Heteroleptic [Os(H)(CO)(N∧N)(tpp)2]+ and [Os(Cl)(CO)(N∧N)(tpp)2]+ complexes-comparative studies of their luminescence properties

Article abstract of DOI:10.1039/c6cp05856f

Two series of cationic heteroleptic osmium(ii) complexes with a coordinated CO molecule, a chloride Cl^- or hydride H^- anion, two monodentate triphenylphosphine (tpp) ligands and one bidentate α-dimine (N^∧N) ligand were prepared from an OsCl₂(CO)₂(tpp)₂ precursor. The investigated complexes, available in the form of PF₆^- salts, have been identified by means of FT-IR, ¹H and ³¹P NMR spectroscopy and X-ray diffraction studies. Their photophysical properties have been investigated in dichloromethane solutions at room temperature and 1:1 ethanol-methanol matrices at 77 K. The investigated complexes exhibit metal to ligand charge-transfer (MLCT) phosphorescence with the emission characteristics distinctly affected by the nature of coordinated α-diimine N^∧N and Cl^- or H^- ligands. Franck-Condon emission spectral band shape analyses and DFT/TD-DFT calculations have been applied to obtain more detailed insight into the nature of emissive ³?[Os(H)(CO)(N^∧N)(tpp)₂]⁺ and ³?[Os(Cl)(CO)(N^∧N)(tpp)₂]⁺ species.

Full text of DOI:10.1039/c6cp05856f

PCCP
View Article Online
View Journal
PAPER
Heteroleptic [Os(H)(CO)(N4N)(tpp)2]+ and
[Os(Cl)(CO)(N4N)(tpp)2]+ complexes – comparative
Cite this:DOI: 10.1039/c6cp05856f
studies of their luminescence properties†
Anna Kamecka,a Kinga Suwinskab and Andrzej Kapturkiewicz*a
´
Two series of cationic heteroleptic osmium(II) complexes with a coordinated CO molecule, a chloride Clꢀ
or hydride Hꢀ anion, two monodentate triphenylphosphine (tpp) ligands and one bidentate a-dimine (N4N)
ligand were prepared from an OsCl2(CO)2(tpp)2 precursor. The investigated complexes, available in the
ꢀ
form of PF6 salts, have been identified by means of FT-IR, 1H and 31P NMR spectroscopy and X-ray
diffraction studies. Their photophysical properties have been investigated in dichloromethane solutions at
room temperature and 1 :1 ethanol–methanol matrices at 77 K. The investigated complexes exhibit metal
to ligand charge-transfer (MLCT) phosphorescence with the emission characteristics distinctly affected by
the nature of coordinated a-diimine N4N and Clꢀ or Hꢀ ligands. Franck–Condon emission spectral band
shape analyses and DFT/TD-DFT calculations have been applied to obtain more detailed insight into the
nature of emissive 3*[Os(H)(CO)(N4N)(tpp)2]+ and 3*[Os(Cl)(CO)(N4N)(tpp)2]+ species.
Received 24th August 2016,
Accepted 25th September 2016
DOI: 10.1039/c6cp05856f
www.rsc.org/pccp
of organometallic luminophores.15 Due to the presence of low-lying
p* orbitals these ligands are good electron acceptors that result in
Introduction
Emissive and photochemically stable coordination compounds the presence of low energy metal-to-ligand-charge-transfer (MLCT)
have been receiving increasing interest since the last few decades electronic states. An appropriate combination of the coordinated
due to their interesting photophysical properties and a number of metal ion(s) and the coordinating a-diimine ligand(s) enables the
potential applications. These include dye-sensitized solar cells design of transition metal based chromophores that absorb and/or
(DSSCs),1–5 organic light-emitting diodes (OLEDs),6–8 light- emit throughout the whole range of UV-vis radiation. Their photo-
emitting electrochemical cells (LECs),9 non-linear optics10 as well physical and photochemical properties can be further modified by
as chemical and biological sensing11 and other bioanalytical introducing into the complex molecule a wide variety of ancillary or
applications.12,13 Room-temperature phosphorescence in both tuning ligands.12,16,17
solution and the solid state is rather frequent for complexes of
Os2+ complexes with a-diimine ligands constitute an impor-
the second and third row transition metal ions possessing d6, tant group of coordination compounds tested in the context of
d8 or d10 electron configuration.6 For these heavy-metal ions the luminescence properties and different potential applications.3 In
presence of the strong spin–orbit coupling leads to an efficient general, homoleptic Os2+ complexes exhibit lower emission energy
intersystem crossing from the singlet excited states to the triplet and much shorter emission lifetimes than their Ru2+ analogues.3,6,12
manifold, and the high quantum efficiencies of phosphorescence Phosphorescent complexes of Os2+ with a-diimine ligands such as
usually observed for these complexes.14
2,20-bipyridine or 1,10-phenanthroline and arsine or phosphine
Among many possible ligands that can be attached to the ligands were studied by Meyer and coworkers18–25 and Carlson
central metal core, a-diimines, derivatives of 2,20-bipyridine and et al.26–31 This group of heteroleptic Os2+ complexes was investi-
1,10-phenanthroline, have been extensively used in the preparation gated also by us and reported earlier.32–34 Among the heteroleptic
Os2+ complexes, described in the papers cited above, those with
the general formula of [Os(Cl)(CO)(N4N)(P4P)]+, where P4P and
a Institute of Chemistry, Faculty of Sciences, Siedlce University of Natural Sciences
N4N denote bidentate phosphine and a-diimine ligands, could be
and Humanities, 3 Maja 54, 08-110 Siedlce, Poland.
