High Stokes shift long-wavelength energy gap regulated fluorescence in the series of nitro/dimethylamino-substituted ortho-analogs of POPOP

Article abstract of DOI:10.2478/s11532-011-0092-9

A series of novel nitro-substituted ortho-analogs of POPOP was synthesized. Like the most of the other known compounds of this class, the synthesized molecules demonstrate high Stokes shift fluorescence emission owing to the planarization of their molecules at electronic excitation. Significant fluorescence quenching in polar solvents was described as the "energy gap law" action rather than the specific effect of the dialkylamino group excited state twisting. Versita Sp. z o.o.

Full text of DOI:10.2478/s11532-011-0092-9

Cent. Eur. J. Chem. • 9(6) • 2011 • 962-971
DOI: 10.2478/s11532-011-0092-9
Central European Journal of Chemistry
High Stokes shift long-wavelength energy
gap regulated fluorescence in the series
of nitro/dimethylamino-substituted
ortho-analogs of POPOP
Invited Paper
Rodion Yu. Iliashenko1, Olexiy S. Zozulia1, Andrey O. Doroshenko1,2
1Kharkov V.N. Karazin National University, Kharkov 61077, Ukraine
2Ukrainian-American Laboratory of Computational Chemistry, Kharkiv 61001
(Ukraine) and Jackson, Mississippi 38217-0510 (United States)
Received 24 May 2011; Accepted 18 July 2011
Abstract:
A series of novel nitro-substituted ortho-analogs of POPOP was synthesized. Like the most of the other known compounds of this
class, the synthesized molecules demonstrate high Stokes shift fluorescence emission owing to the planarization of their molecules at
electronic excitation. Significant fluorescence quenching in polar solvents was described as the “energy gap law” action rather than the
specific effect of the dialkylamino group excited state twisting.
Keywords: Ortho-analogs of POPOP • High Stokes shift fluorescence • Solvatochromism • Energy gap law • ESS-analysis.
© Versita Sp. z o.o.
1. Introduction
at the early 1990s [12-15]. Bulky substituents in the
The planar and rigid molecular structure is usually ortho-positions of the central benzene nucleus result in
considered as the necessary requirement for the the significantly non-planar molecular structure of ortho-
synthesis of potentially effective organic fluorescent POPOPs, that is why these compounds usually absorb
compounds [1-3]. However, low Stokes shifts, typical light in the lower-wavelength spectral region compared
of such molecules, in many cases play the role of the to that of their planar para-isomers [14,16]. High
limiting factor at their practical application owing to Stokes shifts of 9000-10000 cm-1 observed in solvents
the significant reabsorption effects resulting from the of various polarity and even in the crystalline state were
overlap of their absorption and emission spectra, see clarified by the definite excited state flattening of their
for example [4-7]. Several photophysical mechanisms molecules resulting in the restoration of conjugation and
were proposed to increase the Stokes shift [8], the most in the lowering of the energy of the structurally relaxed
prospective among them seems the effect of the excited excited state [17,18]. Low solvatochromic effects are
state planarization of the initially non-planar fluorescent generally typical of most of the earlier investigated ortho-
molecules [9].
analogs of POPOP owing to the electronic symmetry of
their molecules in the structurally relaxed lowest excited
singlet state [19,20].
N
N
O
O
O
O
The hypothesis about the possibility of further Stokes
shift increase at combination of at least two different
N
N
POPOP
ortho-POPOP
Ortho-analogsofthewidelyknownorganicfluorophore photophysical mechanisms in the same molecule
POPOP {1,4-bis-(2-phenyl-5-oxazolyl)benzene, which mentioned in [8] was formulated in [9] and tested in
is extensively used in plastic and liquid scintillating several of our earlier publications [19,21]. In this paper
compositions [10, 11]} were started in our research group we try to combine the excited state structural relaxation
* E-mail: andrey.o.doroshenko@univer.kharkov.ua
962

R. Yu. Iliashenko, O. S. Zozulia, A. O. Doroshenko
typical to ortho-analogs of POPOP with the solvent
Carboxylic acid chlorides were synthesized before
relaxation around the molecule increasing its dipole their subsequent use at boiling of the correspondent acid
moment at electronic excitation.
solution in SOCl2 during 3 hours, than dry toluene was
added and the excess of thionyl chloride was removed
by distillation.
