Four different emissions from a Pt(Bodipy)(PEt3)2(S-Pyrene) dyad

Article abstract of DOI:10.1039/c8dt04823a

The Pt(bodipy)-(mercaptopyrene) dyad BPtSPyr shows four different emissions: intense near-infrared phosphorescence (Φ_ph up to 15%) from a charge-transfer state pyrS⁺-Pt-BDP^-, additional fluorescence and phosphorescence emissions from the ¹ππ? and ³ππ? states of the bodipy ligand at r.t., and phosphorescence from the pyrene ³ππ? and the bodipy ³ππ? states in a glassy matrix at 77 K.

Full text of DOI:10.1039/c8dt04823a

Dalton
Transactions
View Article Online
View Journal
COMMUNICATION
Four different emissions from a Pt(Bodipy)
(PEt3)2(S-Pyrene) dyad†
Cite this: DOI: 10.1039/c8dt04823a
b
Peter Irmler,a Franciska S. Gogesch,a Christopher B. Larsen,b Oliver S. Wenger
Received 6th December 2018,
Accepted 7th December 2018
a
and Rainer F. Winter
*
DOI: 10.1039/c8dt04823a
rsc.li/dalton
The Pt(bodipy)-(mercaptopyrene) dyad BPtSPyr shows four
BPtSPyr was synthesized from trans-Pt(bodipy)I(PEt3)2
different emissions: intense near-infrared phosphorescence (BPtI)7 by halogenide abstraction with AgOTf and subsequent
(Φph up to 15%) from a charge-transfer state pyrS•+-Pt-BDP•−
,
addition to a solution of deprotonated 1-mercaptopyrene (pyrS).
additional fluorescence and phosphorescence emissions from the The same protocol was also used to prepare the complex
1ππ* and 3ππ* states of the bodipy ligand at r.t., and phosphor- MesPtSPyr, where the BDP chromophore is replaced by mesityl
escence from the pyrene 3ππ* and the bodipy 3ππ* states in a (Mes, Mes = 2,4,6-trimethylphenyl) from newly synthesized
glassy matrix at 77 K.
trans-PtBr(Mes)(PEt3)2 (for synthetic details and NMR spectro-
scopic characterization see ESI and Fig. S1–S18† therein).
The electronic absorption spectrum of MesPtSPyr is domi-
nated by the ππ* band of the mercaptopyrene ligand at 431 nm
(Fig. 1). Upon excitation into this band, MesPtSPyr exhibits
dual fluorescence and phosphorescence emissions, at r.t. and
at 77 K. Both emissions emanate from the mercaptopyrene
ligand as follows from the typical shapes of the emissions and
their lifetimes (Fig. S19–S25, ESI† and Table 1).5,16,17 ISC in
MesPtSPyr is obviously triggered by the heavy-metal effect of
the attached Pt ion, similar to what has been observed in Pt
complexes with a σ-bonded pyrenyl ligand.5,16,17
In accordance with Kasha’s rule, the vast majority of com-
pounds emits solely from their lowest excited electronic state.1
Dual fluorescence and phosphorescence emissions may,
however, be observed if intersystem crossing (ISC) occurs at a
competitive timescale to fluorescence and if the rate constant
for phosphorescence emission is increased by a proximal
heavy atom with a large spin–orbit coupling constant.2–5 By
capitalizing on this remote heavy-atom effect we and others
have recently observed dual fluorescence and phosphorescence
emissions at r.t. from metal complexes with a σ-bonded6–11 or
remotely attached bodipy (BDP) ligand.12,13 Multiple emissions
from a single compound can also result from inefficient
Förster resonance energy transfer in dyads with two electroni-
cally decoupled, orthogonally aligned chromophores.14
Intriguing examples are multiply emissive Ir(ppy)2(Q) com-
plexes with two cyclometalated phenylpyridine (ppy) and a
2-hydroxyquinoline ligand Q, which show ligand-localized
fluorescence from 1nπ* and phosphorescence from 3MLCT
(MLCT = metal–ligand charge-transfer) states of the different
ligands.15 We report here on the new meso-Pt bodipy complex
BPtSPyr (Scheme 1), which combines the above principles and
presents four different emissions owing to the coexistence of
energetically close-lying ligand-localized and charge-separated
excited states.
