Photophysical properties of Sn (IV)tetraphenylporphyrin-pyrene dyad with a β-vinyl linker

Article abstract of DOI:10.1142/S1088424615500108

A Sn(IV)tetraphenylporphyrin (T) has been functionalized with a β-vinyl pyrene (P) and the photophysical properties of the formed dyad (T-P) with its corresponding precursors were studied in three solvents with different polarities using steady-state and

Full text of DOI:10.1142/S1088424615500108

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2015; 19: 288–300
DOI: 10.1142/S1088424615500108
Published at http://www.worldscinet.com/jpp/
Photophysical properties of Sn(IV)tetraphenylporphyrin-pyrene
dyad with a b-vinyl linker
P. Silviya Reeta*a, Adis Khetubola,Tejaswi Jellab, Vladimir Chukhareva,
Fawzi Abou-Chahinea,Nikolai V.Tkachenkoa◊,L. Giribabu*b
and Helge Lemmetyinen*a◊
a Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541,
FIN-33101 Tampere, Finland
b Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road,
Tarnaka, Hyderabad 500007, Telangana, India
Dedicated to Professor Shunichi Fukuzumi on the occasion of his retirement
Received 30 October 2014
Accepted 25 November 2014
ABSTRACT: A Sn(IV)tetraphenylporphyrin (T) has been functionalized with a b-vinyl pyrene (P) and
the photophysical properties of the formed dyad (T-P) with its corresponding precursors were studied
in three solvents with different polarities using steady-state and time-resolved measurements in ps
and fs timescales. When the pyrene moiety is excited at lex = 340 nm, the fluorescence spectroscopy
experiments indicate in all the studied solvents, an efficient quenching of the pyrene emission. When
excited at either lex = 340 nm or lex = 405 nm, where porphyrin absorbs, a new emissive excited state
complex (T-P)* is observed at wavelenghts close to the parent porphyrin emission. The emission is
more pronounced in nonpolar hexane showing a mono-exponential decay, but bi-exponential decays are
observed in more polar dicloromethane and acetonitrile. When the porphyrin moiety is excited at lex =
425 nm, the fs transient absorption analysis shows two different intermediate species (~7–11 ps and
80–100 ps) with broad absorption in the near-IR region. This implies either the existence of two different
excited conformers (T-P)*, which decay to the ground state via a charge separated state (CSS), or the
formation of the (T-P)* state via the second excited state of the porphyrin moiety, yielding first an excited
emissive v(T-P)* state, with a lifetime of 80–100 ps.
KEYWORDS: Sn(IV)tetraphenylporphyrin, b-vinyl pyrene donor, optical properties, fs transient
absorption, kinetics.
Though extensive reports are available describing the
INTRODUCTION
intramolecular excitation energy transfer (EET) and
Porphyrin and their metallo derivatives render as
photoinduced electron transfer (PET) in a variety of
potential chromophores in a wide variety of organo-
porphyrin-based donor-acceptor (D-A) systems with
electronic applications [1–8]. While their close
resemblance with the natural photosynthetic reaction
center [9–13] represent them as unique model systems
a donor/acceptor attached either at a peripheral (-meso
[14–19] or b-pyrrolic [20–24]) or at axial position
of resident metallo/metalloid center [27–31], there
to understand the more complex photoinduced electron
endures a continual interest in exploring the excited state
and energy transfer reactions occurring in nature [7].
behavior of such D-A systems. Markedly, functionalizing
the pyrrole-b position with donors/acceptors has gained
special importance due to the favorable in-plane orbital
◊ SPP full member in good standing
overlap of the peripheral donor with the p-conjugated
macrocycle, which sequentially enhances the effective
*Correspondence to: Helge Lemmetyinen, email: helge.
lemmetyinen@tut.fi, tel: +358 40-581-1347
Copyright © 2015 World Scientific Publishing Company

