Influence of solvent polarity and viscosity on nonradiative processes in oligothiophenes with intramolecular charge transfer

Article abstract of DOI:10.1023/A:1016137218378

By the methods of luminescence, picosecond spectroscopy, and quantum-chemical calculations the mechanisms of electron excitation energy deactivation in some oligothiophenes with intramolecular charge transfer depending on the solvent polarity and viscosity have been investigated. While for 2-N-piperidino-5-(2',2'-dicyanovinyl)thiophene (PDCVT) the main channel of nonradiative deactivation is the transition to a lower intermediate state with a "twisted" double bond controlled by the medium viscosity, in the case of (E)-{2-[2-5-piperidino-2-thienyl]-6-(triftuoridemethyl)-4H-4-pyranylidene}propanedinitryl (PTFDN) fluorescence quenching is initiated by the solvent polarity. For two other oligothiophenes, 2-N-piperidino-5-cyanothiophene (PCT) and 2-N-piperidino-5"-cyanoterthiophene (PCTT), differing in the length of the thiophene chain, we have revealed, along with the effective quenching of fluorescence in short-chain PCT (independent of the solvent polarity and viscosity), an increase in the radiation capacity in PCTT with increasing polarity of the solvent. The possible mechanisms of nonradiative deactivation in the investigated oligothiophenes are discussed.

Full text of DOI:10.1023/A:1016137218378

Journal of Applied Spectroscopy, Vol. 69, No. 2, 2002
INFLUENCE OF SOLVENT POLARITY AND VISCOSITY ON
NONRADIATIVE PROCESSES IN OLIGOTHIOPHENES WITH
*
INTRAMOLECULAR CHARGE TRANSFER
a**
a
a
S. L. Bondarev, V. N. Knyukshto, S. A. Tikhomirov,
UDC 539.19
a
b
b
b
I. I. Kalosha, D. N. Bobrov, N. V. Masalov, N. M. Nevar,
b
b
b
c
′
V. I. Tyvorskii, A. V. Kel’in, O. G. Kulinkovich, and K. Dzilinski
By the methods of luminescence, picosecond spectroscopy, and quantum-chemical calculations the mechanisms
of electron excitation energy deactivation in some oligothiophenes with intramolecular charge transfer depend-
ing on the solvent polarity and viscosity have been investigated. While for 2-N-piperidino-5-(2′,2′-dicy-
anovinyl)thiophene (PDCVT) the main channel of nonradiative deactivation is the transition to a lower
intermediate state with a "twisted" double bond controlled by the medium viscosity, in the case of (E)-{2-[2-
5
-piperidino-2-thienyl]-6-(trifluoridemethyl)-4H-4-pyranylidene}propanedinitryl (PTFDN) fluorescence quench-
ing is initiated by the solvent polarity. For two other oligothiophenes, 2-N-piperidino-5-cyanothiophene (PCT)
and 2-N-piperidino-5′′-cyanoterthiophene (PCTT), differing in the length of the thiophene chain, we have re-
vealed, along with the effective quenching of fluorescence in short-chain PCT (independent of the solvent po-
larity and viscosity), an increase in the radiation capacity in PCTT with increasing polarity of the solvent.
The possible mechanisms of nonradiative deactivation in the investigated oligothiophenes are discussed.
Keywords: fluorescence, change transfer, polarity, viscosity, fluorescence quenching.
Introduction. Conjugate oligothiophenes (OT) are known as one-dimensional electron-conductive systems with
a high photo- and thermostability [1], and organic polymers based on them are being widely investigated with the aim
of developing various optoelectronic devices for generating and detecting optical radiation (light-emitting diodes, micro-
lasers, wide-format displays, photodiodes, solar batteries, etc.) [2–4]. In the last few years, the attention of researchers
has been focused on OTs with limit electron-donor and electron-acceptor substitutes in which effective intramolecular
charge transfer (CT) from the electron-donor to the electron-acceptor part of the molecule along the chain of thiophene
rings occurs. Such compounds are characterized by a marked difference between the dipole moments in the ground
(
µ ) and singlet-excited (µ ) states and exhibit, in a number of cases, pronounced nonlinear-optical properties [5].
g
e
The mechanisms of the photophysical and photochemical processes proceeding in systems with charge transfer
after the absorption of a light quantum are highly diverse and depend on the chemical structure of substances, their
aggregate state, and the nature of the environment. At the present time, there are divergent points of view about the
mechanism of charge transfer in electron-excited electron-donor-acceptor (EDA) molecules [6–9]. The most illustrative
example of this is such a relatively simple molecule as para-dimethylaminobenzonitryl (DMABN), which in the last
forty years has been serving as a model compound for investigating processes with charge transfer. However, in the
literature the discussion about the structure of the excited fluorescent state is still going on. It is believed [7] that a
polar solvent in this case stabilizes the fluorescent state with charge transfer resulting from the rotation of the dimethy-
o
lamino group about the single bond by 90 (the so-called TICT-state). In [8, 9], the fluorescence from the ground state
*
Reported at the International Conference on Luminescence, October 17–19, 2001, Moscow, Russia.
*
*
To whom correspondence should be addressed.
a
Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70 F. Skorina Ave.,
b
c
2
20072, Minsk, Belarus; e-mail: bondarev@imaph.bas-net.by; Belarusian State University, Minsk; Polytechnical Insti-
tute, Czestochowa, Poland. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 69, No. 2, pp. 200–206, March–
April, 2002. Original article submitted January 16, 2002.
230
0021-9037/01/6902-0230$27.00  2002 Plenum Publishing Corporation

