Triplet energy management between two signaling units through cooperative rigid scaffolds

Article abstract of DOI:10.1039/c5cc08102e

Through-bond triplet exciplex formation in donor-acceptor systems linked through a rigid bile acid scaffold has been demonstrated on the basis of kinetic evidence upon population of the triplet acceptors (naphthalene, or biphenyl) by through-bond triplet-

Full text of DOI:10.1039/c5cc08102e

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Triplet energy management between two
signaling units through cooperative rigid
Cite this:DOI: 10.1039/c5cc08102e
scaffolds†
Received 28th September 2015,
Accepted 6th November 2015
Paula Miro, Ignacio Vaya, German Sastre, M. Consuelo Jimenez, M. Luisa Marin
´
´
´
and Miguel A. Miranda*
DOI: 10.1039/c5cc08102e
www.rsc.org/chemcomm
Through-bond triplet exciplex formation in donor–acceptor systems exciplex formation has been investigated in flexible benzophenone
linked through a rigid bile acid scaffold has been demonstrated on (BZP)/naphthalene (NPT) bichromophoric systems, in which decay
3
the basis of kinetic evidence upon population of the triplet acceptors of NPT* occurs through intramolecular exciplex formation
3
6–38
(
naphthalene, or biphenyl) by through-bond triplet–triplet energy with ground state BZP.
transfer from benzophenone.
To our knowledge, the possibility of TB triplet exciplex (TB-TEX)
formation in D–A systems linked through a rigid bridge has not yet
Energy and electron transfer (ET and et, respectively) are two general been explored. Herein, this issue has been addressed by means of a
1
non-radiative pathways in the quenching of a triplet excited state,
novel approach based on a ‘tandem’ TB-TTET with subsequent
which require effective orbital overlap within the van der Waals radii formation of a TB extended exciplex from the triplet acceptor. The
2
of donor (D) and acceptor (A) chromophores. Actually, it is well structural design of the newly synthesized D–A systems (1a,b and
established that kET drops to negligible small values when the D–A 2a,b, Fig. 1) includes a rigid hydrocarbon skeleton provided by the
1
distance is longer than 5–10 Å. When D and A are linked by a natural lithocholic acid (LA) that exhibits an unusual cis fusion
spacer, triplet–triplet energy transfer (TTET) could proceed through- between rings A and B and has a carboxylic acid moiety at the
space or through-bond (TB) mechanisms depending on the lateral chain that allows covalent linking of the acceptor chromo-
nature of the bridge. In flexible bridge-linked D–A systems, a phores. The D is attached at C-3 on the a or b face of the skeleton to
rapid conformational equilibrium leads to spatial arrangements, in analyze the influence of the stereochemistry on the kinetics of the
which the two chromophores are close enough for orbital overlap processes. The selected chromophores, BZP as D and NPT or
within the lifetime of the donor triplet, so that TTET happens biphenyl (BIP) as A, fulfill the following requirements: (i) selective
3–11
through-space.
Conversely, in the case of rigid saturated hydro- excitation of BZP is possible at 355 nm; (ii) the intersystem crossing
carbon bridges, in which D and A distances are substantially longer quantum yield of BZP is very high (ca. 1); (iii) the triplet energies of
À1
À1
than the sum of the van der Waals radii, TB mechanism is largely NPT (60.9 kcal mol ) or BIP (65.4 kcal mol ) are lower than that
12–21
predominating.
In these systems, the observed TTET happening
TB is facilitated by the s and s* orbitals of the bridge and, in
22–24
general, is strongly dependent on the conformations.
Exciplex formation constitutes an alternative channel for excited
1
states deactivation, whose involvement has been reported in the
3,4,25–28
intramolecular singlet quenching of flexible systems.
Inter-
estingly, several examples have been reported, in which photo-
induced electron transfer in D–A systems linked through a rigid
bridge leads to ‘exciplex’ type fluorescence, more appropriately
29–35
referred to as extended charge separated state.
Moreover, triplet
Departamento de Qu ´ı mica/Instituto de Tecnolog ´ı a Qu ´ı mica UPV-CSIC,
Universitat Polit `e cnica de Val `e ncia, Camino de Vera s/n, 46022 Valencia, Spain.
E-mail: mmiranda@qim.upv.es
†
Electronic supplementary information (ESI) available: Synthesis, exact mass
1
13
and spectroscopic characterization ( H, C and DEPT-NMR) of all compounds as
well as additional steady-state, transient absorption spectra and computational
methods. See DOI: 10.1039/c5cc08102e
Fig. 1 Chemical structures of the synthesized D–A systems and control
compounds. In 1–3, ‘‘a’’ stands for the 3a- and ‘‘b’’ for the 3b-epimers.