PHOTOPHYSICAL PROPERTIES OF SN(IV)TETRAPHENYLPORPHYRIN-PYRENE DYAD WITH A b-VINYL LINKER
289
OH
Sn
OH
Sn
OH
Sn
N
N
N
N
N
N
N
N
N
N
N
N
HO
SnTPP(OH)2
HO
SnTPP(OH)2-vinyl
HO
SnTPP(OH)2-PYR
Fig. 1. Molecular structure of the Sn(IV)porphyrin-pyrene dyad and its precursors
electron/energy transfer between the peripheral substi-
tution with the porphyrin ring [20–24].
GmbH) system consisted of PicoHarp-200 controller, a
PDL-800-B driver and pulsed LED (PLS-8-2-295) and
pulsed diode (LDH-P-C-405B) exciting samples at 340
and 405 nm, respectively. The detection part consisted
of micro-channel photomultiplier tube (R3809u-50
Hamamatsu) coupled with a monochromator. The time
resolution of the system was roughly 300 and 60 ps with
excitation at 340 and 405 nm, respectively. The data
were analyzed using a freely available software package
Decfit. The quality of the fit was ascertained from the c2
values and the distribution of residuals. Differential pulse
voltammetric (DPV) technique was performed using a
PC-controlled electrochemical analyzer (CH instrument
model CHI620C) using Ferrocene as the standard and
0.1 M TBAP (vs.) SCE under standard experimental
conditions.
The fluorescence decay associated spectra (DAS) were
obtained by global fitting of the fluorescence decays,
measured at every 10 nm over the region of the emission
spectrum (550–800 nm) (c2 ~ 1.1), at a fixed excitation
intensity and measurement time. The relative amplitudes
of the decay components were further corrected by a
function corresponding to the detector sensitivity [33].
Ultrafast fluorescence decays were measured using
an up-conversion method described elsewhere [34] with
a temporal resolution of ~150 fs. In brief, fundamental
pulses of 840 nm produced by a Ti:Sapphire laser (TiF50,
CDP-Avesta) at 80 MHz repetition rate were split into
two beams. One portion underwent second harmonic
generation to excite the sample at 420 nm, generating
emission. The second portion of the fundamental pulse
beam was passed to a delay line and then mixed with
the emission to achieve frequency up-conversion. The
resulting UV photons were detected by a photon counting
photomultiplier coupled with a monochromator with a
typical averaging of 10 s at each delay time.
The fascinating photophysical aspects of Sn(IV)
porphyrin supramolecular architectures were investigated
previously [29] by employing a wide range of axially
ligated donors and acceptors. Recently, we investigated
photophysical properties of porphyrin-pyrene dyad
in which pyrene is connected at b-pyrrolic position
of either a free-base or Zn(II) porphyrin [24]. In this
report, the macrocyclic Sn(IV)porphyrin conjugation
is extended with a pyrene chromophore via a vinylene
spacer at the b-pyrrolic position (Fig. 1) and its influence
on the structural, redox, and excited state properties of
the macrocycle have been dealt in detail using various
spectroscopic techniques in the subsequent sections.
Further research on axial substitution of the Sn(IV)
porphyrin-pyrene dyad with specific acceptor moities is
being continued in our laboratory.
EXPERIMENTAL
General
All the reagents and solvents used for the reactions
were purchased from Sigma-Aldrich. The solvents
utilized for the electrochemical and spectral analysis
were further purified by following standard procedures
[32]. Moisture sensitive reactions were performed under
an inert atmosphere of nitrogen. Chromatographic
purifications were performed using Silica gel (60–120
mesh) and neutral or basic alumina (activity grade I).
The UV-visible spectra were recorded on a Shimadzu
(Model UV-3600) spectrophotometer using 1 × 10-6
M
concentrations.Thesteady-statefluorescencemeasurements
were performed on a Fluorolog-3 spectrofluorometer (Spex
Model, JobinYvon).Theconcentrationofthesolutionswere
maintained constantly at OD < 0.1 for all the compounds
in order to eliminate the possibility of quenching caused by
the aggregation and to minimize nonlinear absorption and
re-absorption effects.
For transient absorption measurements, a sub-
picosecond resolution setup was used and described in
detail elsewhere [34, 35]. Briefly, 800 nm laser pulses
at 1 kHz repetition rate were generated by a Ti:Sapphire
laser system (Libra F, Coherent Inc.). The fundamental
pulses were split into two beams; one pump beam was
guided through an optical parametric amplifier (Topas C,
Light Conversion Ltd.) to generate the excitation pulses
The time correlated single photon counting (TCSPC)
method was used for measuring the fluorescence lifetime
ofthemoleculesintheirexcitedstate.Thebase(PicoQuant
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290