E-mail: andrzej.kapturkiewicz@uph.edu.pl; Tel: +48-25-643-10-97
considered as extremely efficient organometallic luminophores.
It was established that the emission from the electronically
excited state possesses distinct 3*MLCT character with charge
transfer from the Os2+ central ion to the N4N ligand. The
reported crystallographic results for these [Os(Cl)(CO)(N4N)-
(P4P)]+ complexes show that the bidentate N4N and P4P ligands
b Faculty of Mathematics and Natural Sciences, Cardinal Stefan Wyszynski
University in Warsaw, K. Woycickiego 1/3, 01-938 Warszawa, Poland
† Electronic supplementary information (ESI) available: Experimental details and
X-ray crystallographic data for [Os(Cl)(CO)(bpy)(tpp)2]PF6 and [Os(H)(CO)(bpy)(tpp)2]PF6
complexes. CCDC 1498690 and 1498691. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c6cp05856f
This journal is ©the Owner Societies 2016
Phys. Chem. Chem. Phys.

View Article Online
Paper
PCCP
have a cis arrangement, whereas the carbonyl is trans to the characterizing radiative S0
- ’
1*MLCT and S0 3*MLCT
a-diimine nitrogen and the chloride ligand is trans to a phosphorus transitions together with data available from the MLCT emission
of the P4P ligand.27,33
band shape analysis. Despite all approximation done during
The bonding of CO and P4P ligands to the metal ion in performed procedures self-consistent description of the excited
[Os(Cl)(CO)(N4N)(P4P)]+ complexes consists of two types of 3*MLCT deactivation was possible in a relatively simple and
interactions. There are s donation of the lone pair contained in intuitive way. To which extent the applied approach may be
CO and P4P ligands to an empty d-orbital of the metal ion and successfully applied in the case of any other organometallic
p acceptance that occurs by back-donation from a filled d orbital emitters exhibiting MLCT emission remains an open question.
of the metal ion to an empty orbital of both CO and P4P Further detailed studies in such directions, completed for
ligands.27,28 This results in larger energy splitting of 5d orbitals different classes of transition metal complexes, are required
of the Os2+ core forcing many intrinsic changes in its photo- to give any decisive answer. This was the main motivation of
physical properties. Increased energy splitting between 5d sub- the present work.
levels leads to a quite well pronounced hypsochromic shift in
Investigations reported in this paper are devoted to the
’
3*MLCT luminescence properties of two series of heteroleptic Os2+
the observed S0
-
1*MLCT (absorption) and S0
(emission) transitions. Changes in the energy splitting between complexes with general formulas of [Os(Cl)(CO)(N4N)(tpp)2]+
filled and empty 5d sublevels also increase the energies of triplet and [Os(H)(CO)(N4N)(tpp)2]+, respectively. The studied com-
3
metal-centred *MC states that makes thermally activated non- plexes contain two monodentate triphenylphosphine – tpp –
radiative deactivation of the emissive 3*MLCT states (through ligands and one bidentate a-diimine N4N ligand (structures
3*MLCT - *MC - S0 sequence) less operative. Both effects depicted in Fig. 1) with remaining coordination sites around the
3
lead to distinctly better emissive properties of these hetero- Os2+ ion occupied by the CO molecule and the Clꢀ or Hꢀ anion.
leptic complexes compared to their homoleptic congeners To some extent the investigated [Os(Cl)(CO)(N4N)(tpp)2]+ set
Os(N4N)32+. On the other hand energy shifts of the 3*MLCT state can be treated as a slight modification of previously studied
3
make their energies comparable with those of the excited *LC [Os(Cl)(CO)(N4N)(P4P)]+ chelates. Taking into account that the
states, typically 2–3 eV for most of the p-aromatic ligands. Thus structural aspects such as the bond lengths in the complex
by appropriately changing a-diimine N4N ligands attached to the molecule, the nature of bonds and the molecule geometry may
central metal core one can monotonically affect the nature of affect the MLCT transition, any changes in the ligand orientation
observed emission from ‘‘pure’’ 3*MLCT to more pronounced would have consequences on the luminescence behaviour.45
3*LC character. This well-known behaviour, arising from compar- Thus, replacing one molecule of bidentate phosphine with
able energies of the excited 3*LC and 3*MLCT states, is characteristic two molecules of monodentate in the complex structure should
of emissive organometallic derivatives, especially for complexes force the different orientation of the remaining ligands and
of 4d6 and 5d6 ions like Re+, Ru2+, Ir3+ as well as Os2+.
modify the complex structure as well as their photophysical
Electronic interactions between 3*LC and 3*MLCT states are properties. In a similar way replacing Clꢀ by Hꢀ (ligands of
also responsible for differences in the energy gap between the different ‘‘structural trans-effect’’) may lead to additional differ-
excited 3*MLCT and 1*MLCT states strongly affecting mixing of ences in the luminescence properties of two investigated series.