1-(5-[4-N,N-dimethylamionophenyl]-2-oxazolyl)-
2-(5-[4-nitrophenyl]-2-oxazolyl)-benzene (compound
I). Solution of 1.6 g (0.0048 mol) of 2-(5-[4-nitro-phenyl]-
2-oxazolyl)-benzoyl chloride in 25 mL of toluene was
I. X =
Y = CH
Y = N
N
N
N
NO2
O
II. X =
O
X
H
N
N
Y = CH
III. X =
Y
For this purpose, a series of the new ortho-analogs mixed with solution of 1.2 g (0.0048 mol) of 4-N,N-
of POPOP possessing the effective electron donor dimethylamino-ω-aminoacetophenone dihydrochloride
and electron acceptor groups was synthesized and in 50 mL of water. Saturated water solution of sodium
characterized by their absorption and emission spectra. carbonate was added dropwise at intensive stirring to
Increased intramolecular donor-acceptor interaction in pH 9-10, then the reaction mixture was stirred over
these molecules should result in the significant increase an hour. Precipitated solid was filtered, washed with
of the excited state polarity introducing the additional water and dried. The obtained arylamide was dissolved
solvatochromic contributions to their Stokes shift values in concentrated sulfuric acid at 50-60ºC, then left
in polar media.
to stay at room temperature during 5 hours, poured
onto ice, neutralized by solid sodium carbonate to pH
8-9, filtered, washed with water and dried. The final
product was purified by hot column chromatography
on Al2O3 with carbon tetrachloride as eluent followed
by double recrystallization from octane to yield 0.88 g
2. Experimental procedure
2.1. Chemicals and materials
Solvents and several semi-products were purchased (0.0019 mol, 49%) of red polycrystalline powder, M.P.
from Aldrich and used without additional purification.
118-119ºC (C28H20N4O4, MW 452.46, molecular ion in
The title compounds were synthesized by the MS, m/z: 452).
following common scheme:
1-(5-[4-N,N-dimethylamionophenyl]-1,3,4-
oxadiazol-2-yl)-2-(5-[4-nitrophenyl]-2-oxazolyl)-
benzene (compound II). Mixture of 2 g (0.0065 mol)
of 2-(5-[4-nitro-phenyl]-2-oxazolyl)-benzoic acid and
1.2 g (0.0067 mol) of 4-N,N-dimethylaminobenzoic
acid hydrazide was boiled in 20 mL of phosphorus
oxychloride over 3 hours. The reaction mass was
NO2
N
O
N
1) SOCl2
O
N
O
O
O
O
NO2
COOH
NO2
O
O
2NH
. HCl
2)
NAlk
2NH
NAlk
. 2HCl
2-(5-[4-nitro-phenyl]-2-oxazolyl)-benzoic poured into ice and after complete hydrolysis of POCl3,
acid. 3.7 g (0.025 mol) of phthalic anhydride and 5 g water was added to 1 liter. Precipitated solid was filtered,
(0.23 mol) ω-amino-4-NO2-acetophenone hydrochloride washed by water to the neutral reaction and dried. The
were dissolved in 50 mL of dry acetone, then 4 g of product was purified by hot column chromatography
potassium carbonate was added in small portions at on Al2O3 and carbon tetrachloride as eluent and then
continuous stirring over 3 hours. At 20 minutes after the recrystallized from toluene to yield 1.27 g (0.00028 mol,
addition of the first amount of carbonate, the reaction 43%) of orange crystals, M.P. 146-147ºC (C25H19N5O4,
mixture became viscous.
MW 453.45, molecular ion in MS, m/z: 453).