BPtSPyr reveals a three-band absorption pattern comprising
of the structured pyrS-based ππ* band at 350–425 nm, the
bodipy (BDP) ππ* absorption at 470 nm, and a partially overlap-
ping, weaker pyrS-to-BDP charge-transfer (PB-CT) band at ca.
aFachbereich Chemie, Universität Konstanz, Universitätsstraße 10,
D-78457 Konstanz, Germany. E-mail: rainer.winter@uni-konstanz.de
bDepartment of Chemistry, University of Basel, St.-Johanns-Ring 19, CH-4056 Basel,
Switzerland
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8dt04823a
Scheme 1 Synthesis of BPtSPyr and MesPtSPyr.
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Communication
Dalton Transactions
Fig. 1 UV/Vis spectra of BPtSPyr, MesPtSPyr and BPtI in toluene.
Table 1 Emission data of MesPtSPyr and BPtSPyr
b
λph
Fig. 2 Emission spectra of BPtSPyr in ca. 1 μM solutions of acetone,
THF or toluene at r.t. on excitation at (a) 520 nm, (b) 450 nm and (c)
405 nm. (d) Emission spectra recorded in 2-MeTHF at 77 K at different
excitation wavelengths.
a
b
c
d
Solvent
λexc
λfl
(assignment) Φph
τph
MesPtSPyr
Toluene
430
460e 664 (3pyrS)
<0.01 448 6 μs
Me-THFf 430
450
658 (3pyrS)
n.d.
1.04 0.06 ms
BPtSPyr
Toluene
6.7 μs in toluene (for lifetime decays see Fig. S28–S30, ESI†) indi-
cate that this emission emanates from the PB-CT state. We note
520
470
420
520
470
724 (3PB-CT)
724 (3PB-CT)
724 (3PB-CT)
784 (3PB-CT)
0.08
0.15
0.15
n.d.
n.d.
6.7 0.4 μs
6.7 0.5 μs
6.4 0.3 μs
n.d.
78.0 0.4 μs,
51.4 0.8 ns
50.4 0.3 ns
n.d.
3
484
that heavy atom-free donor-substituted BDPs were also shown to
undergo efficient ISC to the T1 state via an excited CT state; the
latter systems are, however, non-phosphorescent.18–20
Excitation into the BDP ππ* absorption band at 470 nm
(Fig. 2b and S31, ESI†) opens two further radiative pathways:
BDP-based fluorescence at 484 nm and phosphorescence at
635 nm are observed in addition to the 3PB-CT emission.
THF
484g 635 (3BDP)
784 (3PB-CT)
420
520
470
784 (3PB-CT)
n.d.
n.d.
n.d.
Acetone
861 (3PB-CT)
484
n.o.
635 (3BDP)
49.9 0.3 μs
2.44 0.02 ns
861 (3PB-CT)
Me-THFf 530
635 (3BDP)
279 1 μs
450
415
n.d.
649 (3pyrS)
1.12 0.02 ms Changing the solvent from toluene to THF or acetone increases
the energy separation between the BDP 3ππ* and the 3PB-CT
a Excitation wavelength in nm. b Wavelength of the fluorescence (fl) or
phophorescence (ph) emissions in nm. c Phosphorescence quantum yield.
states and allows for tuning of the emission colour from red
in toluene (CIE coordinates 0.6052/0.3211 in toluene) to
almost white in THF (CIE coordinates: 0.2209/0.3206) or acetone
(CIE coordinates 0.3357/0.2901). Concomitantly with its red-
d Lifetime of the phosphorescence emissions. e τfl = 1.113 0.005 ns, Φph
0.24. f Measured in a 2-MeTHF glass at 77 K. g τfl = 3.70 0.2 ns.
=
3
shift, the lifetime of the PB-CT emission decreases from 6.7 μs
500 nm. The latter has no equivalent in the precursors BPtI and in toluene solution to only 2.4 ns in acetone (Fig. S30a, ESI†).
