Vibrational and optical characterization of s-triazine derivatives

Article abstract of DOI:10.1016/j.saa.2017.04.053

The optical and electron properties of three different s-triazine derivatives are investigated to ascertain the role of the donor acceptor character in amine-triazine systems depending on the bridging radical of the ammine group. The three derivatives wer

Full text of DOI:10.1016/j.saa.2017.04.053

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 183 (2017) 348–355
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Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Vibrational and optical characterization of s-triazine derivatives
a
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Luigi Stagi , Daniele Chiriu , Marek Scholz , Carlo Maria Carbonaro , Riccardo Corpino , Andrea Porcheddu ,
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a
a,
Suvi Rajamaki , Giancarlo Cappellini , Roberto Cardia , Pier Carlo Ricci ⁎
Dipartimento di Fisica, Università di Cagliari, sp. N°8 Km 0.700, 09042 Monserrato, Cagliari, Italy
Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic
Dipartimento di Scienze Chimiche e GeologicheUniversità degli Studi di Cagliari.Cittadella UniversitariaSS554 bivio per Sestu, 09042 Monserrato (Ca), Italy
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a r t i c l e i n f o
a b s t r a c t
Article history:
The optical and electron properties of three different s-triazine derivatives are investigated to ascertain the role of
the donor acceptor character in amine-triazine systems depending on the bridging radical of the ammine group.
The three derivatives were obtained starting from three different ammine compounds allowing to achieve a
structure with a triazine core, surrounded by three ammine group and terminated with cyano or methyl or
oxy-methyl functional group. Experimental optical data were interpreted in view of the electronic insights gath-
ered by means of density functional theory simulations on base compounds. As compared to the reference elec-
tron donating triazine core, the resulting compounds show different donor acceptor character, from electron
accepting to electron donating feature. Among the analyzed derivatives, the cyano terminated ammino-triazine
compound shows the most promising optical features for photonics and lighting applications.
© 2017 Elsevier B.V. All rights reserved.
Received 6 January 2017
Received in revised form 24 March 2017
Accepted 18 April 2017
Available online 20 April 2017
Keywords:
Amine-triazine systems
Raman spectra
Experimental vibrational properties
Experimental optical properties
Density functional theory simulations
1
. Introduction
Organic molecular materials are receiving an increasing interest for a
[12]. Indeed, the physical properties of organic compounds largely de-
pend on the strength of the donor - acceptor interactions: since organic
compounds are π-electron-deficient aromatic systems, the reactivity
with triazine is widely fostered, thus facilitating the formation of nu-
merous triazine-based compounds [13,14].
wide range of optoelectronic applications in which they can be success-
fully applied, ranging from the lighting sector, as emitting material for
OLED (Organic Lighting Emitting Diodes) [1,2], PLED (Polymer Lighting
Emitting Diodes) and inorganic based-White LED [3,4], to photovoltaics
As already mentioned, the possibility to synthesize a highly symmet-
ric electron-donor accepting system through an easy and efficient
chemical route, largely increases the possibility to apply these systems
in lighting devices [15]. In particular the conjunction of amine systems
with a triazine core can easily form a strong donor - acceptor (D - A)
compound with high optical quantum yield. In addition, the compound
can show highly improved non-linear optical properties, because of the
formation of a donor - π - acceptor (D - π - A) structure through a π-con-
jugated bridge [15,16].
In this view, we performed a detailed analysis of the optical proper-
ties of different triazine/amine systems where, depending on the
starting amines (4-aminobenzonitrile, p-anisidine, p-toluidine), the
resulting compounds have different donor acceptor character, from
the electron accepting to the electron donating feature as compared to
the reference triazine core [15,17–19]. In particular, the N2,N4,N6-tri-
p-tolyl-1,3,5-triazine-2,4,6-triamine (CNTT), terminating with a CN
group, represents a high electron accepting system. The N2,N4,N6-tri-
p-tolyl-1,3,5-triazine-2,4,6-triamine (MeTT), where a methyl group is
the radical bridged to the triazine core, is expected to show an interme-
diate electron – donating character. On the contrary, the N2,N4,N6-
tris(4-methoxyphenyl)-1,3,5-triazine-2,4,6-triamine (OMeTT), where
a OMet group is attached to the triazine molecule through the π-
[5], sensors and thin film transistors (TFT) [6,7]. The main advantage is
the general low fabrication cost and the ease of synthesis, whilst some
drawbacks are connected to the lower stability with respect to other in-
organic compound-based devices, in particular in extreme working
conditions.