Then the reaction mass was neutralized by
ω-amino-1-(1,2,3,4-tetrahydroquinolin-6-yl)
concentrated hydrochloric acid, the yellow solid was ethanone. Quinoline (38.7 g, 0.3 mol), formamide
filtered off, washed with water and dried.
(78 g, 1.65 mol) and 85% formic acid (7.5 g) in 200 mL
The obtained amide was dissolved in 50 mL of flask was heated to 175-180ºC on the metal bath during
concentrated sulfuric acid and left to stay in a dark 6 hours, then 2.5 hours – to 190ºC. The reaction mixture
place at room temperature during 12 hours. The was cooled, poured into 750 mL of water and basified
reaction mixture was poured on ice, the precipitated by aqueous NaOH. After 3 hours of periodical stirring
solid was filtered, washed with water, dried and doubly neededforthedecompositionoftheexcessiveformamide
recrystallized from toluene to yield 4.96 g (68%) of 2-(5- the mixture was extracted 4 times by diethyl ether. The
[4-nitro-phenyl]-2-oxazolyl)-benzoic acid (pale yellow ether was removed by distillation, 30 mL of 85% formic
powder, M.P. 168-170ºС).
acid was added to the dark residue, and the mixture was
963

High Stokes shift long-wavelength energy gap regulated
fluorescence in the series of nitro/dimethylamino-
substituted ortho-analogs of POPOP
boiled under reflux over 30 minutes. Then it was basified dihydrochloride with M.P. 246-249 ºC was obtained in
and extracted four times by diethyl ether. The extracts such a way.
were combined, dried under anhydrous sodium sulfate
6-(2-{2-[5-(4-nitrophenyl)-1,3-oxazol-2-yl]
and then ether was removed by distillation in the water phenyl}-1,3-oxazol-5-yl)-1,2,3,4-tetrahydroquinoline
bath. The residue was distilled under reduced pressure (compound III). 1.2 g of 2-(5-[4-nitro-phenyl]-2-oxazolyl)-
(10-15 mm Hg). The fraction boiling at 190-200ºC was benzoic acid was boiled in 20 mL of phosphorus
collected yielding 26 g (0.162 mol, 53%) of yellow liquid, oxychlorideduring1hour, then1gofω-amino-1-(1,2,3,4-
N-formyl-1,2,3,4-tetrahydroquinoline.
20 (0.124 mol) of
tetrahydroquinoline, 50
tetrahydroquinolin-6-yl)ethanone dihydrochloride was
N-formyl-1,2,3,4- added and the mixture was additionally boiled up to
of anhydrous AlCl3 3 hours. The reaction mixture was cooled, poured
g
g
in 100 mL of methylene chloride was cooled on the ice into ice and neutralized by saturated aqueous sodium
bath,then0.15molofmonochloroaceticacidchloridewas carbonate. Precipitated solid was filtered, washed with
added dropwise. The reaction mixture was stirred during water and dried. The product was purified by hot column
12 hours and then poured on ice. The organic layer was chromatography on silica gel and benzene as eluent,
separated, several times washed with 5% hydrochloric then the room temperature column chromatography was
acid, and then methylene chloride was removed under applied (silica gel – benzene + 1% triethylamine mixture)
reduced pressure. The residue was hydrolyzed at to yield 0.5 g (0.0011 mol, 28%) of red crystals with M.P.
boiling with 50 mL of concentrated hydrochloric acid 102-103ºC (C27H20N4O4, MW 464.47, molecular ion in
until homogenization. The mixture was neutralized by MS, m/z: 464).
solid sodium carbonate, cooled. The yellow crystals of
2-chloro-1-(1,2,3,4-tetrahydroquinolin-6-yl)ethanone was checked by TLC (Silufol UV-254) and confirmed
was precipitated (16 g, 0.076 mol, 70%). additionally by the single-banded fluorescence
14 g of the above semi-product (0.069 mol), 10 g of synchronous scan spectra in octane, recorded
hexamethylenetetramine and 40 mL of chloroform were with ∆λEX/EM
50 nm. No normal Stokes shift
Purity of all the synthesized fluorescent compounds
=
stirred during 5 hours at room temperature, then white emitting components were registered for the case
crystals were filtered, washed with cool chloroform and of ∆λEX/EM = 10 nm.