them. It results in quite huge changes in the experimentally The hydride ligand, which is a powerful s-donor, is known to be
observed values of kr and knr rate constants characterizing the very efficient in compensating for electron deficiency at the
radiative and non-radiative S0 ’ 3*MLCT transitions. As it has metal central ion in complexes.46 Thus one could expect that
been shown in our recently published work34,35 the observed both investigated series will display somewhat different photo-
changes in kr and knr values can be quantitatively interpreted physical behaviour. In both the investigated series interactions
within the Mulliken–Hush formalism36–39 combined with the between the central Os2+ ion and all of the coordinated ligands
Marcus–Jortner description40–44 of the radiative and non- should be somewhat dissimilar because the formation of Os–H
radiative intramolecular electron transfer in the inverted Marcus or Os–Cl bonds should engage empty osmium(II) d orbitals in a
region. The applied approach was based on the examination different way. This is expected because Clꢀ and Hꢀ ions are
of experimental values of the transition dipole moments possessing different s-donation properties.
Fig. 1 Structures of N-N ligands employed: 3,4,7,8-tetramethyl-1,10-phenanthroline – tmphen, 4,7-dimethyl-1,10-phenanthroline – 47dmphen,
5,6-dimethyl-1,10-phenanthroline – 56dmphen, 1,10-phenanthroline – phen, 4,7-diphenyl-1,10-phenanthroline – dpphen, 4,40-di-t-butyl-2,20-bipyridine –
dtbbpy and 2,20-bipyridine – bpy.
Phys. Chem. Chem. Phys.
This journal is ©the Owner Societies 2016

View Article Online
PCCP
Paper
Results and discussion
Synthesis and characterization
The investigated [Os(Cl)(CO)(N4N)(tpp)2]+ complexes have been
synthesized following a published procedure.47 In the performed
syntheses OsCl2(CO)2(tpp)2 has been used as a starting material.
This intermediate is not commercially available but can be synthe-
sized easily by refluxing a K2OsCl6 mixture with two-fold excess of
tpp in deoxygenated N,N-dimethylformamide solution.48 Due to Fig. 2 X-ray structures of [Os(Cl)(CO)(bpy)(tpp)2]+ (left) and [Os(H)(CO)-
(bpy)(tpp)2]+ (right) cations. The PF6ꢀ counterions, solvent (toluene) molecules
the lability of CO and Clꢀ ligands within the OsCl2(CO)2(tpp)2
and the hydrogen atoms (except hydride) have been omitted for clarity.
molecule this intermediate successfully reacted with the investi-
Atoms forming the presented structures are coloured as follows: C – grey,
N – blue, P – yellow, O – red, Cl – green, H – cyan, and Os – purple.
gated N4N ligands in the presence of (CH3)3NO which is required
to initiate the substitution of CO and Clꢀ ligands by N4N diimines
at low temperatures. The formation of [Os(Cl)(CO)(N4N)(tpp)2]+
in agreement with the results from FT-IR and NMR studies.
Here we present data only for complexes with the bpy ligand
but preliminary results from X-ray investigations of their 1,10-
phenanthroline analogues point to the same type of coordination.
Thus the spectroscopic behavior of the studied complexes is
affected solely/mainly by the nature of the coordinated N4N ligand.
complexes results from the nucleophilic attack of (CH3)3NO at CO
in OsCl2(CO)2(tpp)2 that is followed by the coordination of the
N4N molecule and the displacement of the cis chloride ligand.49
In addition, the above mentioned synthetic procedure results
in the formation of minor by-products, hydrido-carbonyl
[Os(H)(CO)(N-N)(tpp)2]+ complexes. When the reaction condi-
tions are changed by applying diethylene glycol monoethyl ether
(BP 202 1C) as the reaction medium, these complexes are major
products whereas their chloride analogues are formed only in
minute amounts. Thus the observed difference in the reactivity of
the OsCl2(CO)2(tpp)2 intermediate in the low- and high-boiling
solvents allows the preparation of two series of Os2+ complexes,
[Os(Cl)(CO)(N4N)(tpp)2]+ and [Os(H)(CO)(N4N)(tpp)2]+, respectively.
The latter series can be prepared, even in the absence of (CH3)3NO,
when the reaction temperature is high enough.22,50
S0 - S1 absorption and S0 ’ T1 emission spectra
Spectroscopic and photophysical data for the investigated chelates
are collected in Table 1 whereas typical examples of the recorded
UV-vis absorption and emission spectra are shown in Fig. 3 and 4
for bpy and tmphen derivatives, respectively.
UV-vis absorption spectra recorded in CH3CN as well as in
CH2Cl2 solutions are characterized by strong absorption at short
wavelengths (o350 nm) and broad low-energy bands in the range
of 400–450 nm. The shapes and molar absorption coefficients
relative to the higher energy bands allow us to assign them as the
p - p* absorption centred mostly in the N4N or tpp ligand (spin
allowed intra-ligand transitions). These bands are somewhat
shifted in comparison to the free ligands due to the coordination
of the heavy metal core. The lowest, broad and structureless
absorption bands can be assigned tentatively to the metal-to-
ligand charge-transfer transitions S0 - 1*MLCT. The assignment
is based on relatively low values of the molar extinction coeffi-
cients (1.2–4.3 ꢁ 103 Mꢀ1 cmꢀ1) and positions of the absorption
maxima depending on the nature of the coordinated N4N ligand.