o
with charge transfer is attributed to either the structure of the DMABN molecule with a cyano group bent by 120 C
2
(
RICT-state) [8] or the structure of DMABN with planar hybridization sp of the nitrogen atom in the dimethylamino
group (PICT-state) [9]. However, the singlet-excited state of molecules with intramolecular charge transfer does not al-
ways exhibit fluorescence and we have to assume its effective nonradiative deactivation [6]. In the EDA molecule, the
photoisomerization processes that are effective, for example, in stilbene-like EDA molecules can compete with the
charge transfer processes [10]. The diversity of phototransformation processes and the necessity of taking into account
their structural features initiate further experimental and theoretical studies of the photodynamics of systems with
charge transfer.
In the present work, we have investigated the spectroscopic and photophysical properties of solutions of a
number of newly synthesized thiophene-containing EDA molecules
and performed quantum-chemical calculations of their spectroscopic and electrophysical characteristics. The EDA OT
presented can be subdivided into two structurally differing groups: compounds 2-N-piperidino-5-cyanothiophene (PCT)
and 2-N-piperidino-5′′-cyanoterthiophene (PCTT) with a different number of thiophene rings (n = 1 and 3), as well as
2-N-piperidino-5-(2′,2′-dicyanovinyl)thiophene (PDCVT) and (E)-{2-[2-(5-piperidino-2-thienyl]-6-(trifluoridemethyl)-4H-
4
-piranylidene}propa- nedinitryl (PTFDN) having a central double bond but different electron-acceptor substitutes. Note
−
28
that PDFDN has a high value of the quadratic polarizability β = 2.6⋅10
ESU units at a wavelength of 1.9 µm [11].
Experimental Technique and Procedure of Quantum-Chemical Calculations. The PTFDN and PDCVT
synthesis is described in [11, 12], and PCT and PCTT were synthesized according to the following scheme:
231