P. S. REETA ET AL.
at 340 and 425 nm. The pump beam and the rest of
fundamentalweredeliveredtoapump-probemeasurement
system (ExciPro, CDP Inc.). The system generated a white
light continuum (WLC) from the 800 nm beam which
was used as a probe pulse. The probe was guided through
a delay line with a moving right angle reflector that
changed the optical path length of the probe with respect
to the pump beam. The maximum time scale available to
monitor absorption changes was ca. 6 ns. The probe pulse
was split into two to obtain signal and reference beams
that both passed through the sample. The signal beam
was overlapped with the pump beam while the reference
beam was not. The excitation was modulated by a chopper
synchronized excitation pulses to detect probe pulse
spectra with and without the excitation and to calculate
differential transient absorbance for each excitation pulse.
Measurements were recorded with a 10 s average for each
delay time, i.e. averaging 10000 excitation shots. The
spectra were acquired in two ranges, 460–780 nm and
850–1050 nm. The measurements around fundamental
wavelength, 800 nm, were unreliable since the continuum
was very uneven close to the fundamental.
The raw data were fit globally by a sum of exponents
to perform data analysis, a procedure which has been
described in more detail previously [34]. Briefly, the
number of exponents needed for a reasonable fit quality
yielded the number of transient species in the photo-
induced processes. The rate constants for the formation
or relaxation of transient species can be calculated from
the respective lifetimes of each component. The results
of the fits are presented as decay component spectra with
the amplitudes of the exponents plotted as functions of
wavelength.
A 10% aqueous NaOH solution was added to the CHCl3
solution of the corresponding dichloro Sn(IV) porphyrin
and the mixture was stirred at room temperature for 2 h.
The organic layer was separated and dried to get violet solid
of the corresponding dihydroxy Sn(IV) porphyrin, which
were recrystallized from CH2Cl2-hexane. Yield upto 90%.
SnTPP(OH)2.Yield90%. ESI-MS:m/zC44H30N4O2Sn,
(765.44): [M + H+] 766 (85%). 1H NMR (CDCl3): d, ppm
9.15 (s, 8H), 8.18 (m, 8H), 7.80 (m, 12H).
SnTPP(OH)2-vinyl. Yield 87%. ESI-MS: m/z
1
C46H32N4O2Sn, (791.50): [M + 2H+] 793 (100%). H
NMR (CDCl3): d, ppm 8.75 (multiplet, 7H), 8.15 (m,
8H), 7.75 (multiplet, 12H), 6.49 (dd, 1H, J = 1.0 Hz, J =
10.8 Hz, J = 17.1 Hz), 5.90 (dd, 1H, J = 2.0 Hz, J = 17.1
Hz), 5.15 (dd, 1H, J = 2.4 Hz, J = 10.8 Hz).
SnTPP(OH)2-PYR. Yield 90%. ESI-MS: m/z
1
C62H40N4O2Sn, (991.44): [M + H+] 991 (50%). H NMR
(CDCl3): d, ppm 9.45 (s, 1H), 9.11 (m, 6H), 8.56 (s, 1H),
8.38 (m, 8H), 8.21 (m, 4H), 8.12 (s, 2H), 8.01 (m, 2H), 7.86
(m, 12H), 7.32 (d, 1H, J = 15.9), 7.16 (d, 1H, J = 15.9 Hz).
RESULTS AND DISCUSSION
Preliminary characterization of the molecules
1
synthesized was performed with ESI-MS, H NMR, and
UV-visible absorption spectroscopy. Mass spectral data
m/z, (Rel.Int.%) of [SnTPP(OH)2-vinyl]: [M + 2H]+, 793
(100) and [SnTPP(OH)2-PYR]: [M + H]+, 991 (50) and
1
the corresponding H NMR spectra recorded in CDCl3
were shown in supplementary material (Fig. S1 to S3) and
the chemical shift data in d, ppm using tetramethylsilane
(TMS) as the internal standard was summarized in the
experimental section. Both the molecules [SnTPP(OH)2-
vinyl] and [SnTPP(OH)2-PYR] displayed the charac-
teristic b-pyrrolic and meso-proton resonance positions of
corresponding SnTPP(OH)2 precursor. Resonance due to
–CH=CH2 in [SnTPP(OH)2-vinyl] appear at d 6.49 (dd,
1H, J = 1.0 Hz, J = 10.8 Hz, J = 17.1 Hz), 5.90 (dd, 1H,
J = 2.0 Hz, J = 17.1 Hz), 5.15 (dd, 1H, J = 2.4 Hz, J =
10.8 Hz). In [SnTPP(OH)2-PYR], the pyrene protons
appear as more overlapping peaks along with the signals
of porphyrin-meso protons in the aromatic region while
the two vinylic protons appear as doublets at d 7.32 (d, 1H,
J = 15.9) and 7.16 (d, 1H, J = 15.9 Hz).
Synthesis
The 5,10,15,20-tetraphenylporphyrinato tin(IV)
[SnTPP(OH)2] [36], 2-vinyl-5,10,15,20-tetraphyenyl-
porphyrinato tin(IV) [SnTPP(OH)2-vinyl] [37], and
2-pyrenyl-5,10,15,20-tetraphyenylporphyrinato tin(IV)
[SnTPP(OH)2-PYR] [20, 22] were synthesized by fol-
lowing the literature reported procedures. The synthetic
details are presented in the experimental procedure below.
General procedure for the synthesis of dihydroxy
Sn(IV)porphyrins
Optical properties
The dihydroxy tin(IV) porphyrins were synthesized by
refluxing the corresponding free-base porphyrin (2.0 g,
3 mM, for representative H2TPP) and SnCl2·2H2O (3.0 g,
13 mM) dissolved in 50 mL of pyridine for 2 h. Then