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À1 39
of BZP (68.6 kcal mol ), thus the exergonicity of the ET process is result was obtained for the b-epimer 1b (Fig. S2A in the ESI†),
3
3
guaranteed; (iv) BZP* has phosphorescence emission as well as where the 530 nm centered band due to BZP* was clearly
3
3
transient absorption bands different from those of NPT* or BIP*. observed in the submicrosecond range. Nevertheless, at longer
As a matter of fact, kinetic evidence obtained by laser flash delays, the BZP* contribution to the overall spectra decreased,
photolysis (LFP) in solution, at room temperature, demonstrates and the band due to NPT* at 420 nm dominated the spectrum.
3
3
the involvement of TB-TEX in the triplet decay of the NPT and BIP Control experiments performed under the same conditions
units in the selected systems 1a,b and 2a,b.
Compounds 1a,b and 2a,b (Fig. 1) were obtained as shown after 200 ns the BZP* band, which was not even detected for the
in detail in the ESI.† The UV-Vis absorption spectra of 1a,b and directly linked system 6 200 ns after the laser pulse (Fig. S3 in the
showed that an intermolecular 1 : 1 mixture of 3a and 4 exhibited
3
2
a,b matched with the sum of the absorption spectra of the ESI†). The TB-TTET process can be analyzed more accurately by
3
isolated chromophores 3a,b + 4 or 3a,b + 5 (Fig. S1 in the ESI†); looking at the kinetic traces monitored at 530 nm ( BZP*). Thus,
the same was true for 6 and 7, lacking the LA spacer. These it can be clearly observed in the inset of Fig. 3, left that the
observations indicate the absence of significant ground state process occurs instantaneously in the case of 1a and 6 (mostly
interactions between the donor and acceptor units.
during the laser pulse), is slower for 1b and takes even longer for
The phosphorescence spectra of 1a and 1b in ethanol at 77 K the intermolecular situation (3a + 4). Accordingly, formation of
3
upon BZP excitation at 355 nm were coincident with that of 3a
Fig. 2, left), indicating that TB-TTET is completely prevented in in the respective time windows. The BZP triplet lifetimes and
the matrix when the chromophores are separated by the LA TB-TTET rate constants are presented in Table 1.
spacer. By contrast, when BZP and NPT were directly connected Moreover, the decay of the 420 nm band in 1a,b should
NPT* (monitored through the growth at 420 nm) was observed
(
through an amide bond in compound 6 the phosphorescence contain relevant information related to the possible involvement of
3
arose exclusively from the NPT moiety, demonstrating that TTET an extended TB-TEX from NPT*. Thus, the kinetic traces monitored
3
within this D–A system is associated with clear phosphorescence at 420 nm are shown in Fig. 4, left, and the NPT* lifetimes are given
spectral changes. The lack of TB-TTET in the matrix may be due in Table 1. The TB-TEX rate constants (Table 1) have been obtained
to the predominance of frozen inefficient conformations of 1a,b. from the difference between the reciprocal lifetimes measured for
A parallel series of experiments where the NPT unit was the dyads and for the intermolecular mixture at the same concen-
replaced with BIP were performed. The smaller BZP/BIP triplet tration. Thus, the obtained values can be safely attributed to the
energy gap in 2a,b was expected to result in a slower TTET intramolecular process. These results demonstrate that extended
3
process and therefore in a broader dynamic range. In fact, the TB-TEX formation is an efficient deactivation pathway for NPT* in
phosphorescence spectra of 2a,b in ethanol matrix, after selective the linked systems 1a,b. the rate constant of this process was even
excitation at 355 nm showed exclusively the typical BZP emission, higher than through-space exciplex formation in 6, where the
indicating that TB-TTET is again blocked in the frozen matrix. With chromophores are directly linked. Interestingly, TB-TEX was faster
this D–A combination, even the directly connected system 7 showed for the b-epimer 1b than for its a-analog 1a, in contrast to the
incomplete TTET, as revealed by spectral changes, which were in tendency found in TB-TTET.
agreement with partial emission from the BIP unit (Fig. 2, right).