TABLE 1. Spectral-Luminescent Properties of OTs at 293 K
a
f
−1
Solvent
λ
, nm
λ
, nm
∆ν , cm
τf, nsec
≤0.001
Φf
max
max
St
PCT
n-Hexane
317
≤10−
6
–
–
n-Propanol
328
PCTT
n-Hexane
425
435
505
645
3730
7370
1.5
3.3
0.48
0.87
Acetonitryl
PDCVT
PTFDN
Ethanol
464
473
488
492
1060
815
0.004
0.0006
0.004
Diethylene glycol
0.004 (0.06)
Benzene
558
567
640
710
2300
3560
1.2
0.28
Acetonitryl
0.02
0.004
The PMR spectrum (CDCl , 200 MHz; δ, ppm; J, Hz; m, multiplet, d, doublet) of PCTT: δ = 1.60–1.82, m (6H),
3
3
4
4
.11–3.23, m (4H), 5.98, d (J = 4, 1H), 6.87, d (J = 4, 1H), 8.94, d (J = 4, 1H), 7.06, d (J = 4, 1H), 7.14, d (J =
, 1H), 7.52, d (J = 4, 1H) and of PCT: δ = 1.52–1.80, m (6H), 3.14–3.30, m (4H), 5.94, d (J = 4, 1H), 7.30 (J =
, 1H). The IR spectrum for both compounds in CCl is characterized by the stretching vibration frequency of the
4
−
1
C N bond of 2220 cm .
Reagent-grade solvents were purified by standard methods. Spectral-fluorescent measurements were made on
the high-transmission set described in [13]. The quantum yield of fluorescence Φ was measured by the relative
φ
method and as standards, solutions of cresyl violet dyes in methanol (F = 0.54 [14]) and 3,6-diaminophthalimide in
f
ethanol (F = 0.6 [15]) were used. The kinetics and the spectra of induced absorption and amplification from the ex-
f
cited S state were recorded on a picosecond spectrometer [16]. Excitation was carried out by the second harmonic of
1
a neodymium laser (λexc = 528 nm) with a pulse duration of 4 psec and probing — by a picosecond continuum. The
time characteristics in the nanosecond time interval (>1 nsec) were recorded on a PRA 3000 fluorimeter operating in
the photon-counting regime.
Low-temperature measurements were made in a cryostat, where the temperature was varied by blowing liquid
nitrogen vapors. A temperature-sensitive element in the form of a DS-1A silicon diode was used. The accuracy of
maintaining the temperature was ±1.0 K. For low-temperature measurements, vitrifying n-propanol systems and an EPA
mixture (diethyl ether + isopentane + ethyl alcohol in a 5:5:2 ratio) were used. The changes in the refractive index of
EPA with decreasing temperature were taken into account by the relation from [17], and the temperature dependence
of the change in the n-propanol viscosity was taken from [18].
The molecule geometry was optimized by the PM3 quantum-chemical method [19]. The ZINDO/S method
[20] was employed to calculate the energies of electron transitions and their oscillator strengths. The dipole moments
in the ground and excited states were calculated by the PM3 semiempirical method on the basis of the HyperChem
6.0 (HyperCube Inc.) software.
Results and Discussion. PCT and PCTT markedly differ from each other in their spectral and photophysical
properties (Table 1).
For instance, the absorption spectra of PCTT at room temperature are shifted to the long-wave region from
the PCT spectra, which is explained by the more extended π-conjugate system in the case of PCTT. We should note
the considerable Stokes shift (∆ν ) between the absorption and fluorescence spectra of PCTT in polar solutions (7570
St
−
1
cm in methanol), which can be attributed to the fact that for PCTT the change in the dipole moment ∆µ = µ –
e
µg is greater. The ∆µ value for PCTT determined by the Lippert equation [12] proved to be equal to 27.2 D, i.e., with
232

Fig. 1. Dependences of the quantum yield of fluorescence (Φ ) of PDCVT on
f
the viscosity of solvents (η) at Troom (upper straight line) [1) n-hexane; 2)
methanol; 3) toluene, 4) dimethyl formamide; 5) carbon tetrachloride; 6) etha-
nol; 7) n-propanol; 8) formamide; 9) ethyleneglycol; 10) diethyleneglycol; 11)
glycerol] and on the viscosity of n-propanol at various temperatures (lower
straight line).
Fig. 2. Absorption (1), fluorescence (4, 5), phosphorescence (6), and fluores-
cence excitation (2, 3) spectra of PDCVT in EPA (1–5) and n-propanol (6) at
7
(
7 K; λexc = 471 (4), 445 (5), and 467 nm (6), λrec = 505 (2) and 475 nm
3); in the inset — the Arrhenius dependence for the logarithm of the nonra-
diative probability on the inverse temperature.
regard for µ = 8.4 D calculated for the ground state the dipole moment in the relaxed singlet-excited S state µ =
g
_
1
e
3
5.6 D. Thus, an increase in the dipole moment for PCTT by 27.2 D in the S state leads to a 0.33e shift throughout
the length of the molecule ( 17 A) from the electron-donor piperidine to the electron-acceptor cyano group.
The most significant difference between PCT and PCTT showed up in investigating the photophysical charac-
1
°
−
6
teristics. For instance, for PCT we did not manage to detect fluorescence at room temperature; for it Φ ≤ 10 . Under
f
the same conditions for PCTT in acetonitryl Φ = 0.87 ± 0.05 and in n-hexane — 0.48 ± 0.03. One would expect the
f
opposite result, because in nonpolar n-hexane the fluorescence spectrum is shifted to the short-wave region from the
spectrum in acetonitryl by 140 nm, and accordingly, the higher-lying S state of PCTT in n-hexane should be charac-
1
terized by a smaller internal conversion constant. Therefore, in this case it may be suggested that such a polar solvent
as acetonitryl stabilizes the polar PCTT molecule in the excited state by means of specific interactions and thus de-
creases the probability of rotary motions in a long, flexible molecule leading to nonradiative deactivation. In so doing,
the fluorescence durations were 3.3 ± 0.1 and 1.5 ± 0.1 nsec, respectively. At 77 K these parameters did not change
markedly. Unlike DMABN [7], for PCTT no dual fluorescence in polar solutions was detected.
PDCVT. The electronic absorption spectrum is characterized by an intense long-wave band in the visible re-
4
gion. The molar extinction coefficient at the absorption band maximum at 464 nm in ethanol is 6.2⋅10 li-
−
1
−1
ters/mole ⋅cm , i.e., the transition is allowed. The absorption spectra of PDCVT feature a bathochromic shift with
increasing solvent polarity. For instance, on passing from n-hexane (λmax = 441 nm) to ethanol the spectral shift is
1
±
120 cm. The fluorescence spectra of PDCVT are shifted to the long-wave region from the absorption spectra by 1625
70 cm in n-hexane and by 1060 ± 50 cm in ethanol, i.e., in polar solvents the shift of the spectra to the long-
−
1
−1
wave region is weaker than in nonpolar solutions.
The quantum yield of fluorescence Φ of PDCVT increases linearly with increasing viscosity of the solvent
f
(Fig. 1), changing from 0.0004 ± 0.0001 in n-hexane (viscosity η = 0.0029 P at 298 K) to 0.019 ± 0.02 in glycerol
(
η = 9.45 P at 298 K). At room temperature in the solid matrix of polymethylmetacrylate (PMMA) Φ = 0.027 ±
f
0
.002. The linear dependence of Φ on η for PDCVT in n-propanol (Fig. 3, lower straight line) was also observed in
f
233