the mixture was cooled and pyridine was removed under
reduced pressure. The solid obtained was dissolved in
CHCl3 and washed several times with water. The organic
layer was dried by passing through anhydrous Na2SO4
and chromatographed over basic alumina. Elution with
CHCl3/CH3OH (98:2 V/V) gave the corresponding
dichloro tin(IV) porphyrins. Yield upto 95%.
The UV-visible absorption spectrum of the dyad is
compared with those of its precursors in Fig. 2 and the
data are summarized in Table 1. The absorption of pyrene
is mainly at the 300–350 nm region. For SnTPP(OH)2-
vinyl and SnTPP(OH)2 the absorption bands are
displayed at the very same wavelength regions with
intense Soret bands appearing at around 425 nm and two
less intense Q-bands around 500 to 620 nm region.
The absorption peaks are slightly broadened and red
shifted by ~5–6 nm for the SnTPP(OH)2-PYR dyad
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PHOTOPHYSICAL PROPERTIES OF SN(IV)TETRAPHENYLPORPHYRIN-PYRENE DYAD WITH A b-VINYL LINKER
291
DEp = 65
3 mV for ferrocenium/ferrocene couple)
reactions. The peaks occurring at anodic potentials are
ascribed to successive one-electron oxidations of the
porphyrin parts of SnTPP(OH)2/SnTPP(OH)2-PYR.
As seen in Fig. S5, the pyrene redox potential appears
as overlapped peaks in SnTPP(OH)2-PYR compared
to the precursor SnTPP(OH)2. Quite interestingly,
when appended with peripheral pyrene (SnTPP(OH)2-
PYR), the first oxidation potential of porphyrin, shows a
cathodic shift by nearly 80 mV compared to its precursor
SnTPP(OH)2. This observation accounts for the influence
of attaching a peripheral electron donating pyrene in
SnTPP(OH)2-PYR on the redox properties of Sn(IV)
porphyrins.
The spectroscopic and electrochemical features of the
Sn(IV)porphyrin-pyrene dyad described above suggest
that in the ground state, an electronic communication
between the porphyrin and pyrene chromphores, though
not significant, but could not be completely ruled out.
Nevertheless, the excited state properties of individual
chromophores could be more deeply exploited by
selective excitation of the individual chromophore units.
Fig. 2. UV-visible absorption spectra of pyrene and the studied
Sn(IV)porphyrin derivatives in CH2Cl2
Table 1. UV-visible data (1 × 10-6 M for Soret and 1 × 10-3
for Q-bands)
M
Compound
Absorption, lmax, nm (log e, M-1.cm-1)a
Porphyrin bands
Pyrene bands
—
Fluorescence quantum yields
SnTPP(OH)2
425
560
600
The emission spectra of the Sn(IV)porphyrin
precursors and dyads were measured in different
solvents with increasing polarity, in hexane (Hex),
dichloromethane (DCM), and acetonitrile (AcCN).
The recorded emission spectra are displayed in Figs 3
and 4, and the data are summarized in Table 2. The
fluorescence quantum yields of the reported compounds
were calculated with reference to ZnTPP (f = 0.036 in
CH2Cl2) [38]. The efficiency of fluorescence quenching
was measured by employing Equation 1;
(5.07) (4.10) (3.85)
SnTPP(OH)2- 426
vinyl (5.01) (4.05) (3.05)
562
601
—
SnTPP(OH)2- 426 557 597
(5.06) (4.03) (3.54) (3.30) (4.30) (4.39)
290 333 376
239
290
347
PYR
Pyrene
—
(4.55) (4.39) (4.34)
aSolvent CH2Cl2, error limits: lmax
,
1 nm, log e, 10%.
Φ(D) − Φ(DA)
Q =
(1)
due to the presence of extended conjugation via the
vinyl spacer. The results indicate a weak ground state
interaction between the porphyrin and pyrene moieties in
SnTPP(OH)2-PYR.
Φ(D)
where f(D) refers to the fluorescence quantum yield
of pyrene or SnTPP(OH)2 whereas f(DA) refers to the
quantum yield of SnTPP(OH)2-PYR.
The emission measurements of the precursor porphyrin
were performed at lex = 425 nm while the dyad molecule
Electrochemical properties
was excited at two different wavelengths, i.e. lex
=
The redox curves of the reported Sn(IV)porphyrins
with their corresponding precursors were measured by
differential pulse voltammetry with CH2Cl2 solvent and
0.1 M TBAP as supporting electrolyte and the data are
summarized in supplementary material (Table S1 and
Fig. S5). Figure S5 and data in Table S1 indicate that
the dyad shows up to two reduction and three oxidation
peaks under the experimental conditions employed in this
study. Wave analysis suggested that, in general, while
the first two reduction steps and the first two oxidation
steps are reversible (ipc/ipa = 0.9–1.0) and diffusion-
controlled (ipc /n1/2 = constant in the scan rate (n) range
50–500 mV/s) one-electron transfer (DEp = 60–70 mV;
340 nm or 425 nm, where the individual subunits pyrene
and porphyrin, respectively, absorb predominantly.
Considering the dyad molecule, the peripheral pyrene
emission (lem = 350–500 nm) overlaps with the porphyrin