As in the case of the NPT analogs, the TB-TTET process was also
To enhance the prospects of TB-TTET, the systems were observed in the LFP experiments performed with 2a,b in solution
investigated in solution at room temperature. First, LFP of 1a (see transient absorption spectra in Fig. 3, right as well as Fig. S2B
with selective excitation of BZP at 355 nm, gave rise to transient and controls in S4 in the ESI†). An important difference between
3
absorption spectra where contribution of the BZP triplet excited 1a,b and 2a,b was found in the kinetics of BZP* decay (Fig. 3,
3
3
state ( BZP*, lmax ca. 530 nm) was almost negligible, yet NPT* insets), which was slower for the BIP derivatives, allowing for more
max = 420 nm) was the predominating species as early as 20 ns accurate determination of the triplet lifetimes and rate constants
(l
3
after the laser pulse (Fig. 3, left). This indicates a very fast TB-TTET (Table 1). The BZP* lifetime of the b-derivative 2b was similar to
from BZP to NPT in this system. However, a markedly different
Fig. 3 Transient absorption spectra obtained upon LFP (lexc = 355 nm,
À4
Fig. 2 Normalized phosphorescence spectra of left: 1a (black), 1b (red),
CH
2
Cl
2
, N
2
, A355 = 0.2, 5 Â 10 M) of left: 1a and right: 2a at 0.02 ms
3
5
a (blue), 4 (orange), and 6 (green), and right: 2a (black), 2b (red), 3a (blue),
(orange), and 7 (green) in ethanol matrix at 77 K. The excitation
(black), 0.2 ms (red) and 1 ms (blue) after the laser pulse. Insets show the
decays of 1a or 2a (red), 1b or 2b (magenta), 3a or 3b (black), 3a + 4 or
3a + 5 at 1 : 1 molar ratio (blue) and 6 or 7 (green) monitored at 530 nm.
wavelength was 355 nm in all cases except for 4 (266 nm) and 5 (285 nm).
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Table 1 Triplet lifetimes, TB-TTET and TB-TEX rate constants for BZP-NPT
and BZP-BIP systems
a
3
b
c
3
3
b
c
System
t( BZP*)
k(TB-TTET)
t( NPT* or BIP*)
k(TB-TEX)
7
5
1
1
2
2
3
3
3
6
7
a
b
a
b
a
o0.06
0.22
0.12
0.52
2.37
0.65
0.67
41.5 Â 10
3.18
2.54
2.96
2.30
N.A.
4.83
4.60
3.81
1.91
1.1 Â 10
6
5
3.0 Â 10
1.9 Â 10
6
5
6.8 Â 10
1.2 Â 10
5
5
4.3 Â 10
2.2 Â 10
N.A.
N.A.
N.A.
N.A.
d
a + 4d
N.A.
N.A.
ea + 5
Fig. 5 Conformational landscape of molecules 2a and 2b, based on the
intramolecular chromophore–chromophore distance (as defined in Fig. S5,
ESI†). The histograms have been produced from two molecular dynamics
calculations of compounds 2a (blue) and 2b (red) in dichloromethane at
300 K.
8
4
o0.01
41.0 Â 10
5.5 Â 10
e
7
5
o0.02
44.8 Â 10
3.1 Â 10
a
À4
b
c
À1
In all cases, the concentration was 5 Â 10 M. Units: ms. Units: s
d
determined as (1/t À 1/t3a+4) or (1/t À 1/t3a+5). Each compound at
À4
e
5
 10 M concentration. Control systems for the through-space
processes.
In summary, the involvement of extended through-bond
triplet exciplex formation in the excited state dynamics of
naphthalene/benzophenone and biphenyl/benzophenone linked
systems has been demonstrated by means of laser flash photolysis
in solution, at room temperature. Kinetic evidence supporting this
concept has been obtained from the decay of the naphthalene or
biphenyl triplet after population via through-bond energy transfer
from triplet benzophenone. All through-bond processes proceed
across a rigid saturated bile acid scaffold and are strongly depen-
dent on the relative spatial arrangement of the chromophores.
Financial support from the Spanish Government (Grants
Fig. 4 LFP decays (lexc = 355 nm, CH
2
Cl
2
, N
2
, A355 = 0.2) of left: 1a (red), SEV-2012-0267, CTQ2012-38754-C03-03, CTQ2013-47872-C2-1-P
1
b (magenta), 3a + 4 at 1 : 1 molar ratio (blue) and 6 (green) monitored at
and JCI-2011-09926), EU (PCIG12GA-2012-334257), Generalitat
Valenciana (Prometeo Program), and Technical University of
Valencia (VLC/Campus, ASIC-UPV for computational facilities and
Predoctoral FPI fellowship for P. Miro) is gratefully acknowledged.
4
20 nm, and right: 2a (red), 2b (magenta), 3a + 5 at 1 : 1 molar ratio (blue)
and 7 (green) monitored at 380 nm.
that of the intermolecular 3a + 5 mixture at the same concen-
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