dried. The obtained acylation product was dissolved
in 40 mL of 96% ethyl alcohol, 10 mL of concentrated 2.2. Spectra measurement
hydrochloric acid was added and the mixture was left Electron impact (70 eV) mass-spectra were determined
at room temperature overnight. The precipitated solid on a Varian 1200L device.
was filtered, some additional amount of the product
was collected at partial evaporation of the filtrate at UV/Vis spectrophotometer Hitachi U3210, fluorescence
reduced pressure. Finally 7 g (0.027 mol, 66%) of spectra and quantum yields on fluorescence
Electronic absorption spectra were recorded on a
–
ω-amino-1-(1,2,3,4-tetrahydroquinolin-6-yl)ethanone spectrometer Hitachi F4010 in the standard 1 cm
Figure 1. The ESS-analysis applied to the long-wavelength electronic transition in the absorption spectra of compound I (ground state optimized
molecular geometry): localization of the electronic excitation (gray circles with radii proportional to localization numbers [27]), net charge
changes (blue circles for increase of the electronic density, red – for increase of the positive charge) and arrow diagrams showing the
main pathways for the electronic density movement at the electronic excitation S0-S1 (charge transfer indices [27]).
964

R. Yu. Iliashenko, O. S. Zozulia, A. O. Doroshenko
quartz cells. Quinine bisulfate in 0.5 M aqueous sulfuric the source and destination groups participating in it.
acid served as quantum yield reference standard These indices are much more informative in comparison
(ϕf = 0.546 [22]). Quadratic refraction indices quantum to the traditional “charge changes” in ∆q format, which
yield correction was implied [1-3].
Fluorescence lifetimes were measured on the charge redistribution at the electronic excitation.
nanosecond single photon counting spectrometer Applying ESSA to the S0-S1 electronic transition of
reflect only the final result of the intramolecular electric
described elsewhere [16], decay curves were compound I gives the results presented in Fig. 1. The
deconvoluted using non-linear least squares method discussed molecule is non-planar like the most of the
[23]. Rate constants of primary photophysical ortho-analogs of POPOP: the b3lyp/cc-pvdz calculated
photoprocesses - radiative (kf) and radiationless (kd) angles between the oxazole cycle planes and the central
decay were calculated on the background of the benzene ring is near 29-30º. No spatial asymmetry
experimental quantum yields and fluorescence lifetimes: typical to this class of organic molecules in the crystalline
kf = φf / τf and kd = (1-φf) / τf [1-3].
state and qualitatively reproduced for them by the most
of semiempirical methods [12-21], was detected with
DFT modeling. Moreover, such a result was unexpected
for the molecule with donor and acceptor substituents,
which themselves could be the reason of the electronic
density distribution asymmetry.
The long-wavelength electronic transition in the
absorption spectrum of I (calculated in TDDFT for the
ground state molecular geometry) is localized mainly on
the terminal benzene rings and substituents introduced
in them. Participation of the heterocycles is much lower,
while the central benzene ring seemingly does not take
part in the electronic excitation. The charge changes
look similar to Li, however in this case the participation
of the substituents is a little more prominent.
2.3. Quantum-chemical modeling
Quantum-chemical calculations were conducted by the
following program packages: Gaussian-03, release E.01
[24](forthegroundstatemolecularstructureoptimization
in DFT scheme b3lyp/cc-pvdz [25,26]), Gamess US [27]
version R1 (for the excited state optimization in TDDFT
scheme TD/b3lyp/cc-pvdz), Gaussian-09, release B.01
[28] (for the ground and excited state optimization in
DFT/TDDFT scheme TD/M06-2X/cc-pvdz [29]) and
NWChem version 5.1 [30] (equipped with special module
for ESSA, “Excited State Structural Analysis”, [31] - for
the electronic spectra calculation and analysis).