Among the wide possibilities within the organic compounds, 1,3,5
triazine derivatives (hereafter triazine) have become one of the most
promising compounds for their exceptional properties connected to
the high structural symmetry and the high electron-donor feature [8].
Moreover, they show a large chemical stability under high temperature
and light irradiation, a characteristic that is fundamental for lighting ap-
plications [9]. Recently, triazine based materials were applied in differ-
ent of the above mentioned optoelectronic applications as well as for
2
the CO absorption [10,11].
One of the features that makes these compounds so fascinating is the
possibility to vary and tune their physical and chemical characteristics
depending on the decorating terminal structure of the triazine core
⁎
Corresponding author.
E-mail address: carlo.ricci@dsf.unica.it (P.C. Ricci).
http://dx.doi.org/10.1016/j.saa.2017.04.053
386-1425/© 2017 Elsevier B.V. All rights reserved.
1

L. Stagi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 183 (2017) 348–355
349
conjugated bridge, it is expected to possess a lower electron affinity
with respect to the s-triazine core that is known as a high electron do-
nating system [19].
The experimental results and computational DFT calculation relative
to the vibrational properties (Raman and Infrared, IR) and optical prop-
erties (absorption and emission) were acquired and discussed with re-
spect to specific donor acceptor character of the compound.
ensure that there were not imaginary frequencies (the structures corre-
spond to true local minima).
Previous studies have shown that the B3LYP functional gives the
overall best agreement with available experimental data for this class
of molecules [23,25].
2.1. Samples
A set of three triazine-based molecules were synthesized starting
from different amine compounds 4-aminobenzonitrile, p-anisidine, p-
toluidine as reported in the following and listed in Table 1.
2
. Experimental Set Up and Simulations
UV–vis absorption measurements were carried out by means of a
spectrophotometer Perkin-Elmer Lambda 15 (spectral bandwidth
Sample name
CNTT
Sample structure
2
nm) in the 250–700 nm range.
Steady state photoluminescence (SSPL) measurements were per-
formed using the filtered light from a laser driven Xenon lamp (EQ-
9-X) with a final bandwidth of about 1 nm. The excitation wavelength
9
for each sample was in correspondence of the relative absorption
maximum.
Time-resolved photoluminescence (TRPL) measurements were per-
formed by means of a streak camera (Hamamatsu C5680) coupled to a
grating spectrograph (Arc-Spectra-Pro 275i). Pulsed laser excitation
source at λexc = 355 nm was provided by a Coherent PS laser module
with a pulse duration of 100 ps and repetition rate of 1 KHz. The laser
was focused on a 500 μm spot with a power of 0.2 mW. PL measure-
ments were obtained in different time range: temporal resolution was
MeTT
2
00 ps with a 20 ns time range and better than 40 ps in the range
OMeTT
lower than 10 ns.
Raman measurements were performed in air at room temperature
with a spectrometer BWTEK i-Raman Ex with a spectral resolution of
−
1
9
(
cm . All the spectra were collected with low power excitation
2
lower than 5 mW) concentrated in a spot of 1 mm . The measurements
were obtained in back scattering geometry with excitation wavelength
at 1064 nm generated by a Nd:YAG laser.
For the synthesized molecules, we performed DFT calculations using
the NWCHEM package [20,21] Following previous works [22,23], geom-
etry optimization was obtained using the Becke three-parameter Lee-
Yang-Parr (B3LYP) hybrid exchange-correlation functional [24,25], in
combination with the 6-31+G* basis set, a valence double-ζ set aug-
mented with d polarization functions and s and p diffuse functions.
We performed a vibrational analysis on each optimized structure to
1) 4,4′,4″-((1,3,5-Triazine-2,4,6-triyl)tris(azanediyl))tribenzonitrile
(CNTT)
Cyanuric chloride (184 mg, 1 mmol) was dissolved in toluene (4 ml)
and 4-aminobenzonitrile (354 mg, 3 mmol) was added, followed by
a solution of NaOH (120 mg, 3 mmol) in water (1 ml). Reaction mix-
ture was heated to reflux for 16 h, and the resulted suspension was
filtered and washed with water to give the product as a white solid,
Table 1
Experimental and calculated Raman and IR frequencies.