absorption maximum (labs = 420 nm) as shown in Fig. S4,
whichshowsthepossibleoccurrenceoftheenergytransfer
from the pyrene to the Sn(IV)porphyrin moiety, which is
supported by the efficient quenching of pyrene emission
(Fig. 3) upto 98% in the dyad at lex = 340 nm. This is
also supported by the spectral overlap of the excitation
spectra of the dyad with its UV-visible absorption spectra
in Hex and AcCN (Figs S6 and S7). More importantly, at
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P. S. REETA ET AL.
600–750 nm the emission is enhanced with a variation
in the intensity ratios accompanied with pronounced
broadening and red-shifted maxima compared to that
of the SnTPP(OH)2 emission (Fig. 3). A similar trend
is also observed when the porphyrin chromophore was
excited at lex = 425 nm (Fig. 4).
Thus, irrespective of the chromophore being excited
the dyad shows distinct emission features, which could be
possibly due to the formation of an intermediate species,
which does not exactly have the emission properties
of the neat porphyrin moiety, but is also influenced by
the pyrene, covalently linked to it by a double bond.
This new intramolecular emissive transient state is a
noticeable interesting phenomena in this report. The
occurrence of this intermediate species is investigated by
the time-resolved measurements at ps and fs time scales,
the details of which will be described in the subsequent
sections.
Fig. 3. Fluorescence spectra of pyrene in hexane and
SnTPP(OH)2-PYR in three different solvents. lex = 340 nm
Time-resolved fluorescence experiments
As evidenced from the fluorescence quantum yield
(QY) data, a relatively efficient quenching of pyrene
monomer emission possibly due to energy transfer,
was observed in the SnTPP(OH)2-PYR. This leads
to the formation of a “new emission band” at a longer
wavelength, which is no longer characteristic of the
pyrene monomer. This new emission band reflects the
formation of new excited state which combines properties
of both pyrene and porphyrin monomers and could be
described as an intramolecular transient state with dual
fluorescence properties (see Figs 3 and 4).
To gain further insight of the intramolecular
photophysical processes between the pyrene and
porphyrin moieties in the dyad, time-correlated single
photon counting (TCSPC) experiments were performed
Fig. 4. Fluorescence spectra of SnTPP(OH)2 in Hex and
SnTPP(OH)2-PYR in Hex, DCM, and AcCN. lex = 425 nm
Table 2. Fluorescence dataa of the studied compounds
lem, nm (f, %Qb)
Compound
Hexane
CH2Cl2
CH3CN
Hexane CH2Cl2 CH3CN
lex = 425 nm
lex = 340 nm
SnTPP(OH)2
605, 669
(0.043)
605, 661 604, 659
(0.048)
605, 661 606, 660
(0.040) (0.045)
622, 674 626, 664 383, 394, 456 386, 397, 456 385, 397, 455
(0.066)
SnTPP(OH)2-Vinyl 606, 661
(0.037)
SnTPP(OH)2-PYR 630, 679
(0.056)
(0.039)
(0.020)
(0.001, 95) (0.002, 98) (0.001, 98)
624, 676 637, 677 630, 673
(0.083) (0.077) (0.023)
Pyrene
—
—
—
381, 391, 403 372, 383, 392 372, 382, 392
(0.021) (0.087) (0.047)
aError limits: lex, 2 nm, f 10%. bQ is defined in Equation 1 (see text).
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PHOTOPHYSICAL PROPERTIES OF SN(IV)TETRAPHENYLPORPHYRIN-PYRENE DYAD WITH A b-VINYL LINKER
293
(a)
(b)
Fig. 5. Fluorescence decays of (a) pyrene and (b) the SnTPP(OH)2-PYR in Hex, DCM, and AcCN. lex = 340 nm and lem = 385 nm
in three different solvents with varied static dielectric
constants. First, we collected the fluorescence decays of
the solutions of the dyad and compared them with those
of the corresponding reference compounds, i.e. pyrene
and SnTPP(OH)2 (Figs 5 and 6) and SnTPP(OH)2-
vinyl (Fig. S8). Each sample was excited either at
340 nm (pyrene absorption) or at 405 nm (porphyrin
absorption) and the decay times were measured at
their corresponding emission maxima in three different
solvents. The corresponding decay curves are shown in
Figs 5 and 6 and the data are summarized in Tables 3 and 4
(complete data in Table S2 of supplementary material).
From Fig. 5 and Table 3, when excited at lex = 340 nm
and monitored the pyrene emission at 385 nm in both
polar and nonpolar solvents, the decays of the dyad are
bi-exponential and the fluorescence lifetimes shorter than
that for the pyrene monomer. The shorter (~1 ns) decay
component, contributes 75–91% to the fluorescence.
The decreased lifetimes correspond to efficient excit-
ation energy transfer (EET) from pyrene à porphyrin.
Moreover, the quenching is more efficient in the nonpolar
solvent (hexane) where the larger contribution (91%) of
this component is observed. The quenching rate constants
were calculated using Equation 2.
Φ0f



