transfer in the excited state from O33 to
N31. In the case of DMF N31 to H34 hydrogen bonding
distance decreases by 0.089 Å from Enol (1.793 Å) to Enol*
(1.704 Å) while the distance between H34 and O33 was in-
creased by 0.019 Å from Enol to Enol*, respectively. This
suggests that in excited state (Enol*) the H34 approaches near
to the N31 via hydrogen bonding for transfer of proton to
enable ESIPT. The bond distance C4-C20 decreases from
Enol (1.449 Å) to Enol* (1.415 Å) by 0.034 Å while O33-
C5 bond distance decreases by 0.023 Å from Enol (1.354 Å)
to Enol* (1.331 Å), which is further decreased by 0.062 Å in
case of Keto* (1.269 Å). In addition to this, the bond angle
H34-O33-C5 increases by 0.5° from Enol (108.5°) to Enol*
(109.0°). In this way, H34 approaches nearer to N31 in Enol*
and further transfer to N31 leads to Keto* with N31-H34 bond
distance 1.025 Å and O33-H34 hydrogen bonding distance of
1.911 Å. The Keto* returns to ground state Keto conformer
with N31-H34 bond distance of 1.045 Å and O33-H34 hydro-
gen bonding distance of 1.718 Å. As H34 approaches to O33
the Keto* hydrogen bonding distance decreases by 0.193 Å
from 1.911 to 1.718 and it immediately converts to the Keto
conformer and then to the Enol.
Result and Discussion
Compound 4
2,5-Bis(benzo[d]oxazol-2-yl)benzene-1,4-diol contains two
hydroxyl groups at one and four position with respect to
benzoxazole ring has two acidic protons. These acidic protons
are in close vicinity of the two benzoxazole rings which leads
to the possibility of various interconvertable conformational
isomers arising from rotamerism and tautomerism. The possi-
ble conformers of the compound 4 with their respective keto
tautomers were optimized by using B3LYP functional and 6-
31G (d) basis set to determine the most stable low energy
conformer. All the possible conformers of the compound 4
in enol-keto form are represented in Fig. 1. These conformers
can co-exist with each other and with their tautomers in solu-
tion depending upon their relative energy difference. Relative
energy difference of the different planar conformers and their
tautomeric structures (Fig. 1) optimized in vapour phase by
using B3LYP/6-31G (d) is given in Table 1. Enol–Keto tau-
tomer of rotamer R1 where both hydroxyl groups are in close
vicinity of N = C bond where acidic proton can have intermo-
lecular hydrogen bond with basic nitrogen atom (OH…N = C)
is thermodynamically most stable according to B3LYP/6-31G
(d) calculations in all polar and non-polar solvents. DFT cal-
culation showed that in all the solvents keto form is less stable
than the enol form due to higher energy which indicates keto is
not a preferred geometry at ground state.
Photo-Physical Data for Compound 4
The thermodynamically most stable optimized structure of
Compound 4 in its enol- keto form is illustrated in Fig. 2. The
rotamer R1 on photo-excitation shows redistribution of charge
densities which gives rise to different geometries in the
Photo-physical interaction of the compound 4 is represented
in Table 3. Compound 4 is sensitive to solvent polarity which
absorbs at 419 nm in n-hexane shows very slight
hypsochromic shift with increase in solvent polarity and ab-
sorbs at 407 nm in DMF. In n-hexane it shows five absorption
maxima at 419 nm, 389 nm, 333 nm, 320 nm and 304 nm
which are in good agreement with the reported values 414 nm,
392 nm, 335 nm, 319 nm and 305 nm [26]. There is a good
agreement with theoretical vertical excitation values obtained
from TD-DFT/B3LYP 6-31G (d) calculations. A similar trend
is observed in case of other non-polar and polar solvents. The
largest difference in wavelength between the computed and
experimental absorption maxima was 7 nm in heptane with
2.3 % deviation. In all the solvents, the prominent intense
absorption can be assigned to HOMO to LUMO, HOMO to
LUMO +1 and HOMO to LUMO +2 transitions of R1-enol
rotamer. The frontier molecular orbital (FMO) 89 is HOMO
and FMO 90 is LUMO in all the cases. The two benzoxazole
rings act as an acceptor which upon excitation undergo
Table 1 Relative energies (kJ/mol) of the different planar conformers
of compound 4 and their tautomeric structures in the gas phase
Rotamer
Compound 4 (Enol)
(kJ/mol)
Compound 4 (Keto)
(kJ/mol)
R1
R2
R3
R4
R5
R6
R7
R8
R9
0.00
25.71
50.55
49.09
96.46
91.32
86.05
46.66
91.32
0.00
26.24
97.98
0.00
45.88
120.05
119.81
44.78
44.78