CNTT
Exp
IR
MeTT
Exp
IR
OMeTT
Exp
Assignment
Calc.
Calc.
Calc.
Raman
Raman
IR
Raman
2
1
1
1
224
696
607
589
2230
1615
2345
1626
CN stretch
1618
1565
1539
1515
1450
1379
1296
1243
1214
1183
976
942
871
780
635
415
346
1626
1578
1564
1531
1489
1390
1253
1216
1210
1185
952
1620
1580
–
1515
1452
–
1300
1262
1230
1184
985
938
877
826
631
396
341
240
176
1637
1578
1564
1542
1479
1391
1277
1249
1211
1185
950
Phenyl quadrant ring stretch
β(NH) + ν (CN)
β(NH)
1576
1530
1539
–
1565
1533
1516
1496
ν (CC)
twisting methyl
1
424
β(CH)
wagging methyl
1
1
465
378
1377
1380
β(CH)
1316
1260
1245
1222
1177
953
903
901
777
694
ν (CC)
β(NH) + β(CH)
β(CH)
1
1
1
9
9
248
219
182
86
1227
987
β(CH)
Ring breathing mode I
op
42
923
903
778
688
933
893
780
686
β(CH)
β(CH)
op
836
850
804
778
642
409
341
827
β(CC)
β(CC) + β(CN)
β(CC)
436
408
op
340
215
187
β(CH)
2
10
246
β(CH)
β(CN)
179
176

350
L. Stagi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 183 (2017) 348–355
3
) N2,N4,N6-tris(4-methoxyphenyl)-1,3,5-triazine-2,4,6-triamine
OMeTT)
(
Cyanuric chloride (184 mg, 1 mmol) was dissolved in THF (10 ml).
DIPEA (0.60 ml, 3.5 mmol) and a solution of p-anisidine (430 mg,
3
.5 mmol) in THF (5 ml) were added and the reaction mixture was
heated to reflux for 72 h. The reaction mixture was concentrated
on silica and column chromatography (40% EtOAc in hexane) gave
the product as a palebrown solid, 100 mg, product yield 22%, melting
point 212–214 °C.
R
f
= 0.41 (1:1 Hexane/AcOEt)
(400 MHz; DMSO-d ) 10.1(3H, m, NH), 7.67–7.49(6H, m, Ar),
.90(6H, d, J = 7.7 Hz, Ar), 3.75(9H, s, CH ).
) 163.7(0), 155.6(0), 131.2(0), 122.7(1),
δ
H
6
6
3
δ
C
(100 MHz; DMSO-d
6
1
13.7(1), 55.2(3).
24 6 3
HRMS Calcd. for C24H N O : 444.1910. Found: 444.1916.
3. Results and Discussion
Fig. 1. Raman spectra of the s-triazine derivatives.
3.1. Vibrational Properties
The experimental Raman features of the triazine derivatives com-
pounds (CNTT, MeTT and OMeTT) are reported in Fig. 1.
2
96 mg, product yield 69%, melting point N300 °C.
The spectra are rather similar, with several bands in the wide spec-
R
f
= 0.32 (1:1 Hexane/AcOEt)
(400 MHz; DMSO-d ) 7.38(6H, d, J = 7.9 Hz, Ar), 6.60(6H, d, J =
.9 Hz, Ar), 6.13 (3H, s, NH).
−
1
tral region between 300 and 1400 cm where some of them are over-
δ
H
6
−
1
lapped and a main vibration at about 1600 cm . Among the vibrational
7
−
1
−1
modes, three strong Raman bands located at about 780 cm , 980 cm
δ
C
6
(100 MHz; DMSO-d ) 153.0(2C, 0), 133.4(1), 120.6(1), 113.4(0),
5.5(0).
−
1
and 1180 cm are detected in the whole set of samples but with differ-
ent relative intensities. Also the group of bands in the 1200–1300 cm−
shows a relative contribution, as referred to the main band at
9
1
HRMS Calcd. for C24
) N2,N4,N6-tri-p-tolyl-1,3,5-triazine-2,4,6-triamine (MeTT)
Cyanuric chloride (1 mmol, 184 mg) was dissolved in dioxane
20 ml), and p-toluidine (642 mg, 6 mmol) was added, followed
15 9
H N : 429.1450. Found: 429.1455.
−
1
2
1600 cm , which depends on the sample. Finally, the Raman spectrum
of the CNTT compound presents a distinctive band at 2230 cm−
1
.