1
1
1
kq =
−1 =
−
(2)

t0f t k
tf t0f


f
f
Here f0f is the fluorescence quantum yield of the donor
in the absence of acceptor. kf is the fluorescence decay
rate constant of the donor. tf and t0f are the decay times
of the donor in the presence and absence of the acceptor,
respectively. The time tf corresponds to the lifetime of
the shorter living component observed for the dyad, while
t0f corresponds to the lifetime of the single exponential
decay observed for the monomer. The calculated rate
constants are summarized in Table 3.
The results in Table 3 are in a relatively good
agreement with the quantum yield data collected from the
steady-state emission experiment (Table 2). Furthermore,
the decay of the dyad emission by two different time
constants might be due to an equilibrium formed between
the excited monomer in the dyad and the excited emissive
state, as will be discussed later. On the other hand it
Table 3. Fluorescence decay times, amplitudes and quenching rate constants (kq, s-1)‡ at
lex = 340 nm and lem = 385 nm analyzed from the fluorescence decay curves in Fig. 5
Compound
Solvent
t1, ns
t2, ns
kq, s-1
Pyrene
Hexane
DCM
8.20 (100%)
28.81 (100%)
15.12 (100%)
0.77 (90.8%)
1.12 (75.3%)
0.94 (86.3%)
—
—
—
—
AcCN
Hexane
DCM
—
—
SnTPP(OH)2-PYR
7.59 (9.2%)
10.91 (24.7%)
10.27 (13.7%)
1.18 × 109
0.86 × 109
1.00 × 109
AcCN
kq ‡ is the quenching rate constant of the pyrene emission observed for the SnTPP(OH)2-
PYR dyad calculated according to Equation 2. Error limits of t and kq ~ 10%. Values in
parenthesis are the relative amplitudes of corresponding decay components.
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P. S. REETA ET AL.
might imply the formation of two distinguished excited
states, which decay to the ground state with different
time constants, which is probably caused by a change of
the molecular conformation [18].
From Fig. 6 and Table 4, when excited at lex = 405 nm,
and monitored at the porphyrin emission maxima,
650 nm–675 nm, the decay curves of the dyad were
bi-exponential in DCM and AcCN, with the shorter
lifetime component having the major amplitude. Whereas
in hexane the decay was mono-exponential. Thus, in
DCM and AcCN, irrespective of the wavelength of the
chromophore being excited, i.e. lex = 340 nm (where
the absorption ratio is about 1.7:1 for pyrene:porphyrin)
or 405 nm (which excites selectively porphyrin
chromophore), the fluorescence decays can be analyzed
as a sum of two exponentials. The decay times and
amplitudes are presented in Table 4.
Comparing the lifetimes in Tables 3 and 4, one
can observe that monitoring either at 385 nm (pyrene
emission) or at >650 nm (porphyrin emission), the
decays for SnTPP(OH)2-PYR are bi-exponential in all
solvents, when monitored at the pyrene or porphyrin
emission wavelengths, except the porphyrin emisssions
in hexane. In the case of nonpolar hexane, the decay
is mono-exponential and the lifetime is the same as
that of SnTPP(OH)2. The results suggest that the final
excited state of the dyad combines the properties of
both pyrene and porphyrin monomers and can no longer
be considered as individual monomers. The relative
amplitudes of the decay components however are slightly
different at different excitation wavelengths. Especially
the longer decay times in the later case (lex = 405 nm) are
considerably shorter.
For the emission at wavelength >410 nm the time
constants are very close to each other regardless of the
chromophore being excited, i.e. in pyrene or porphyrin
absorption band. This could be attributed to the
stabilization of only one intermediate species in nonpolar
hexane, whereas in polar solvents, there is a probability
for the formation of different excited states, e.g. a charge
separated or a conformeric state, which both could
account for the two decay components.
Decay associated spectra
As mentioned above the final excited state of the
dyad combines properties of both the chromophores and
can no longer be considered as pyrene (P) or porphyrin
(T) moieties individually. The relative amplitudes of
the decay components are slightly different at different
excitation wavelengths. Namely, the shorter living
component has little larger contribution when excitation
occurred at 340 nm, while the longer living component
has little larger contribution when excitation occurred at
405 nm. The decay of the SnTPP(OH)2-PYR (T-P) dyad
emission by two different time constants might be due, on
Fig. 6. Fluorescence decays of SnTPP(OH)2 (, lex = 405 nm,
lem = 650 nm) and SnTPP(OH)2-PYR (DDD, lex = 405 nm,
lem = 675 nm) and SnTPP(OH)2-PYR (+++, lex = 340 nm, lem
=
675 nm) in (a) Hex, (b) DCM, and (c) AcCN
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PHOTOPHYSICAL PROPERTIES OF SN(IV)TETRAPHENYLPORPHYRIN-PYRENE DYAD WITH A b-VINYL LINKER
295
Table 4. Fluorescence decay times, amplitudes and quenching rate constant (kq, s-1)‡ at lex = 340 or 405 nm, and lem at
porphyrin emission maxima analyzed from the fluorescence decay curve in Fig. 6
Compound
Solvent
lexc, nm
lem, nm
t1, ns
t2, ns
kq, s-1
SnTPP(OH)2
Hexane
DCM
405
650
1.54 (100)
1.39 (100)
1.67 (100)
1.36 (100)
1.52 (100)
0.91 (88.8)
0.63 (52.5)
0.50 (84.5)
0.57 (87.4)
—
—
—
—
AcCN
Hexane
—
—
SnTPP(OH)2-PYR
340
405
340
405
340
405
675
675
675
675
675
675
—
No quenching
No quenching
0.38 × 109
0.87 × 109
1.40 × 109
1.15 × 109
—
DCM
2.07 (11.2)
1.22 (47.5)
1.67 (15.5)
1.65 (12.6)
AcCN
kq ‡ is the quenching rate constant of the pyrene emission observed for the SnTPP(OH)2-PYR dyad calculated according
to Equation 2. Error limits of t and kq ~ 10%. Values in parenthesis are the relative amplitudes of corresponding decay
components.
(a)
(b)
Fig. 7. The fluorescence decay associated spectra of T-P dyad in Hex (a) and in AcCN (b) at 405 nm excitation. The time constants
with negative amplitudes, about 30 and 50 ps, are on the limit of the time resolution of the instruments
one hand, to an equilbrium formed between the excited
monomer in the dyad and the excited emissive state.
This could be especially in the case of pyrene emission
(Table 3). On the other hand it might imply the formation
of two distinguished excited states, which decay to the
ground state with different time constants, which could be
caused either by a change of the molecular conformation
[18] or by reactions of different excited state, e.g. the first
and second excited state.
The results of DAS measurements are shown in Fig. 7.
Two different components are formed in times of few
tens of picosecond (31 and 50 ps) and decay mono-
exponentially (1.3 ns) in hexane, whereas bi-exponentially
(280 ps and 1 ns) in AcCN. Although the formation times
of 30–50 ps are on the limit of the time resolution of
TCSPC-method, and thus that of the DAS-method, these
components could imply a formation of the (T-P)* state