For [Os(Cl)(CO)(dtbpy)(tpp)2]+ and [Os(H)(CO)(dtbpy)(tpp)2]+ com-
plexes the MLCT absorption bands are located, due to weaker
accepting character of the dtbpy ligand, at shorter wavelengths in
comparison to their bpy analogues. A similar effect is clearly seen
when in [Os(Cl)(CO)(phen)(tpp)2]+ and [Os(H)(CO)(phen)(tpp)2]+
cations the phen ligand is replaced by tmphen or other methyl
substituted phen derivatives. In the case of [Os(Cl)(CO)-
(dpphen)(tpp)2]+ and [Os(H)(CO)(dpphen)(tpp)2]+ counter-
parts the bathochromic shift of the MLCT absorption is clearly
seen. This is caused by the stronger electron affinity of the
dpphen ligand compared to that of the unsubstituted 1,10-
phenanthroline.
Structures of the synthesized complexes were confirmed
1
using H NMR, 31P NMR and FT-IR spectroscopic techniques.
According to the FT-IR and 31P NMR results two coordinated
tpp ligands are expected to be located on the z-axis whereas the
remaining N4N, CO and Clꢀ (or Hꢀ) ligands are positioned in
the xy-plane of the pseudo-octahedral structures of the studied
[Os(Cl)(CO)(N4N)(tpp)2]+ or [Os(H)(CO)(N4N)(tpp)2]+ cations.
Such orientation around the central Os2+ core is nicely reflected
in the recorded 1H NMR spectra showing a double set of resonances
from protons in bpy, phen and their derivatives.51 The additional
general feature of the 1H NMR spectra recorded for each complex
of [Os(H)(CO)(N4N)(tpp)2]+ series is the presence of well-resolved
triplet signals of hydride-H protons observed in the range of
ꢀ11.92 to ꢀ12.35 ppm. The observed splitting of the signal
arises from coupling of hydride-H protons with two equivalent
phosphorus nuclei (JP–H = 17.7–18.1 Hz).21,22,52–54
The results from the performed FT-IR and NMR spectroscopic
studies seem to determine unambiguously the molecular structures
of the investigated complexes. Despite that we have decided to carry
out X-ray diffraction studies as the final decisive proof. It was
possible because the slow evaporation of acetonitrile/toluene solu-
tions allows the growth of crystals with quality suitable for X-ray
measurements. The performed X-ray investigations revealed that in
[Os(H)(CO)(bpy)(tpp)2]+ as well as in [Os(Cl)(CO)(bpy)(tpp)2]+ cations
all ligands are coordinated to the central metal core in a conven-
All the Os2+ complexes under study are luminescent both at
room temperature and at 77 K. The luminescence spectra
tional fashion. The found orientation for bpy, tpp, CO and Clꢀ recorded for deaerated solutions in CH2Cl2 at room temperature
(or Hꢀ) ligands around the central Os2+ ion (cf. Fig. 2) remains are broad and show no vibronic structure. The spectral positions,
This journal is ©the Owner Societies 2016
Phys. Chem. Chem. Phys.

View Article Online
Paper
PCCP
Table 1 Spectroscopic and photophysical properties of [Os(H)(CO)(N^N)(tpp)2]+ and [Os(Cl)(CO)(N4N)(tpp)2]+ complexes (data in CH2Cl2 solutions at
room temperature and methanol/ethanol 1 : 1 glasses at 77 K). MLCT absorption maxima n~mabasx, molar extinction coefficients eM, and transition dipole
moments Mabs of S0
moments Mem of S0
-
1*MLCT transitions, MLCT emission maxima n~emmax, emission quantum yields fem, emission lifetimes tem, and transition dipole
’
3*MLCT transitions
S0
-
1*MLCT (298 K)
S0
’
3*MLCT (298 K)
S0
em/ms Memb/D n~memax/cmꢀ1
’
3*MLCT (77 K)
Complex type
Ligand N4N n~mabasx/cmꢀ1 eM/Mꢀ1 cmꢀ1 Mabsa/D n~emmax/cmꢀ1 fem
t
tem/ms
[Os(H)(CO)(N4N)(tpp)2]+ tmphen
47dmphen
25 000
24 270
23 530
23 810
23 580
23 420
22 980
3.2 ꢁ 103
2.7 ꢁ 103
2.5 ꢁ 103
2.2 ꢁ 103
4.3 ꢁ 103
2.1 ꢁ 103
1.7 ꢁ 103
1.8
1.8
1.7
1.7
2.3
1.5
1.5
16 300
15 890
15 970
15 570
15 240
15 720
15 250
0.12
24.0
11.9
7.3
4.9
7.8
1.1
0.5
0.033
0.043
0.062
0.050
0.049
0.072
0.061
18 230
17 900
17 820
17 740
16 940
17 990
17 500
87.0
55.0
35.0
37.0
36.0
14.0
22.0
0.077
0.074
0.042
0.060
0.020
0.0058
56dmphen
phen
dpphen
dtbbpy
bpy
[Os(Cl)(CO)(N4N)(tpp)2]+ tmphen
24 400
23 920
23 150
23 810
23 310
22 620
22 470
2.0 ꢁ 103
3.0 ꢁ 103
2.5 ꢁ 103
1.5 ꢁ 103
4.1 ꢁ 103
1.4 ꢁ 103
1.2 ꢁ 103
1.5
1.9
1.6
1.3
2.3
1.2
1.2
16 650
16 200
16 000
15 970
15 560
15 970
15 640
0.081
0.14
0.15
0.17
0.20
0.085
0.048
10.6
12.5
10.6
9.7
13.9
2.2
0.043
0.053
0.061
0.070
0.064
0.103
0.099
17 900, 19 110 42.0
17 740, 18 870 50.0
17 500, 18 550 30.0