TABLE 2. Energies (∆E) and Oscillator Strengths (f) of Transitions (ZINDO/S) and Dipole Moments in the Ground (µg) and
Excited (µe) States (PM3) for PDCVT as a Function of the Angle of Rotation (ϕ) of the Cyano Groups about the 1′−2′ Bond
∆E, cm− (nm)
26,101(383)
25,982(385)
25,613(390)
24,950(401)
23,898(419)
20,006(500)
11,329(883)
430(23,275)
1
ϕ, deg
f
µg, D
µe, D
0
0.912
0.914
0.917
0.915
0.895
0.709
0.242
0.000
7.01
7.04
7.11
7.21
7.34
7.89
9.09
15.68
9.95
9.94
9.87
9.71
9.37
8.17
7.20
7.04
1
2
3
4
6
8
9
0
0
0
0
0
0
0
the case where the solvent viscosity increased with decreasing temperature of the solution. At 77 K in the EPA mix-
ture Φ = 1.00 ± 0.05. Thus, Φ increases 2500 times on passing from n-hexane at 298 K to EPA at 77 K.
f
f
It should be noted that the solvent polarity did not influence Φ . Indeed, in methanol and toluene, solvents
f
differ so widely in permittivities (32.6 and 2.4) but having equal viscosities, the quantum yields of fluorescence coin-
cide (Fig. 1).
Since the absorption spectra of PDCVT in n-propanol remain practically unaltered with a change in the tem-
perature in the 293–180 K range, it may be suggested that the probability of fluorescence radiation k in this range
f
does not change either. Then, proceeding from the Arrhenius linear dependence for the logarithm of the probability of
−
E ⁄ RT
A
nonradiative transitions k = k (1 – Φ )/Φ = ΣA e
on inverse temperature 1/T (Fig. 2, inset), we can determine
nr
f
f
f
n
the activation energy EA of the processes of nonradiative transitions, which was measured to be 7.9 ± 0.2 kJ/mole.
This value is in good agreement with the data obtained in investigating the mechanism of nonradiative processes of
trans-stilbene in various solvents [22]. It may be suggested that the nonradiative process in the PDCVT molecule in
liquid solutions is also associated with the rotation about the 1′–2′ bond in the excited state. As a result of such a ro-
tation, the molecule goes to the lower "twisted" state, in which, under the action of the polar solvent, the conformation
with intramolecular charge transfer is stabilized or, as in the case of stilbene, the double bond acquires the character
of a single bond and the electronic integration is minimum. The energy of such states sharply decreases to provide an
effective nonradiative exchange of the electron excitation energy.
The absorption, fluorescence, and phosphorescence spectra of PDCVT at 77 K narrow and acquire a vibra-
tional structure (Fig. 2, curves 3–6). For instance, in the absorption spectrum in the short-wave region a maximum at
445 nm appears and in the fluorescence spectrum, symmetrically to it, a maximum at 505 nm is positioned. The en-
ergy gap between the 0–0 band in the absorption spectrum (Fig. 2) at 471 nm and the vibrational satellite at 445 nm
−
1
−1
is 1180 ± 50 cm , and in the fluorescence spectrum — 1170 ± 50 cm . Thus, there exists a mirror symmetry be-
tween the absorption and fluorescence spectra of PDCVT in both the frequencies and intensities. The phosphorescence
spectrum of PDCVT in n-propanol at 77 K (Fig. 2, curve b) is similar in form to the fluorescence spectrum and its
maximum lies at 645 nm. The fluorescence excitation spectrum coincides with the S ← S absorption spectrum. The
1
0
−
1
distance between the maxima of the fluorescence and phosphorescence spectra is 5400 cm and the triplet energy
−
1
level of PDCVT is 15,500 cm . The spectral data for PDCVT at 77 K suggest the presence in the solution at a low
temperature of one conformer, whose structure undergoes no special changes in the ground and excited singlet and
triplet states.
To determine the electron transition energy for the PDCVT molecule with cyano groups rotated about the
1′−2′ bond, we have performed quantum-chemical calculations, which have shown (Table 2) that the transmission en-
ergy, the oscillator strength, and the dipole moment depend on the angle of rotation of the cyano groups about the
o
1
′−2′ bond. The changes are especially significant at rotation angles larger than 60 .
From Table 2 it is also seen that the transition energy decreases with increasing angle of rotation about the
o
−1
1
′−2′ bond and at 90 it is equal to 430 cm . These data agree with the above suggestion that the energy of the in-
234