from the primary excited T*-P state.
Since the two distinguished decay components were
observed from the dyad in the polar solvents, it is worth
to further investigate how these species contribute over
the range of the emission band. Hence the fluorescence
decay associated spectra (DAS) of the T-P dyad in hexane
and AcCN were measured.
Fluorescence up-conversion experiments
Thefluorescenceup-conversionlifetimemeasurements
for T-P dyad in AcCN gave decay curves with three
major decay components; a fast component, 10 ps, a
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296
P. S. REETA ET AL.
Fig. 8. The fluorescence up-conversion decay curves of T-P dyad in Hex, excited at 420 nm and monitored at 615 nm, with time
windows of (a) 1.1 ns and (b) 200 ps. The decay can be analysed as three-exponential, yielding t1 = 10 ps (84%), t2 = 25 ps (-36%),
and t3 = 1.3 ns (52%). The growing of 25 ps component is indicated by the blue curve
Fig. 9. Transient component spectra of T-P dyad in Hex (a) and AcCN (b) at lex = 340 nm
longer-living component, about 1.3 ns, and a component
with a negative amplitude, 25 ps (see Fig. 8 and its
caption). The determined lifetimes coincide reasonably
well with those obtained by the DAS-method. The very
fast component (10 ps, in up-conversion) could be the
fluorescence lifetime of the T moiety, while the longer-
living (about 1.3 ns) component should correspond to the
emission of the (T-P)* complex. Moreover this 1.3 ns
componentfitsverywellwiththesingle-exponentialdecay
of the dyad in hexane, observed in TCSPC experiment.
The additional intermediate component with a negative
amplitude and lifetime of ~25 ps resembles the 30–50 ps
components observed in the DAS experiments and
corresponds to the formation of (T-P)*.
with excitation both at 340 nm (pyrene absorption) and at
425 nm (porphyrin absorption). The transient absorption
component spectra, at 340 nm excitation, in hexane and
acetonitrile are shown in Fig. 9.
In hexane (Fig. 9(a)) a ~2 ps component corresponds
to the singlet excited state of the pyrene moiety. This is
followed by a 25 ps component, and a formation of the
excited dyad, (T-P)*. This fits very well with the 25 ps
component observed in the fluorescence up-conversion
experiment. (T-P)* has a broad absorption from 620 nm
to 850 nm, with a lifetime of 1.4 ns. These results also
coincide well with fluorescence up-conversion and
TCSPC results. The negative bands (1.4 ns) at 500–620 nm
and 860–940 nm describe the formation of the final
transient state, with lifetime longer than 6 ns, the upper
limit of the instrument, and a broad intense absorption from
500 to 950 nm, with two main maxima. This state could be
cation radical (at shorter wavelength) of SnTPP and anion
radical (at longer wavelength) of pyrene moieties.
Ultra-fast pump-probe experiments
The transient absorption behavior of T-P compound
was studied by the femtosecond pump-probe method
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PHOTOPHYSICAL PROPERTIES OF SN(IV)TETRAPHENYLPORPHYRIN-PYRENE DYAD WITH A b-VINYL LINKER
297
Fig. 10. Transient component spectra of T-P dyad in Hex (a) and AcCN (b) at lex = 425 nm
In acetonitrile (Fig. 9(b)) the two fast components
correspond, as previously mentioned, to the singlet
state decay (2.6 ps) of the porphyrin moiety and the
simultaneous formation (19 ps) of the excited dyad,
(T-P)*. It has a broad absorption band from 600 nm
to 910 nm and considerably shorter (350 ps) lifetime
than in hexane. The final state is the charge transfer
state with the absorption characteristic for porphyrin
cation (600–800 nm) and that of the pyrene anion
(870–950 nm).
The transient absorption component spectra, at 425 nm
excitation, in hexane and acetonitrile are shown in
Fig. 10. In hexane (Fig. 10(a)) the result shows a strong
and broad absorption band with a lifetime of 7 ps and
the maximum at ca. 750 nm, and clear bleaching of
the ground state SnTPP absorption at 580 and 630 nm.
This could correspond to the singlet excited state of the
porphyrin. The next component, 80 ps, does not exist at
340 nm excitation, neither in fluorescence experiments.
It has a very broad absorption band from the visble to
the NIR region. That is followed by 1.3 ns and 8 ns
components, which are similar to those observed with the
340 nm excitation, and assigned to the (T-P)* state (1.3 ns)
and the cation and anion radical (8 ns) of the SnTPP and
pyrene moieties, respectively.
the porphyrin monomer [SnTPP(OH)2]. Moreover, the
single component decay could also suggest that in a
nonpolar solvent the excited state can be regarded as a
state, in which the excitation is located in the porphyrin
moiety, although the emission is red-shifted compared
to that of SnTPP(OH)2 and is less perturbed by the
surrounding solvent molecules.
When the pyrene moiety is excited at 340 nm and
monitored at the pyrene emission wavelength at 385 nm,
the bi-exponential emission decay of the T-P dyad
indicates an equilibrium between the excited ground
state complex, (T-P*), and a newly formed excited
complex between T and P. Energetically this is possible
because the energy of the first excited state of pyrene is
about the same as the energy of the Soret band of the
porpyrin moiety. This is actually shown quite clearly in
Fig. 5, where the decay of the fluorescence monitored
at 385 nm is evidently bi-exponential with lifetimes of
about 1 and 10 ns.
The dynamics when monitored at the pyrene emission
wavelength could be described by an “equilibrium”
kinetics as in Fig. 11.
At 425 nm excitation (Fig. 10(b)) inAcCN the transient
absorption responses are quite similar to those in hexane.
A broad absorption with about 100 ps lifetime was not
observed at the 340 nm excitation nor in the fluorescence
experiments.
FLUORESCENCE KINETICS
In the nonpolar solvent (hexane), independent of
the excitation wavelengths, the fluorescence decays of
the dyad at the porphyrin emission wavelength, can be
analyzed as a single exponential decay with the time
constant around 1.4–1.7 ns, which resembles that of
Fig. 11. The “equilibrium” dynamics when excited and moni-
tored at the pyrene absorption and emission wavelengths,
respectively
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298
P. S. REETA ET AL.
Table 5. Kinetic parameters calculated for SnTPP(OH)2-PYR monitored at pyrene emission. lex = 340 nm. All the rate
constants are presented in × 108 s-1
Solvent
A1
A2
b1
b2
km
X
Y
k1[B]
k-1
ke
Hexane
DCM
0.91
0.75
0.86
0.09
0.25
0.14
13.0
8.93
10.6
1.32
0.92
0.97
1.22
0.35
0.66
11.90
6.95
9.31
2.39
2.90
2.30
10.70
6.60
8.65
1.06
1.81
1.28
1.33
1.09
1.02
AcCN
By using the data in Table 3, the kinetic parameters in
Fig. 11 could be calculated based upon Equations 3–5:
A*
[
]
X −b e−b t + b − X e−b t