17 420, 18 470 28.0
16 450, 17 760 35.0
47dmphen
56dmphen
phen
dpphen
dtbbpy
bpy
17 580
17 420
11.0
9.5
1.4
a
max 1/2
Mabs values have been estimated using the so-called lazy man’s approximation according to the Mabs = (0.092eMDn~1/2/nn~
)
relationship,55,56
abs
b
where Dn~1/2 and n are the full width at half maximum of the absorption band and the solvent refraction index. Mem values have been calculated
from the experimental radiative rate constant values according to the kr = fem/tem = (64p4/3h)(nn~emmax)3|Mem|2 relationship,57,58 where h is the Planck
constant.
Fig. 4 Band profiles of the UV-vis absorption and emission spectra recorded
for the [Os(H)(CO)(tmphen)(tpp)2]+ (top) and [Os(Cl)(CO)(tmphen)(tpp)2]+
complexes (bottom). Room temperature (red lines) and 77 K (blue lines)
emission in CH2Cl2 solutions and 1 : 1 methanol/ethanol matrices, respectively.
Room temperature absorption in acetonitrile (black lines) and CH2Cl2
solutions (dotted black lines).
Fig. 3 Band profiles of the UV-vis absorption and emission spectra
recorded for the [Os(H)(CO)(bpy)(tpp)2]+ (top) and [Os(Cl)(CO)(bpy)(tpp)2]+
(bottom) complexes. Room temperature (red lines) and 77 K (blue lines)
emission in CH2Cl2 solutions and 1 : 1 methanol/ethanol matrices, respectively.
Room temperature absorption in acetonitrile (black lines) and CH2Cl2
solutions (dotted black lines).
emission quantum efficiencies and emission lifetimes are distinctly
At room temperature the luminescence properties of the
dependent on the nature of the N4N ligand present in struc- [Os(Cl)(CO)(N4N)(tpp)2]+ and [Os(H)(CO)(N4N)(tpp)2]+ series are,
tures of the studied [Os(Cl)(CO)(N4N)(tpp)2]+ as well as for the given N4N ligand, very similar to one another. The main
[Os(H)(CO)(N4N)(tpp)2]+ series. The observed trends in the difference is a rather small (300–400 cmꢀ1) bathochromic shift
emission maxima follow that found for the S0
-
1*MLCT of a somewhat broader emission band of all hydride complexes
absorption. It allows us to conclude that the observed emissions with respect to their chloride analogues. Much more pronounced
also arise from the excited states possessing MLCT character. differences can be seen, however, in the emission recorded
Taking into account the large Stokes shift between emission and at 77 K in the methanol/ethanol matrices. Whereas the
absorption bands as well as differences in the transition dipole [Os(H)(CO)(N4N)(tpp)2]+ complexes exhibit broad and structure-
moments characterizing both processes (cf. data in Table 1) one less emission some of their [Os(Cl)(CO)(N4N)(tpp)2]+ analogues
can postulate the triplet nature of the emissive species. Thus the (with N4N = phen and phen derivatives) demonstrate structured
room temperature luminescence can be attributed to the metal- emission bands. The differences in the spectral positions of
3
to-ligand charge-transfer S0 ’ *MLCT transitions.
luminescence observed in both temperature regimes, being
Phys. Chem. Chem. Phys.
This journal is ©the Owner Societies 2016

View Article Online
PCCP
Paper
ca. 2000–2500 cmꢀ1, are pretty similar for both the investigated
3
series. It allows concluding similar stabilization of the *MLCT
emitting states at 298 K in a fluid environment in comparison to
the glassy samples at 77 K, where the extremely high viscosity
hinders the solute/environment relaxation. The observed rigido-
chromism of the studied complexes can be hardly related to the
temperature induced changes in the nature of the emissive states
3
3
from the excited *MLCT to the excited intra-ligand states *IL
because the latter ones are located at distinctly higher (in the
range of 2.59–2.88 eV)59,60 energies. Thus the 77 K emissions
observed for the excited 3*[Os(H)(CO)(N4N)(tpp)2]+ as well as
3*[Os(Cl)(CO)(N4N)(tpp)2]+ species can be attributed to the S0 ’
3*MLCT transitions similarly as it has been postulated for the
room temperature solutions. The differences in the emission band
profiles characteristic for two investigated series can be explained
by dissimilar values of the Franck–Condon factors describing the
S0 ’ 3*MLCT transitions. This hypothesis has been confirmed by
means of more detailed analysis of the emission band profiles
done within the framework of the Marcus–Jortner approach
(vide infra).