1
MesPtSPyr (Fig. 1, Table S1, ESI†). The above band assignments This in turn increases the intensity ratio of the ππ*/3ππ* to the
are supported by time-dependent density functional theory 3PB-CT emissions (Fig. 2b). We note that the BDP-based emis-
(TD-DFT) calculations (Fig. S26, ESI†). According to our calcu- sions of BPtSPyr are not due to contamination with the BPtI pre-
lations all transitions are ligand-based with only minor Pt con- cursor (Fig. S32, ESI†). Triple emission upon excitation into the
tributions to the corresponding donor and acceptor orbitals BDP ππ* band at 470 nm indicates that ISC from the 1ππ* to the
1
(Table S2†). In line with a CT character, the PB-CT excitation is 3ππ* state of the BDP ligand as well as formation of the PB-CT
3
solvatochromic while the pyrS and BDP ππ* absorptions are state and subsequent ISC to the PB-CT state occur at similar
only modestly influenced by solvent polarity (Fig. S27, ESI†).
rates as fluorescence from the BDP 1ππ* state. When the sample
Importantly, and in contrast to MesPtSPyr, BPtSPyr exhibits is excited into the pyrS ππ* band at 415 nm, again solely 3PB-CT
four different emissions depending on excitation wavelength, emission is observed, but with an even increased quantum yield
solvent polarity and temperature (Fig. 2). As shown in Fig. 2a, exci- of 15% (λexc = 420 nm) as compared to 8% for λexc = 520 nm
tation into the PB-CT absorption band at 520 nm leads to broad (Fig. 2a and c), rendering BPtSPyr a powerful NIR emitter. All
and unstructured NIR emission with a quantum yield of 8% in the above conclusions are fully supported by excitation vs. emis-
toluene. The emission shifts red with increasing solvent polarity, sion maps and excitation spectra (Fig. S33 and S34, ESI†).
from 724 nm in toluene to 784 nm in THF and to 816 nm in
Emission spectra recorded in a glassy Me-THF matrix at
acetone (Table 1). Its strong solvatochromism and the lifetime of 77 K show only two features, irrespective of the excitation wave-
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Communication
length: the 3ππ* BDP emission with a maximum at 635 nm and
a typical lifetime of 279 μs (Fig. S35, ESI†) as well as a struc-
tured emission enveloping the region in from 649 to 750 nm
(Fig. 2d). This latter feature and its lifetime of 1.12 ms
(Fig. S35, ESI†) are typical of a pyrene-based 3ππ* emis-
3
sion.5,16,17 The ππ* excited states of the BDP and pyrS ligands
are hence both accessible from excitation into any band in the
rigid matrix. This indicates that, at 77 K, population of the
close-lying BDP and pyrS 3ππ* from the 1PB-CT state of BPtSPyr
is more rapid than direct intersystem crossing to the 3PB-CT
state and that deactivation to the 3PB-CT state is blocked
Fig. 3 Transient absorption spectra of BPtSPyr at r.t. (a) in deaerated
20 μM THF solution at different time delays after excitation at 470 nm;
and (b) in a 20 μM toluene solution directly after laser excitation at the
under these conditions. One possible explanation is that the given wavelength. All spectra are time-integrated over 200 ns.
3
1PB-CT and PB-CT states possess very similar nuclear coordi-
nates and the BDP and pyrS 3ππ* potential wells have more
easily accessible intersections with the 1PB-CT potential well. (pyrS), 470 (BDP), or 532 nm (PB-CT absorption) (Fig. 3 and
Previous studies have similarly demonstrated direct singlet-to- Fig. S36, ESI†). Three main features are observed: the bleached
triplet electron-transfer by spin–orbit coupling.21–23
pyrS and BDP ground state absorption signals at ca. 400 nm or
The Jablonski diagram of Scheme 2 summarizes the pro- 470 nm, and an excited state absorption (ESA) signal at 550 nm,
cesses following the different excitations of BPtSPyr. Electronic which we assign to the pyrS•+ radical cation (vide infra). Identical
decoupling of the pyrS- and the BDP-localized 1ππ* states, as monoexponential kinetics with a common time constant of
indicated by the confinement of the HOMO to the pyrS and of 50 ns were found for all three transients when excited at 532 nm
the LUMO to the BDP ligand (Fig. S26, ESI†), explains the (for single point kinetics for each TA feature see Fig. S37 and
absence of pyrS to BDP energy transfer from the pyrene-based S38, ESI†). When excited into the BDP absorption at 470 nm the
1ππ* state S3 and of any BDP emission after excitation into the features at 415 nm and 550 nm still have a time constant of
1
3
pyrS ππ* band at 415 nm. The BDP-based ππ* and ππ* states 50 ns, but the bleach recovery at 475 nm has much longer kine-
S2 and T3 are thus exclusively populated by excitation into the tics of 16.5 μs in THF. This is in accordance with the long-lived
BDP 1ππ* band. In contrast, the 3PB-CT state, which constitutes BDP 3ππ* excited state in that solvent (red trace in Fig. S37a,
the energetically lowest excited triplet state T1, is accessible ESI†). Transient absorption bleach recoveries and decays match
from all three excited singlet states, either via IC to the 1PB-CT the luminescence lifetimes in all cases. This corroborates that
and subsequent ISC, or via ISC and subsequent electron trans- none of the observable emissions originate from impurities but
fer from the higher-lying pyrS-based T2 and BDP-based T3 represent inherent photoluminescence from BPtSPyr.