Fig. 3. Dependence of the logarithm of the nonradiative deactivation rate con-
stant knr on the solvent polarity function ∆f. The knr values were calculated
8
−1
from the equation knr = k (1 – Φ )/Φ at k = 1.7⋅10 sec , T = 293 K; 1)
r
f
f
r
benzene; 2) toluene; 3) dibutyl ether; 4) chloroform; 5) diethyl ether; 6) tolu-
ene + DMSO (90:10); 7) tetrahydrofuran; 8) toluene + DMSO (80:20); 9)
toluene + DMSO (75:25); 10) amyl alcohol; 11) toluene + DMSO (50:50); 12)
n-propanol; 13) toluene + DMSO (25:75); 14) diethylene glycol; 15) dimethyl
sulfoxide (DMSO); 16) methanol; 17) ethyleneglycol; 18) acetonitryl.
termediate excited state decreases at rotation about the "double" bond and, accordingly, the probability of nonradiative
transitions increases.
PTFDN. As opposed to the previous compounds, for PTFDN a strong fluorescence quenching with increasing
solvent polarity has been revealed. Figure 3 shows the dependence of the logarithm of the nonradiative deactivation
rate constant knr of PTFDN on the solvent polarity function ∆f [21]:
2
ε − 1
n − 1
∆f =
−
,
2
2
ε + 1 2n + 1
where ε and n are the permittivity and the refractive index of the solvent. The small slope in the dependence of ln
knr on ∆f is indicative of a low quenching efficiency and is observed in low-polarity solvents (toluene, benzene, chlo-
roform, diethyl and dibutyl ethers). With further increase in ∆f the fluorescence quenching efficiency sharply increases.
Such a trend of the dependence of ln knr on ∆f can be explained by the existence of two states: a weakly polar state
(with a small intramolecular charge transfer) and a strongly polar state (with a significant charge transfer). The second
state stabilizes mostly in polar solvents, and in weakly polar solvents emission occurs from the weakly polar excited
state. In so doing, the dipole moments of the polar and nonpolar states can differ widely. For example, for such a
known dye as neutral red the dipole moments of these states are 17.5 and 4.8 D, respectively [23].
It is necessary to note the deviations from the general linear dependence in Fig. 3 for a number of polar pro-
ton-containing solvents (10 — amyl alcohol, 12 — n-propanol, 16 — methanol). This fact can be attributed to the
ability of these solvents to interact with a fluorophore molecule, i.e., form a complex with a hydrogen bond. The spe-
cific interaction thereby leads to a decrease in the polarity of the excited complex fluorophore + solvent and, as a con-
sequence, to a decrease in the nonradiative deactivation rate constant knr (see Fig. 3).
The radiative rate constants calculated from the absorption spectra on the basis of the approximate equation
[24]
2
ν ε
∆ν0.5
8
abs
kr
0 max
=
3
.5⋅10
235