Fig. 5 Plots of the isodensity surfaces (with contour value Z = 0.02) for
the HOMO (left) and the LUMO (right) of the [Os(H)(CO)(bpy)(tpp)2]+ (top)
and [Os(Cl)(CO)(bpy)(tpp)2]+ (bottom) complexes.
Analysis of the distribution of atomic charges in the com-
plexes (cf. data in Table 2) shows the Mulliken atomic charges
on the metallic centres equal to +0.36 and ca. +0.41 for the
[Os(Cl)(CO)(bpy)(tpp)2]+ and only +0.14 and +0.17 for the
[Os(H)(CO)(bpy)(tpp)2]+ complexes in their ground state and
the lowest excited triplet state configurations. These values, dis-
tinctly smaller than naively expected +1.0 or +2.0, can be explained
by the presence of the s-donation bonds between the ligands and
the central Os2+ ion. Consequently, the electron donating ligands
(bpy and tpp molecules) become positively charged whereas the
coordinated Clꢀ or Hꢀ ions lost their negative charges. The latter
effect is distinctly stronger for the Hꢀ ion, definitely a better s donor
as compared to the Clꢀ ion. Due to the well pronounced accepting
properties of the CO molecule this particular ligand becomes
negatively charged through accepting charge from the central
Os2+ ion. One can also observe that the positive charge (in both
Nature of the lowest excited T1 state
In the case of the complexes under study it seems to be obvious
that their UV-vis spectroscopic and photophysical properties can
1
be attributed to the presence of low energetically lying *MLCT
and 3*MLCT states. The quantum-mechanical DFT/TD-DFT
calculations (done within B3LYP/LANL2DZ/3-21G + combination)
provide additional confirmation of this anticipation. The calcula-
tions have been performed for two selected [Os(Cl)(CO)(bpy)-
(tpp)2]+ and [Os(H)(CO)(bpy)(tpp)2]+ cations for the optimized
geometries of the ground state and the lowest triplet states.
Here only results from the quantum-mechanical calculations
done for the complexes with the bpy ligand are presented, but the
general picture should be very similar for the other investigated
complexes as it can be expected from the similarity of their
spectroscopic properties. Preliminary results from the similar
TD-DFT calculations performed for the [Os(H)(CO)(phen)(tpp)2]+
and [Os(Cl)(CO)(phen)(tpp)2]+ remain congruent with their bpy
analogues.
3
S0 and *MLCT states) is to a large extent delocalized into the
phosphor atoms of the tpp ligands, due to their strong ability to
donate s electrons. Similar behaviour has also been observed
for the heteroleptic Ru2+ and Os2+ complexes with cis-1,2-
bis(diphenylphosphino)ethylene and bpy ligands.61
For both geometries the lowest S0 - S1 and S0 - T1
transitions are described mainly (with CI coefficients 0.68–0.70)
by electron transfer between the HOMO and the LUMO. The
calculations performed in a vacuum and in CH2Cl2 solutions gave
the same results pointing to the localization of the HOMO and
the LUMO on the Os2+ core and the bpy ligand (cf. Fig. 5) in the
case of [Os(Cl)(CO)(bpy)(tpp)2]+ as well as [Os(H)(CO)(bpy)(tpp)2]+
cations. Although the HOMO is mainly of the metallic d orbital
character, some contributions from the ligand orbitals are clearly
seen. The LUMO is essentially a p* orbital localized on the bpy
ligand with only minor contributions from the Os2+ ion and other
components forming the complex structures. Thus the lowest
excited triplet states of the discussed complexes can be treated
hardly as 100%-pure MLCT states and can be better described
taking into account an admixture of the intra-ligand excitations,
Table 2 Distribution of the Mulliken charges on the Os2+ ion and ligands
in the [Os(H)(CO)(bpy)(tpp)2]+ and [Os(Cl)(CO)(bpy)(tpp)2]+ complexes.
Dipole moment values of their ground (mS ) and excited (mT ) states. Results
from the DFT calculations for optimized geometries of the ground S0 and
lowest exited triplet T1 states. Data as computed for the equilibrated
configurations in CH2Cl2 solutions
0
1
Complex
Os2+/ligand
S0 state
T1 state
[Os(H)(CO)(bpy)(tpp)2]+
Os2+
bpy
tpp
CO
+0.14
+0.35
+0.17
ꢀ0.21
mS = 10.3 D
+0.42, +0.42
ꢀ0.39
+0.63, +0.65
ꢀ0.35
0
mT = 2.1 D
1
Hꢀ
+0.04
+0.11
[Os(Cl)(CO)(bpy)(tpp)2]+
Os2+
bpy
tpp
CO
+0.36
+0.36
+0.41
ꢀ0.12
mS = 12.7 D
+0.49, +0.51
ꢀ0.37
+0.62, +0.62
ꢀ0.33
0
mT = 5.7 D
1
Clꢀ
ꢀ0.35
ꢀ0.23
3
e.g. contribution from the lowest *IL state of the bpy ligand.