(
In order to deeply understand the vibration properties and to give a
clear assignment of the Raman spectra, the infrared (IR) and Raman vi-
brational properties of phenyl-s-triazine-2,4,6-triamine derivative were
simulated by means of DFT calculations. The peak position of the exper-
imental Raman and IR bands as well the results obtained by the DFT cal-
culation and the relative assignment are summarized in Table 1.
The general approach to solve the Raman attributions is based on the
consideration of the molecular systems as composed of separated units,
the phenyl groups and the triamine derivatives, and to take into account
their bonding only for the effects in the Raman shift, as previously re-
ported by Palafox et al. [26]. The approach also allows a wider overview
of the proposed vibrational assignments. Indeed, the vibrational proper-
ties of phenyl groups are well documented in the literature, and a num-
ber of studies can be found on both experimental and theoretical studies
on vibrational modes [30–34] of several cyanoanilines molecules [26–
by potassium carbonate (830 mg, 6 mmol). The reaction mixture
was heated to reflux overnight, concentrated under reduced pres-
sure and the residue was partitioned between water and ethyl ace-
tate. The organic phase was washed with water and brine, dried
2 4
over Na SO and concentrated. The obtained off-white solid was
suspended in hot ethanol (75 ml) and filtered. The product precipi-
tated from ethanol as a white solid, 114 mg, product yield 29%, melt-
ing point 238–240 °C. Lit. [4] 238–239 °C.
R
f
= 0.45 (8:2 Hexane/AcOEt)
(400 MHz; DMSO-d ) 9.07(3H, s, NH), 7.67(6H, d, J = 7.6 Hz, Ar),
.09(6H, d, J = 8.0 Hz, Ar), 2.28(9H, s, CH ).
164.0(0), 137.4(0), 130.7(0),
δ
H
6
7
3
δ
C
6
(100 MHz; DMSO-d )
128.7(1),120.3(1), 20.4(3).
24 6
HRMS Calcd. for C24H N : 396.2062. Found: 396.2057.
Fig. 2. Absorption (A) and emission (B) spectra of the triazine compounds. The emission spectra were recorded by exciting the samples at their absorption maximum.

L. Stagi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 183 (2017) 348–355
351
Fig. 3. HOMO (left) and LUMO (right) charge distribution of CNTT compound. Red orbitals correspond to positive values of the wave function, blue ones to negative values.
3
1]. Among the others, the IR and Raman active modes of 4-
the linear response of optical properties and to exclude phenomena re-
aminobenzonitrile (4ABN) were previously assigned and leaded us in
the discussion for the reported compounds [26].
lated to the concentration, such as concentration induced quenching ef-
fects of the luminescence, decrease of the quantum yield or re-
absorption effects. It is worth to underline that the optical properties
of triazine compounds were unaffected by the nature of the solvent
and the solid state samples did not show any variation with respect to
optical spectra of the solutions.
As shown in Fig. 1, the three triazine compounds present the stron-
−
1
−1
gest Raman mode at 1615 cm
1
(CNTT), 1618 cm
(MeTT) and
−
1
620 cm (OMeTT). This vibration can be attributed to ν (CC) mode
in the form of the characteristic phenyl quadrant ring stretching with
a weak interaction with bending β(CH) [32]. Another feature, character-
istic of the phenyl groups, is the C\\H bending mode that, according to
our calculations, is assigned to the vibration at about 1180 cm−1 with a
negligible contribution of C\\C stretching [26]. Being characteristic of
para-substitutes benzenes, it is substantially unaffected by substituent
groups [32]. The remaining frequencies involve in-plane bending
mode of C\\H with the exception of the frequency at about
Fig. 2a reports the absorption spectra of triazine compounds in
−
4
DMSO solution (10
M). The absorption maxima are located at 274
(MeTT), 280 nm (OMeTT) and 310 nm (CNTT), evidencing a large red-
shift in the CNTT compounds of about 40 nm. The insets report the ab-
sorption spectra down to 700 nm showing that no other bands are
detected.