The electron density redistribution pathways (lij)
analysis allows to conclude that the most important
among them is the direct exchange between the
donor and acceptor substituents and their neighboring
benzene rings. The oxazole cycles and central benzene
ring play the role of the bridge groups; their involvement
in the general redistribution of the electronic density is
auxiliary.
All these results allow for the classification of the
S0-S1 electronic transition in I molecule as the classical
intramolecular charge transfer one. Indeed, the
calculated dipole moment of the discussed molecule in
its lowest singlet Frank-Condon excited state exceeds
that of the ground state on 26 D.
3. Results and discussion
The title compounds contain both the most efficient
neutral electron donor (N,N-dialkylamino) and electron
acceptor groups (nitro) in para-positions of the terminal
benzene rings. This allows for increased intramolecular
donor-acceptor interactions in their molecules at the
electronic excitation. In addition to the excited state
planarization typical to the sterically hindered molecules
oftheortho-POPOPanalogs,theinducedsolvatochromic
effects should further enlarge their abnormally high
Stokes shifts observed even for the unsubstituted
representatives of the investigated series [12-21].
The intramolecular electron density redistribution in
the molecules of dimethylamino/nitro substituted ortho-
POPOPs was analyzed with the ESSA approach [27],
which is a generalization of the earlier scheme initially
elaborated for the π-electronic calculations methods in
the mid- 1970s [32]. The electronic excitation localization
indices (Li) help to elucidate the functional groups
and structural fragments of the studied molecules
participating in the electronic transitions. Charge
transfer indices (lij) show the details of the electronic
density redistribution between the atoms and more
complicated fragments of the whole molecule displaying
All the synthesized donor-acceptor substituted ortho-
analogs of POPOP are characterized by substantial
sensitivity to the polar nature of their environment: the
fluorescence color changes from green in the low polar
octane to the red in solvents of intermediate polarity (1,4-
dioxane, ethyl acetate). However, fluorescence intensity
of the investigated compounds decreases with solvent
polarity down to complete disappearance in acetonitrile
or DMF: generally, no emission lower than 14000 cm-1
was observed for the investigated compounds (Table 1,
Fig. 2).
965
965

High Stokes shift long-wavelength energy gap regulated
fluorescence in the series of nitro/dimethylamino-
substituted ortho-analogs of POPOP
27000
25000
23000
21000
19000
17000
15000
13000
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
N
Solvent polarity, ET
Figure 2. Solvatochromic behavior of compounds I (red lines: solid - absorption, dashed - fluorescence) and III (blue lines) presented versus
the Reichardt solvent polarity function ETN [33].
The analogous tendency is typical to various internal rotation thus making formation of TICT states
organic dialkylamino-substituted molecules with impossible [36]. However, the discussed structural
strong intramolecular donor-acceptor interactions, for modification could influence not only the conformation,
which the fluorescence quenching in polar and proton but also electronic effects of such a substituent. Thus,
donor solvents was often observed, for example, we performed preliminary quantum-chemical modeling,
see [34-39]. The probable reason for such behavior aimed to outline the difference between such structurally
could be the specific excited state relaxation process fixed alkylamino group introduced in molecule III and
leading to the formation of TICT (Twisted Intramolecular dialkylamino groups (compounds I,II). The results of
Charge Transfer) states [40-43], emissive for only a this modeling are presented in the Table 2. The simple
limited number of rather small organic molecules like the systems containing the discussed substituents, benzene
classical dimethylamino-benzonitrile, DMABN, [36,44] ring and electron accepting nitro-group were examined
and dimethylamino substituted pyridines [45,46]. In most with modeling in the b3lyp/cc-pvdz and TD/b3lyp/cc-
cases the structural fixation of the dialkylamino-group by pvdz schemes for the ground and lowest singlet excited
the alkyl chains resulted in prohibition of intramolecular states, respectively.
twisting and made fluorescence quenching hardly
possible [36].