states (note that MesPtSPyr also undergoes ISC from the
The common CT state after excitation should exhibit spec-
1
pyrene-based ππ* state). At 77 K, direct singlet to triplet elec- troscopic features similar to the oxidized and reduced forms of
tron-transfer from the 1PB-CT state allows population of the BPtSPyr. Cyclic voltammograms show a reversible oxidation at
pyrS- and BDP-based triplet states T2 and T3 (blue arrows in +0.18 V and a reduction at −1.97 V against the ferrocene/ferro-
Scheme 2) and renders the pyrS-based phosphorescence cenium couple (Fig. S39 and Table S3, ESI†). This allowed us
emission observable, which, at r.t., is quenched by rapid decay to experimentally probe the UV/Vis/NIR features of the
into the lower-lying 3PB-CT state.
oxidized and reduced forms of BPtSPyr by means of spectro-
The proposed excited state cascade is further supported by electrochemistry. Upon oxidation, the pyrS-based ππ* absorp-
transient absorption (TA) spectroscopy in THF. TA spectra tion band bleaches while the BDP ππ* band exhibits an only
recorded over the first 200 ns after excitation are very similar to minor red-shift of 316 cm−1 (Fig. 4). Based on literature
each other, irrespective of whether the sample is excited at 410 data24,25 and the results of our TD-DFT calculations (Fig. S40
and Table S4, ESI†), the new absorption bands at ca. 520, 550,
750, and 1000 nm can be assigned to ππ* transitions of the
pyrS•+ radical cation. When BPtSPyr is reduced, the BDP ππ*
and the PB-CT absorption bands bleach while the pyrS-based
ππ* band experiences a moderate red-shift. The weak new band
at 518 nm constitutes mainly a ππ* transition of the reduced
BDP ligand (Fig. S41 and S42, Table S5, ESI†); it is essentially
identical in shape and energy to that observed for BPtI•− 9
.
Calculated spin densities for the BPtSPyr radical cation and
anion also agree with a pyrS-based oxidation and a BDP-based
reduction (Fig. 5 and Table S6, ESI†). The results of our spectro-
electrochemical experiments and our calculations thus confirm
our assignment of the ESA signal to the pyrS•+ constituent as
well as the character of the HOMO and LUMO of BPtSPyr.
Scheme 2 Qualitative Jablonski diagram.
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Communication
Dalton Transactions
3 C.-C. Hsu, C.-C. Lin, P.-T. Chou, C.-H. Lai, C.-W. Hsu,
C.-H. Lin and Y. Chi, J. Am. Chem. Soc., 2012, 134, 7715–
7724.
4 Y.-C. Chang, K.-C. Tang, H.-A. Pan, S.-H. Liu,
I. O. Koshevoy, A. J. Karttunen, W.-Y. Hung, M.-H. Cheng
and P.-T. Chou, J. Phys. Chem. C, 2013, 117, 9623–9632.
5 W. Y. Heng, J. Hu and J. H. K. Yip, Organometallics, 2007,
26, 6760–6768.
6 F. Geist, A. Jackel and R. F. Winter, Inorg. Chem., 2015, 54,
10946–10957.
7 P. Irmler and R. F. Winter, Dalton Trans., 2016, 45, 10420–
10434.
8 F. Geist, A. Jackel, P. Irmler, M. Linseis, S. Malzkuhn,
M. Kuss-Petermann, O. S. Wenger and R. F. Winter, Inorg.
Chem., 2017, 56, 914–930.
Fig. 4 Changes of UV/Vis/NIR spectra of BPtSPyr on oxidation (THF,
NBu4PF6, 293 K).