The emission spectra are also reported in Fig. 2b. The PL spectra were
excited in continuous wavelength regime at the maximum of absorp-
tion of each sample and clearly evidence the lower quantum efficiency
of the OMeTT compound (its spectrum is magnified of 2 order of magni-
tude) with respect to the cyano and methyl terminating compounds
(CNTT and MeTT respectively). By means of a coumarin standard we es-
timated a quantum yield of about 0.3 for the CNTT, 0.20 for the MeTT
and only of 0.02 for the OMeTT compound [33]. The measured variation
in the quantum yield is related to the optically excited charge transfer
process among different groups of the compounds: the transfer is very
efficient in the s-triazine – amino or methyl system, leading to a quite
high quantum yield, whilst the optical efficiency in the s-triazine –
methoxy system is lowered to fractions of percentage. This trend is re-
lated to the different affinity between the terminating groups and the
triazine core, where the s-triazine is a high electron donating systems,
the cyano group (CN) represents a high withdrawing electron system
and the methoxy group (OMet) is a weak electron accepting. In this
scale, the methyl group (Met) is a less withdrawing electron system as
compared to CN group but larger than OMet one [19].
−
1
1
320 cm , generated by C\\C sextant stretching [32]; since the out-
of-plane C\\H bending modes do not involve the motion of substitu-
ents, they are located in the 800–950 spectral range for the three
compounds.
On the other hand, the vibration at about 780 cm−1, assigned to in-
plane C\\C quadrant bend, interacts weakly with substituents giving a
vibration almost unaltered in its spectral position irrespective of the
considered compound.
Concerning the radical groups bonded to the phenyl ring, the dis-
tinctive modes of the methyl group (OMeTT and MeTT compounds)
are located in the region between 1350 and 1500 cm−1 whilst the char-
acteristic vibration modes of the CN group (CNTT compound) are clearly
−
1
visible between 2000 and 2300 cm , respectively.
The bands at 1379 cm and 1450 cm−1 in the experimental spec-
trum of the methyl compounds are assigned to the β(CH) twisting
and wagging modes of the methyl group, respectively. The mode at
−1
−
1
2
230 cm , characteristic of the CNTT compound, is attributed to the
stretching C≡N vibration of the CN terminal group, according to the
characteristic vibrations of cyanide ions.
In order to understand the withdrawing character of the cyano
group, we calculated the charge distribution for the highest and lowest
occupied molecular orbital (HOMO and LUMO respectively) states. The
charge distribution was calculated within the molecular orbital approx-
imation. Fig. 3 reports the HOMO and LUMO charge distribution of the
cyano compound. The terminating C≡N group presents localized and
−
1
Finally, the mode at 986 cm
is assigned to the ring-breathing
mode I of the s-triazine and it is characteristic of the central core of
the triazine compounds [26–29,32].
3
.2. Optical Properties
The optical properties of the CNTT, METT and OMeTT compounds
were studied by means of experimental measurements and reproduced
by DFT numerical calculations.
Table 2
Calculated optical values for the CNTT and MeTT compound.
A V A V
IE and IE = ionization energy (vertical and adiabatic); EA and E = electron affinity
(vertical and adiabatic); QPGap = Quasi-Particle Gap; EOPT = Optical Gap; EBind = Exciton
Absorption and emission properties were measured in the powder
solid samples as well as in solutions by dissolving the compounds in
proper solvents (acetone and dimethyl sulfoxide, DMSO). Concerning
solubility, CNTT resulted soluble in both acetone and DMSO, whilst
MeTT and OMeTT are soluble in DMSO and, weakly in acetone. The so-
Binding Energy.
IE
A
EA
[eV]
A
IE
[eV]
V
EA
[eV]
V
QPGap
[eV]
E
[eV]
OPT
E
Bind
[eV]
[
eV]
CNTT
METT
8.02
6.83
1.28
−0.12
8.05
6.87
1.20
−0.14
6.85
7.01
4.16
4.35
2.69
2.66
−
5
−3
lutions were studied in the range of 10 –10 M, in order to verify

352
L. Stagi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 183 (2017) 348–355
vacant π orbitals, tending to introduce a vacant orbital between the
highest occupied and the lowest unoccupied π orbitals of the
unsubstituted molecule. This charge distribution results in a smaller
HOMO-LUMO energy gap with respect to the other two compounds, un-
derlying the larger electronegativity value of the CN compound. As evi-
denced in Table 2, the calculations agree well with the experimental
findings with an optical energy gap of 4.16 eV and 4.35 eV for the
\
\CN and\\Me, respectively.