0
1
2
(3)
(4)
(5)

A* =
(
)
(
)
[
[
]
2
1


b1 −b2
k A*
[
]
1
−b2t
E * =
e
− e−b t

0
1

]


b1 −b2
1
2




b1,2
=
X +Y  Y − X + 4k1k−1
(
)
 
2
Fig. 12. The kinetic scheme for the decay processes followed
the excitation of the pyrene moiety of the T-P dyad based on
assumption on two conformers of the T-P dyad
In Equations 3–5, X = km + k1 and Y = ke + k-1, where
km is a relaxation rate (radiative + non-radiative) of the
excited pyrene monomer, (T-P*); k1 is the formation rate
of the excited dyad,ke and k-1 are the relaxation (radiative +
non-radiative) rates of the excited dyad, (T-P)*. The
calculated kinetic parameters are presented in Table 5.
With the excitation at 405 nm, where the pyrene
moiety absorbs significanly less than porphyrin, the time
resolved fluorescence decays were also bi-exponential,
but the longer-living components were 5–6 times shorter
in time, 1.6–2.1 ns (Table 4), than those obtained by the
P absorption and emission (Table 3). Thus the emission
at >600 nm does not originate from the equilibrium state,
which was formed and observed with the P excitation and
emission, respectively. Also the relative amplitudes of the
two emission bands (Fig. 4) at about 625 nm and 675 nm
varied depending on solvent polarity and monitoring
wavelength.
Fig. 13. The mechanisms at the 425 nm excitation. The scheme
is based on the assumption on the electron transfer from the
second excited state of porphyrin and the results of the pump-
probe measurements shown in Fig. 10
MECHANISMS
The processes followed after the excitation of the T-P
dyad can be described in all the solvents with practically
the same mechanism (Fig. 12). First, when mainly the
pyrene moiety in the T-P dyad is excited, the excitation
is followed by an energy transfer to the porphyrin
moiety. Then an excited dyad is formed and decays to
a partial charge transfer complex, more efficiently in
polar DCM and acetonitrile, than in nonpolar hexane.
The existence of two different conformers and their
influence on the ground and excited state properties of
a porphyrin dimer linked by a vinylene spacer has been
studied previously [18]. Thus, two possible conformers
due to the vinyl chain, should be taken into account.
Namely, two types of excited dyads, (a-T-P)* and
(b-T-P)* are formed depending on the relative ground
state concentration ratio of the conformers. The excited
dyads have different lifetimes, from about 0.6 to 1.6 ns,
depending on the environment. Finally the excited state
dyads relax to charge transfer complexes and to the
ground state.
Based on the time-resolved fluorescent (Figs 5 and 6)
and pump-probe measurements (Figs 9 and 10) with
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PHOTOPHYSICAL PROPERTIES OF SN(IV)TETRAPHENYLPORPHYRIN-PYRENE DYAD WITH A b-VINYL LINKER
299
either pyrene or porphyrin excitation, it seems evident,
that the mechanisms differ from each other at the
very beginning of the photo-induced processes. The
differences between 340 nm and 425 nm excitations
can be qualitatively explained as follows (Fig. 13).
The porphyrin excitation yields the excitation of the
second excited state of porphyrin in the dyad, (2T*-P).
This opens a possibility for an intramolecular charge
an additional contributing species. However, from the
pump-probe measurements, when excited at 425 nm, two
different intermediate species are observed (~7–11 ps
and 80–100 ps) with broad absorptions in the near-IR
region, which imply the existence of two different
conformers, which decays to the ground state via a CSS.
An alternative explanation is that the intramolecular
electron transfer and the formation of the final CSS state
occurs via the second excited state of the porphyrin.
The dyad synthesized serves as a molecular platform
to demonstrate the influence of the flexible vinyl spacer
on the excited state photophysics of the chromophores
involved. Moreover, the presence of Sn(IV) metal center
could assist in extending the axial coordination of the
dyad molecules, further studies of which is in progress.
2
transfer to occur from T* state of the porphyrin to
the pyrene moiety. This is quantitatively discussed by
Mataga et al. [39] and recently by Fujitsuka et al. [40],
and explains well the observed differences depending
on the excitation wavelength. As the electron transfer
2
would take place from the T* state, it decays, in 1.1–
1
1.7 ps (Fig. 10) to the T* state, (1T*-P), and to an
v
excited dyad, (T-P)*. This state, which is not observed
with the 340 nm excitation (Fig. 9), relaxes in 80–
100 ps to a longer living excited dyad, (T-P)*, which
also is formed via the (1T*-P) state in 7–11 ps. (T-P)*
relaxes in 0.33–1.3 ns to the charge seaprated state,
(Td+-P d-). The mechanism presented in Fig. 13 explains
well the observation of five different component spectra
in Fig. 10.
Acknowledgements
HL acknowledges the financial support from the
Academy of Finland (UVIADEM) and the Finnish
Funding Agency for Technology and Innovation
(Biosphere). And LG acknowledges the Department of
Science and Technology (DST, SB/S1/IC-14/2014) for
financial support of this work.
Analogically to the porphyrin excitation, the pyrene
excitation (Fig. 9) could create an excited complex u(T-P)*
in few picosecond, which relaxes to the (T-P)* state in
19–25 ps. The latter has lifetime 0.3–1.4 ns, depending
on the solvent. The CT state formed after (T-P)* has a
lifetime >10 ns.
Supporting information
Figures S1–S8 and Tables 1–2 are given in the
supplementary material. This material is available free of
charge via the Internet at http://www.worldscinet.com/
jpp/jpp.shtml.
v
Thepropertiesofthetransientstatesof (T-P)*,u(T-P)*,
and (T-P)* resemble those known as intramolecular
exciplexes, because they have emission bands, which
differ from those of porphyrin or pyrene (Figs 4 and 8).
The exciplexes, however, do not have fine structure, as
the emission of the (T-P)* state has two bands. This could
be explained by the formations of two types of (T-P)*
states due to the different conformers [18] of the dyad
in solution (Fig. 12). More intriguing explanation is,
however, formation of different kinds of intramolecular
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