The pyramidalization degree of alkylated amino-
group was estimated as a sum of three valence angles
An alternative reason for the decrease of fluorescent formed by its nitrogen atom. The selected DFT schemes
ability in the highly polar surrounding could be the (b3lyp/cc-pvdz) optimal to predict electronic spectra [27]
critically close approaching the energy of the structurally seems to slightly overestimate conjugation effects, thus
and solvent relaxed S1* excited state to the ground no substantial pyramidality was found for dialkylamino
state. This leads to an increase of the rate of internal group at our modeling. The recently developed
conversion S1*-S0 in the framework of the Energy gap “universal” Minnesota DFT functionals [29] qualitatively
law [47-49].
To make the grounded choice between these
gave the same results.
The space lability of the trimethylene chain forming
two possible mechanisms of solvent polarity induced the partially unsaturated six-membered nitrogen
fluorescence quenching in the series of the investigated heterocycle leads to the most energetically favorable
ortho-analogs of POPOP we have specially synthesized chair-like conformation of the latter. This resulted in
the compound III with the saturated alkyl bridge rigidly substantial general deplanarization of the corresponding
fixing its electron donor center, alkylamino-group, in the model molecules and pyramidalization of their nitrogen
planeoftheneighboringbenzenering.Thiscircumstance atoms accompanied by slight increase of N-C(Ar)
excludes the possibility of alkylated amino-group interatomic distances in the model molecules C, D in
966
966

R. Yu. Iliashenko, O. S. Zozulia, A. O. Doroshenko
Table 1. Spectral and photophysical properties of the dialkylamino/nitro substituted ortho-analogs of POPOP (I-III) in solvents of different polarity.
Solvent, ETN [29]
νа, cm-1 λа, nm νf, cm-1 λf, nm ΔνST, cm-1
φf
τf, ns kf×108, s-1 kd×108, s-1
1-(5-[4-N,N-dimethylamionophenyl]-2-oxazolyl)-
2-(5-[4-nitrophenyl]-2-oxazolyl)-benzene (I)
Octane, 0.012
24860
402
19740
507
5120
0.25
1.65
1.50
4.56
Toluene, 0.099
23700
24040
422
416
16340
15220
612
657
7360
8820
0.22
0.07
2.39
1.26
0.91
0.54
3.28
7.40
1,4-Dioxane, 0.164
Ethyl Acetate, 0.228
24040
416
14900
671
9140
0.001
-
-
-
1,2-Dichloroethane, 0.327
DMF, 0.386
23240
23280
23440
430
430
427
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Ethyl Alcohol, 0.654
1-(5-[4-N,N-dimethylamionophenyl]-1,3,4-oxadiazol-2-yl)-
2-(5-[4-nitrophenyl]-2-oxazolyl)-benzene (II)
Octane, 0.012
28740
26440
27380
27140
25980
348
378
365
368
385
20900
17820
16940
14980
15220
478
561
591
667
657
7840
8620
0.10
0.08
1.29
2.33
0.80
0.36
0.31
-
6.95
3.93
4.54
-
Toluene, 0.099
1,4-Dioxane, 0.164
Ethyl Acetate, 0.228
1,2-Dichloroethane, 0.327
10440
12160
10760
0.065 2.06
-
0.07
-
0.53
-
-
-
-
DMF, 0.386
26400
26720
27520
379
374
363
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Acetonitrile, 0.460
Ethyl Alcohol, 0.654
-
-
-
-
6-(2-{2-[5-(4-nitrophenyl)-1,3-oxazol-2-yl]phenyl}-
1,3-oxazol-5-yl)-1,2,3,4-tetrahydroquinoline (III)
Octane, 0.012
24960
24680
24160
24120
23840
23920
23280
23420
23660
401
405
414
415
419
418
430
427
423
20320
17360
15920
14660
15200
14880
-
492
587
628
682
658
672
-
4640
7320
8024
6420
8640
8340
-
0.14
0.12
0.05
3.55
4.11
3.30
0.39
0.29
0.15
0.001
0.02
0.06
-
2.42
2.14
2.88
2.32
2.45
3.04
-
Toluene, 0.099
1,4-Dioxane, 0.164
Ethyl Acetate, 0.228
Chloroform, 0.259
1,2-Dichloroethane, 0.327
DMF, 0.386
0.006 4.28
0.01
4.04
0.02
3.22
-
-
-
-
-
-
Acetonitrile, 0.460
Ethyl Alcohol, 0.654
-
-
-
-
-
-
-
-
-
-
967

High Stokes shift long-wavelength energy gap regulated
fluorescence in the series of nitro/dimethylamino-
substituted ortho-analogs of POPOP
Table 2. Quantum-chemical modeling of the space structure and electronic effects of the trimethylene chain fixed alkylamino group.