The functional group of CNTT molecule derives from a 4ABN system,
a benzene derivative substituted by the electrondonating NH group
2
and an electron-withdrawing CN group [17,18]. Then, light absorption
is expected to promote an electron transfer from a π orbital of aromatic
ring to a vacant π orbital of CN group [34]. Table 2 reports also the cal-
culated optical parameters of the two compounds with the more inter-
esting optical properties. The adiabatic and vertical electron affinity
underlines the above-mentioned withdrawing character of the\\CN
compound, and evidences, on the contrary, the almost neutral character
3
of the\\CH molecules.
In order to correlate the withdrawing character with the recorded
optical properties, we estimated the optical transitions through the qua-
siparticle energy gap. In order to achieve the required values we calcu-
lated the adiabatic electron affinities and ionization energies (EA
A
and
IA in Table 2) via total energy differences obtained after geometrical re-
A
laxation of each molecule. The vertical ionization energies (IEV) and
vertical electron affinities (EAV) were evaluated at the relaxed geome-
try of the neutral molecules. This procedure enabled the calculations
of the quasiparticle energy gap as defined in the so-called delta-self-
consistent-fields (ΔSCF) approach [35]. This method was successfully
applied to obtain the quasiparticle energies for several clusters [for ex-
ample, C60] [36] and consists in evaluating total energy differences be-
tween the self-consistent calculations performed for the system with N
and N ± 1 electrons, respectively.
Using Time Dependent DFT (TDDFT) [37], we have also obtained the
excitation energies and the electronic absorption spectra in the visible
and near-UV regions. The calculations were performed with the
NWCHEM package adopting the same B3LYP/6-31+G* level of theory.
We used the so-called Casida approach based on the linear response of
the density matrix, in which the poles of the linear DFT calculations
Fig. 4. Calculated and experimentally measured absorption spectra of CNTT (A), MeTT (B)
and OMeTT (C) compounds. Calculated spectra are the sum of Gaussian bands peaked at
vertical excitation energies with a width of 0.4 eV.
Fig. 5. Deconvolution of PL spectra of CNTT and MeTT (black and red squares respectively) by means of two Gaussian bands (Gaussian bands and resulting fit are reported as solid lines).
The blue lines are assigned to recombination at LE states, pink curves are related to ICT bands (see text for details). (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)

L. Stagi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 183 (2017) 348–355
353
Fig. 6. Time behavior of the CNTT compound. Time decays and time dependent spectral profiles (0–0.3 ns and 4.5–5 ns) acquired with excitation at 355 nm (A), and 405 nm (B).
response function correspond to vertical excitation energies and the
pole strengths to the corresponding oscillator strengths [38]. Knowing
the first optically active transition EOPT, the exciton binding energy has
been estimated through the difference:
the deconvolution process indicate two bands peaked at 3.44 eV and
2.99 eV for the compound\\Me. The redshift for the optical properties
of the\\CN compound, as already observed in the optical absorption,
fix the relative components at 3.06 and 2.96 eV, respectively.
The efficient emission in similar systems was previously assigned to
the presence of intramolecular charge transfer (ICT) emission and to lo-
cally excited state (LE) [8,15]. In case of\\CN compound the ICT state
can be related to the πσ*C≡N state, arising from the promotion of an
electron from the aromatic π orbital to the σ* orbital localized on the
cyano (C≡N) group [40].
The presence of the ICT bands is connected to the presence of elec-
2 2 3
tron-donating groups (e.g.\\NH ,\\NMe , –CH O) conjugated to an
electron-withdrawing group (e.g. NC_O,\\C≡N) and they are, hence,
visible in the\\CN,\\Met compounds whilst are almost absent in the
OMeTT compound [14,15].
The two emitting states identified in the deconvolution process of
the two species are assigned to radiative recombination at LE and ICT,
being the components at higher energy related to the LE band (at
3.44 eV and 3.06 eV for the MeTT and CNTT compounds, respectively).
On the contrary, the ICT recombinations are related to emission at
lower energies (at 2.99 eV and 2.96 eV for the MeTT and CNTT com-
pounds, respectively). However, the exact mechanism of the recombi-
nation process is still debated in similar compounds [40]. Several
models, in particular on the origin of the ICT bands, have been proposed
and need to be better confirmed [40,41]. Five different models have
been recently proposed: twisted ICT, planar ICT, wagged ICT, re-hybrid-
ized ICT and partially twisted ICT, depending on the molecular structure
and interactions among the molecules. It should be noted that the ICT
Ebind ¼ QPgap−Eopt
The method, successfully applied in the original ΔSCF form on clus-
ters of different size and atomic composition [36], was applied in a
more systematic way on different organic systems. Indeed, it was suc-
cessful in testing different choices of families of PAHs or different com-
putational scheme with respect to the exchange-correlation functional
of the total energy, or even the effect of molecule functionalization
[25,39]. More recently the present method was demonstrated suitable
to attack medium-size/large clusters and robust in comparison to recent
many-body schemes [39].