Fig. 4. Stationary absorption (1) and fluorescence (2) spectra and nonstationary
spectra of PTFDN in n-propanol at 293 K. The fluorescence spectrum 2 was
recorded at λexc = 560 nm. The time delays are 8 (3), 90 (4), and 160 psec
(5).
(
∆
ν0 and εmax are the wave number and the molar decimal absorption coefficient at the absorption-band maximum and
−
1
ν0.5 is the absorption-band half-width in cm ) and from the experimental data on the lifetimes τ and quantum yield
f
Φ of fluorescence
f
Φf
em
kr
=
,
τf
8
−1
within the measurement error are independent of the solvent nature and their mean value is (1.7 ± 0.3)⋅10 sec . This
fact indicates that the long-wave absorption band of PTFDN is formed by one electron S ← S0 transition and the
1
processes of long-wave absorption and fluorescence are associated with the S state alone.
1
With the aim of investigating the spectral-kinetic properties of the S state of PTFDN in a polar solvent, we
1
made picosecond measurements of the spectra of this OT in n-propanol at various delay times after the pulse excita-
tion (Fig. 4). With increasing delay time the amplification bands (curves 3–5) shift into the long-wave region (from
665 nm at ∆t = 8 psec to 700 nm at ∆t = 160 psec) and the bleaching bands — into the short-wave region (from 600
to 565 nm). It should be noted that the stationary fluorescence (curve 2) and fluorescence excitation (not shown) spec-
tra of PTFDN in n-propanol at 293 K remain unchanged in both the shape and the position at different excitation and
recording wavelengths, i.e., the observed changes in the time-resolved amplification and bleaching spectra are associ-
ated with the excited S state.
1
We revealed an interesting fact of a hypsochromic shift of the bleaching spectra of PTFDN in n-propanol
−
1
with increasing delay time ∆t: at ∆t = 160 psec the dynamic Stokes shift ∆ν = 3415 cm . For comparison, in the
St
−
1
case of the stationary fluorescence and absorption spectra ∆νSt = 2600 cm . Thus, the time-resolved amplification and
bleaching spectra of PTFDN in n-propanol are characterized by a Stokes shift increasing with time. This points to the
fact that the molecular system displays a marked spectral inhomogeneity and consists of a set of absorbing and emit-
ting centers with time-varying surroundings from the solvent molecules. Inhomogeneity also appears in the kinetic
characteristics of PTFDN and consists of the fact that the kinetics of the intermediate processes of PTFDN in n-
propanol are characterized by different decay times depending on the recording wavelength. For instance, in amplifica-
tion spectrum recording, the decay time increases with increasing λrec (43 psec at 660 nm and 93 psec at 700 nm).
Similar changes in the fluorescence decay kinetics at various λrec were also observed for phthalimides [25] and cou-
marin [26] in alcohols in investigating their nanosecond [25] and subpicosecond solvation relaxation [26].
Conclusions. On the basis of the obtained experimental and theoretical data for the investigated OTs the fol-
lowing conclusions can be drawn.
236

The photophysical properties of PCT and PCTT strongly depend on the chain length. For instance, in the PCT
molecule (n = 1) practically the whole of the electron excitation energy (≥99%) is deactivated by means of internal
conversion. In the case of PCTT (n = 3), an increase in the solvent polarity leads to an increase in Φ , and in ace-
f
tonitryl Φ = 0.87. We associate this fact with the stabilization of the singlet-excited PCTT molecule by the polar sol-
f
vent due to the specific interactions and the corresponding decrease in the probability of nonradiative transitions
through rotary motions in a long, flexible molecule.
The effective nonradiative deactivation of the electron excitation energy of PDCVT in low-viscosity solvents
at room temperature is due to the transition of the molecule from the singlet-excited state to the intermediate lower
state with a "twisted" conformation (about the 1′−2′ bond) from which a nonradiative transition to the ground state oc-
curs. An increase in the solvent viscosity increases the activation barrier for the transition to this intermediate state, as
a result of which the quantum yield of fluorescence for the PDCVT molecules increases and at 77 K in EPA it is
equal to unity. In this case, the solvent polarity plays no important role.
In the case of the PTFDN molecule, the main state responsible for the nonradiative process is the state with
charge transfer, whose energy decreases with increasing polarity of the solvent. The PTFDN molecule polarized in the
S1 state features a large dynamic Stokes shift caused by the time-varying inhomogeneous surroundings of the polar
solvent.
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