This journal is ©the Owner Societies 2016
Phys. Chem. Chem. Phys.

View Article Online
Paper
PCCP
Table 3 Selected bond lengths (values in Å) for the [Os(H)(CO)(bpy)(tpp)2]+
and [Os(Cl)(CO)(bpy)(tpp)2]+ complexes in their ground S0 and excited T1
states. Data from DFT calculations for structures optimized in a vacuum
For the excited MLCT states one can also expect that the atomic
charge is increased by about +1.0 in the central Os2+ ion, since one
electron is transferred from the metallic d orbital to the ligand
p*orbital. However, after one electron excitation and relaxation of
Complex
Bonda
S0 state
T1 state
3
the *MLCT state, the formation of the bpyꢀ anion takes place
[Os(H)(CO)(bpy)(tpp)2]+
Os–N
Os–P
Os–C
Os–H
2.156, 2.131
2.410, 2.410
1.876
2.111, 2.112
2.463, 2.424
1.890
making the bpy ligand still a better s-donor. The presence of the
s donation bond will move electron density back toward the
metallic ions, which will reduce the amount of positive charge
on the Os2+ core. Consequently, the negative charge on the bpy
ligand becomes smaller than ꢀ1.0 as it could be expected for the
transformation of the neutral bpy molecule into the negatively
charged bpyꢀ radical anion. In the studied cases the comparison
of the charges located on the bpy ligands allows us to conclude
1.651
1.669
[Os(Cl)(CO)(bpy)(tpp)2]+
Os–N
Os–P
Os–C
Os–Cl
2.141, 2.073
2.453, 2.454
1.878
2.126, 2.034
2.533, 2.525
1.895
2.467
2.405
a
The computed bond lengths in the optimized S0 state geometries are
close (within ꢂ0.01–0.03 Å) to those from the X-ray data. Only the Os–P
and C–O bond lengths are somewhat larger (ꢂ0.05–0.07 Å and ꢂ0.04–
0.05 Å, respectively) than the experimental ones.
3
that during the S0 - *MLCT excitation an effective charge shift
between Os2+ and bpy of ca. ꢀ0.48–0.56 takes place.
Another set of data from the performed DFT calculations also
3
support the dominant MLCT nature of the excited *[Os(H)(CO)-
(bpy)(tpp)2]+ as well as 3*[Os(Cl)(CO)(bpy)(tpp)2]+ states. Upon upon excitation from S0 to T1. In general, all the computed
inspecting the calculated spin densities one can see that one of geometric changes are also consistent with the electron transfer
the unpaired electrons is nearly localized on the metallic ion. from the Os2+ d orbital(s) to the p* orbital(s) of the bpy ligand.
The obtained Mulliken atomic spin densities as high as 0.7–0.8
1
3
for the Os3+ core are mainly compensated by the sum of the spin
densities localized on the carbon and nitrogen atoms of the bpy
ligand.
In a similar way the computed values of the dipole moments
also point to the MLCT character of the lowest excited
3*[Os(H)(CO)(bpy)(tpp)2]+ and 3*[Os(Cl)(CO)(bpy)(tpp)2]+ states.
During electronic excitation the initial dipole moments charac-
Energetics of S0 - *MLCT and S0 ’ *MLCT transitions
The performed DFT/TD-DFT calculations allowed us to construct
energy diagrams presenting energetic levels of the excited
1*MLCT and 3*MLCT states under study. Energies of the vertical
S0 - 1*MLCT and S0 ’ 3*MLCT transitions have been computed
for two molecular geometries, optimized for the ground S0 state
and the lowest excited T1 state, respectively. Energies of both
transitions have been computed for the [Os(Cl)(CO)(bpy)(tpp)2]+
or [Os(H)(CO)(bpy)(tpp)2]+ complexes in vacuum as well as in
solvent (CH2Cl2) environments. The latter has been done using
the equilibrated and non-equilibrated solvation options of the
PCM (polarizable dielectric continuum model) implemented in
the applied Gaussian 09W software. The computed energies of
the S0 - 1*MLCT as well as S0 ’ 3*MLCT transitions correspond
to the experimental values of the absorption and emission
maxima. In the case of absorption, the computation errors are
as small as 0.02–0.03 eV. In the case of emission, the errors are
larger (0.15–0.23 eV) but the agreement can also be regarded as
fairly satisfactory for the theory level applied in the computations.
Values of the computed energies of T1 and S1 states (related to
the S0 state energy taken as zero) are presented in Fig. 6. Data
shown in Fig. 6 correspond to the equilibrated solvation of the
ground and excited states. The computed energies of the excited
1*MLCT and 3*MLCT states, higher in CH2Cl2 solutions than in a
vacuum, indicate that the solvation energies of the ground state
terizing the ground S0 states (mS = 10.3 or 12.7 D, respectively) are
0
changed to mT = 2.1 or 5.7 D in the excited states. The performed
1
calculations indicate nearly parallel orientations of mS and mT
0
1
vectors that allow us to conclude that Dm changes close to 7–8 D.