Fig. 4 focus on the calculated vertical excitation energies and the os-
cillator strengths for singlet states in comparison with the experimental
data. For the sake of clarity, calculated absorption spectra by means of
Gaussian convolutions of theoretical results and experimental spectra
are also reported. Calculated absorption spectra are obtained as the
sum of Gaussian bands peaked at the theoretical vertical excitation en-
ergies and assuming Gaussian width of 0.4 eV. The position of the re-
ported vertical transitions and the absorption bands calculated on the
hypothesis of Gaussian convolution agree quite well with the experi-
mental data. Slight variations from the experimental data can be associ-
ated to the role of the environmental and of the intramolecular
interactions, which were not accounted for in the simulated spectra. In-
deed, TDDFT calculations reproduced correctly the trend of optical ab-
sorption depending on functional groups. It is worth to evidence that
the calculations correctly estimated a higher wavelength absorption
for CNTT (λexp = 310 nm and λcalc = 295 nm). We also successfully es-
timated the influence of methyl groups in lowering the wavelength of
the absorption peak as compared to the cyano group (λexp = 274 nm
and λcalc = 277 nm for the MeTT compound; λexp = 278 nm and λcalc
Table 3
Fitting parameters of the time decay behavior of the CNTT and MeTT compounds.
CNTT
y = A1 ∗ exp(−x / t1) + A2 ∗ exp(−x / t2) + y0 - Adj. R-Square 0.99972
Excitation 355 nm
Excitation 405 nm
y0
A1
t1
A2
t2
94.2 (± 2.9)
2552(± 24)
0.254 (± 0.002)ns
1882 (± 23)
1.12 (± 0.01) ns
50.5 (± 1.8)
829 (± 14)
0.249 (± 0.005) ns
1760 (± 14)
1.10(± 0.01) ns
=
287 nm for the OMeTT compound).
Concerning the emission properties of the compounds, we focused
our attention on the CNTT and MeTT compounds, because of their larger
quantum yield with respect to OMeTT compound. The experimental
features of \\CC and \\Me compounds present a main peak and a
broad shoulder extending to the lower energies (Fig. 5). The non-sym-
metric shape of the emission spectra suggests the contribution of
more than one emitting center to the recorded spectra. Indeed, the
spectra can be easily fitted by means of two Gaussian curves for both
the compounds. Gaussian bands represent the energy distribution of
emissions because of environment induced broadening. The results of
MeTT
y = A1 ∗ exp(−x / t1)
Excitation 405 nm
y0
A1
t1
23 (± 2.8)
2433 (± 7)
4.11 (± 0.03) ns

354
L. Stagi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 183 (2017) 348–355
Fig. 7. Time behavior of the MeTT compounds: decay time (left) and time dependent spectral profiles, acquired at different time delays (right).
bands are easily to observe in polar solvents whilst are quenched in
non-polar solvent where, on the other hand, the LE bands are still active
triazine core, the resulting compounds did show different donor accep-
tor character, from electron accepting to electron donating feature.
Among the analyzed derivatives, the cyano terminated ammino-tri-
azine compound shows the most promising optical features for photon-
ics and lighting applications, with emission spectrum peaked in the blue
range, fast decay time (in the nano and sub-nanosecond domain) with
an estimated quantum yield of about 0.3.
[40]. The impossibility to find a non-polar solvent for the compounds
and the needed to perform transient absorption measurement do not
permit to clearly assign the emission bands to a specific model of intra-
molecular charge transfer process. Dedicated studies on this topics are
mandatory but out from the main scope of this article.
Finally, the time resolved behavior of the luminescence was studied
under excitation at 355 nm. Fig. 6A reports the luminescence decay ob-
served in the CNTT compound. As evidenced in the Table 3, the experi-
mental results can be well fitted by using two exponential decays, a fast
one with time decay of 0.25 ns and a slower one with time decay of
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