Model molecules
H
H
N
N
NO2
N
N
NO2
Calculated characteristics
C
D
A
B
Nitrogen atom pyramidalization,
S0 state
360°
360°
360°
360°
348°
359°
357°
354°
Nitrogen atom pyramidalization,
S1 state
rC-N, S0 state
1.386 Å
1.423 Å
274 nm
1.374 Å
1.381 Å
315 nm
1.397 Å
1.388 Å
273 nm
1.376 Å
1.385 Å
323 nm
r
C-N, S1 state
Electronic transition S0 - S1
Charge transfer at the electronic
transition S0 - S1
0.431
6.1 D
0.603
0.351
4.8 D
0.430
∆µ at the electronic transition S0 - S1
15.0 D
10.2 D
comparison with that of the A, B molecules. Introduction of donor-acceptor substituted molecules. Probably,
of the strong electron accepting nitro group smoothes the energy gap lowering between the structurally and
this effect, however does not eliminate it entirely.
solvent relaxed electronically excited state and the
The above results reveal deterioration of the corresponding ground state plays the key role in the
conjugation between the benzene ring and the amino dramatic decrease of fluorescence quantum yields in
group nitrogen atom at its space fixation with the alkyl the studied series of the ortho-analogs of POPOP, in the
chain. The tendency for hypsochromic shifts in the analogy to the cases described, for example in [50-53].
electronic absorption spectra and decrease of the charge
However, there exists another possible hypothesis,
transfer at the electronic excitation from the alkylamino- which does not contradict the above declared Energy
substituents towards the rest of the molecule followed gap regulated fluorescence quenching in polar solvents.
from the above discussed circumstances.
Systematic decrease of the experimentally estimated
In the structurally relaxed electronically excited state fluorescence emission rate constants (Table 1) and
the enforced mesomeric effects dominate over the steric our quantum-chemical modeling of the excited state
ones: the pyramidality of the nitrogen atoms decreases, structurally relaxed conformation of the donor-acceptor
making the model molecules more planar and more substituted ortho-POPOP indirectly pointed to this
polar than in the ground state.
possibility.
The quantum-chemical calculation of the structurally
The above results reveal that the structural fixing
of amino group with trimethylene chain leads to the relaxed excited I molecule was conducted in the TDDFT
disturbance of coplanarity of this substituent with scheme. The final “relaxed” molecular structure and the
the neighboring benzene ring and a decrease in the electron density redistribution in the ESSA approach
conjugation between the nitrogen atom unshared are shown in the Fig. 3. It is considered, that the
electron pair with the aromatic π-system. This also leads b3lyp functional is not the best one for calculation of
to the decrease of the electron donor ability of such a the systems with intramolecular charge transfer [54]
structurally fixed substituent, lowering the intensity of (it underestimates the energy yet gives, at the same
intramolecular charge transfer, short-wavelength shifts time, reasonable prediction of molecular geometry),
in the electronic absorption spectra and a decrease in that is why we have recalculated the optimized excited
the sensitivity of the spectral parameters to the polar state geometry also in the Truhlar’s M06-2x [29]
influence of the environment. According to our modeling, functional, generally showing much better results in
the outlined tendencies were partially compensated predicting highly polar excited states [55,56]. Indeed,
by the excited state structural relaxation, thus the the b3lyp-calculated excited state energy was quite
hypsochromic effects should be less expected in the underestimated, however, the optimized b3lyp and M06-
fluorescence emission spectra.