9 P. Irmler and R. F. Winter, Organometallics, 2018, 37, 235–
253.
10 W. Wu, J. Zhao, H. Guo, J. Sun, S. Ji and Z. Wang, Chem. –
Eur. J., 2012, 18, 1961–1968.
11 F. Zhong, A. Karatay, L. Zhao, J. Zhao, C. He, C. Zhang,
H. G. Yaglioglu, A. Elmali, B. Küçüköz and M. Hayvali,
Inorg. Chem., 2015, 54, 7803–7817.
12 J. Sun, F. Zhong, X. Yi and J. Zhao, Inorg. Chem., 2013, 52,
6299–6310.
13 W. Wu, J. Sun, X. Cui and J. Zhao, J. Mater. Chem. C, 2013,
1, 4577–4589.
Fig. 5 Calculated spin densities of (a) the radical cation and (b) the
radical anion of BPtSPyr.
14 B. Ma, P. I. Djurovich, M. Yousufuddin, R. Bau and
M. E. Thompson, J. Phys. Chem. C, 2008, 112, 8022–8031.
15 T. Tsuboi, D.-F. Huang, T. J. Chow and W. Huang, Opt.
Mater., 2014, 36, 1734–1738.
16 J. Hu, J. H. K. Yip, D.-L. Ma, K.-Y. Wong and W.-H. Chung,
Organometallics, 2009, 28, 51–59.
17 W. Wu, W. Wu, S. Ji, H. Guo and J. Zhao, Eur. J. Inorg.
Chem., 2010, 2010, 4470–4482.
18 M. A. Filatov, S. Karuthedath, P. M. Polestshuk,
S. Callaghan, K. J. Flanagan, M. Telitchko, T. Wiesner,
F. Laquai and M. O. Senge, Phys. Chem. Chem. Phys., 2018,
20, 8016–8031.
In summary, the BPtSPyr dyad combines several remarkable
features. It constitutes an extremely rare case of a compound
showing emissions from four different excited states and acts
as a powerful NIR emitter. Its emission profiles and colour can
be tuned by the excitation wavelength and solvent polarity
from deep red to almost white. The crucial role of the
pyrS•+−Pt-BDP•− charge-transfer state within the excited state
cascade was identified by TA spectroscopy, spectroelectro-
chemical methods and TD-DFT calculations.
19 X.-F. Zhang and N. Feng, Chem. – Asian J., 2017, 12, 2447–
2456.
Conflicts of interest
The authors declare no competing financial interests.
20 S. Ji, J. Ge, D. Escudero, Z. Wang, J. Zhao and
D. Jacquemin, J. Org. Chem., 2015, 80, 5958–5963.
21 M. R. Wasielewski, D. G. Johnson, W. A. Svec, K. M. Kersey
and D. W. Minsek, J. Am. Chem. Soc., 1988, 110, 7219–7221.
22 S.-H. Lee, A. G. Larsen, K. Ohkubo, Z.-L. Cai, J. R. Reimers,
S. Fukuzumi and M. J. Crossley, Chem. Sci., 2012, 3, 257–
269.
23 T. Higashino, T. Yamada, M. Yamamoto, A. Furube,
N. V. Tkachenko, T. Miura, Y. Kobori, R. Jono,
K. Yamashita and H. Imahori, Angew. Chem., Int. Ed., 2016,
55, 629–633.
Acknowledgements
This work was supported by Deutsche Forschungsge-
meinschaft (grant Wi1262/10-2) and the Swiss National
Science Foundation (grant number 200021_176780).
Notes and references
24 T. Shida and S. Iwata, J. Am. Chem. Soc., 1973, 95, 3473–
3483.
1 M. Kasha, Discuss. Faraday Soc., 1950, 9, 14–19.
2 C.-W. Hsu, C.-C. Lin, M.-W. Chung, Y. Chi, G.-H. Lee, 25 J. Maurer, M. Linseis, B. Sarkar, B. Schwederski,
P.-T. Chou, C.-H. Chang and P.-Y. Chen, J. Am. Chem. Soc.,
2011, 133, 12085–12099.
M. Niemeyer, W. Kaim, S. Záliš, C. Anson, M. Zabel and
R. F. Winter, J. Am. Chem. Soc., 2008, 130, 259–268.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018