2x molecular geometries were practically the same.
The fact that spatial fixation of amino-group in the Thus, we could make several conclusions based on our
molecule of compound III could not prevent the polarity TDDFT calculations. In the lowest singlet excited state
induced fluorescence quenching allows us to conclude of I the planes of the terminal benzene rings are nearly
that TICT state formation is not the main reason for the parallel with minimal distance between them close to
decrease of fluorescent ability in the investigated series the mean thickness of the aromatic cycle. Donor and
968

R. Yu. Iliashenko, O. S. Zozulia, A. O. Doroshenko
Ground (b3lyp/cc-pvdz) and excited state (TD/b3lyp/cc-pvdz) optimized molecular geometry of compound I and the electron density
redistribution at electronic transition S0-S1 in the latter (ESS-analysis).
Figure 3.
acceptor substituents are on the minimal distance one strength) was low, reflecting the charge transfer origin
from another reflecting their significant electrostatic of the structurally relaxed excited state. The tendency to
attraction resulting from the excited state intramolecular decrease the “experimental” kf values calculated based
charge transfer. Generally, the excited state optimized on fluorescence quantum yields and lifetimes observed
structure of I looks like intramolecular exciplex with for compounds I-III (Table 1) is in line with the above
substantial stacking interaction of the terminal benzene fact. No one should expect fluorescence emission
rings.
in the case when the excited molecule radiative rate
The ESSA approach confirmed the increase of the constant dramatically decreases owing to the charge
intramolecular donor-acceptor interactions: charge transfer nature of the structurally relaxed S1 state, at the
transfer between the most important moieties intensifies same time as the radiationless rate constant increases
significantly in the structurally relaxed lowest excited following the Energy Gap Law at energies approaching
singlet state of I. At the same time, difference in the the structurally relaxed excited and the corresponding
ground and excited state dipole moments was lower in Franck-Condon ground state.
the S1-optimized structure in comparison with that of S0-
Thus, the excited state formation of intramolecular
state one (∆μ ~18 D and ~26 D correspondingly). This exciplex-like conformation should be considered among
fact reflects the spatial approaching of the nitro and the thepossiblereasonsforthepolarity-inducedfluorescence
dimethylamino groups, which decreases the length of quenching in the series of the investigated compounds.
the electric dipole even while increasing the difference Recently, a possibility for the intramolecular exciplexes
in electric charges and thus resulting in total decrease of the sterically hindered dendrimer-like molecules
of ∆μ = ∆q×l.
composed from the residues of the oxadiazolic analogs
The calculated charge transfer indices revealed of ortho-POPOP was discussed in [57].
increased direct electron density redistribution between
the donor and acceptor centers of the excited structurally
relaxed I molecule: nitro and dimethylamino groups and
theirneighboringbenzenerings.Theelectronicexcitation
localization indices also evidenced the decreased
participation of the three-cycle oxazole-phenyl-oxazole
bridge moiety in the formation of the lowest structurally
relaxed singlet excited state of I.
The energy of the long-wavelength electronic
transition (calculated with m062x/cc-pvdz scheme) lies
at ~750 nm, practically on the margin of near-infrared
region. The calculated transition intensity (oscillator
4. Conclusions
Fluorescence ability of the newly synthesized
dialkylamino-nitrosubstitutedderivativesofortho-POPOP
was examined in several solvents of different polarity. The
significant solvatochromic effects both in the absorption
and in the emission spectra and fluorescence quenching
in polar solvents are described as a result of increased
intramolecular excited state donor-acceptor interactions.
The observed decrease of fluorescence quantum yields
969
969

High Stokes shift long-wavelength energy gap regulated
fluorescence in the series of nitro/dimethylamino-
substituted ortho-analogs of POPOP
in polar surroundings was shown to be regulated by
the Energy Gap Law rather than TICT-mechanism.
Structurally relaxed excited state conformation looking
like intramolecular exciplex was predicted for the studied
compounds by the quantum chemical modeling.
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