This remains in reasonable agreement with that expected from
the estimated 0.5–0.55 values describing an effective charge shift
from the metallic centre to the bpy ligand.
The prevailing MLCT character of the lowest triplet state for
both complexes is also reflected in the effects of electron excita-
tion on the computed key bond parameters (data collected in
Table 3). The electronic excitation S0 - 3*MLCT has a significant
influence on the metal–ligand distances with distinct elongation
of the Os–P and Os–C bonds together with shortening of the Os–N
and Os–Cl bonds. In contrast to shortening of the Os–Cl bond in
the excited 3*[Os(Cl)(CO)(bpy)(tpp)2]+ state, the corresponding
Os–H bond in the excited 3*[Os(H)(CO)(bpy)(tpp)2]+ state is some-
what lengthened as compared to the ground state of this complex.
This remains in accordance with changes in effective electric
charges localized on the chloride and hydride ligands. Electronic
excitation also has a marked influence on geometric structures of
the bpy ligand. The computed changes in the CQN and CQC
bond lengths are characteristic of conversion from the conjugated
to the alternate single/double bond distributions associated with
transition between the ground S0 and excited T1 states. This is
consistent with the introduction of an additional electron on the
p* orbitals of the bpy ligand. On the other hand, the structures
of the tpp and CO ligands in the [Os(H)(CO)(bpy)(tpp)2]+ or
DGSso0lv are distinctly larger compared to the solvation energies of
both excited states, DGTso1lv and DGsSo1lv, respectively. In the case of
3
S0 ’ *MLCT transitions, they remain in nice agreement with
the computed values of dipole moments characterizing the
ground state mS and the T1 state mT .
Assuming that the observed differences in the solvation
energies arise from the difference in the dipole moment of
the solvated species one can separate the computed DGsTo1lv
0
1
ꢀ
[Os(Cl)(CO)(bpy)(tpp)2]+ complexes remain almost unchanged DGsSo0lv values into individual contributions from the solvation
Phys. Chem. Chem. Phys.
This journal is ©the Owner Societies 2016

View Article Online
PCCP
Paper
with the values of the dielectric constant (e = 8.93) and the
refraction index (n = 1.4244) of CH2Cl2 one can simply estimate
solv
DE
terms using the available DGTso1lv ꢀ DGsSo0lv values and
FC
solv
FC
(mS ꢀ mT )/(mS + mT ) ratios. The resulting, rather small DE
E
0
1
0
1
0.05 eV values correspond well with those obtained in the
performed DFT/TD-DFT calculations (ca. 0.02–0.03 eV). Such
solv
FC
small DE
values also indicate that the intramolecular
changes (in bond lengths and angles) are dominantly contri-
buting to the energies of Franck–Condon states reached in the
3
3
discussed S0 - *MLCT absorption (DEaFbCs) and S0 ’ *MLCT
emission (DEeFmC ) transitions. The Franck–Condon energies
for the 3*[Os(H)(CO)(bpy)(tpp)2]+ state (DE
= 0.39–0.40 eV;
abs
FC
em
DE = 0.43–0.47 eV) are distinctly larger compared to those
FC
abs
em
FC
(DE
= 0.29–0.32 eV; DE = 0.33 eV) for the 3*[Os(Cl)(CO)-
FC
(bpy)(tpp)2]+ state. This remains in agreement with the experimen-
tally found differences in the emission spectra recorded for the
[Os(H)(CO)(N4N)(tpp)2]+ and [Os(Cl)(CO)(N4N)(tpp)2]+ series in
both temperature regimes applied.
Emission band shape analysis
Additional information on the properties of the excited 3*MLCT
states can be obtained from the band shape analysis of the
S0 ’ 3*MLCT spectra. The emission profile, i.e. the relationship
between the emission intensity Iem(n~em) (in photons per second
Fig. 6 Energies (values in eV) of the excited 3*MLCT and 1*MLCT states as
computed for the [Os(H)(CO)(bpy)(tpp)2]+ (top) and [Os(Cl)(CO)(bpy)(tpp)2]+
(bottom) complexes. Data in CH2Cl2 solutions calculated for the equilibrated
solvent polarization. Values in parentheses correspond to the experimentally
found emission and absorption maxima.
per unit of spectral energy) and the emitted photon energy hcn~em
,
is given by40–43
2
Iðn~em
Þ
64p4 Mem
3h
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4plOkBT
¼
3
of the ground and excited state in view of the Onsager
relationship62
ðnn~em
Þ
"
#
j
2
ꢀSH
X
e
S
ðEMLCT ꢀ lO ꢀ jhnH ꢀ hcn~em
4lOkBT
Þ
ꢁ
H exp ꢀ
2
DG
ꢀ DG
¼
2 ꢁ
(1)
mS ꢀ mT
e ꢀ 1
T1
solv
S0
solv
j!
0
1
j¼0
3
a0
2e þ 1
(3)
where e and a0 are the static dielectric constant of the solvent and
the effective radius of the Onsager cavity. Taking into account the
available (mS /mT )2 ratios one can obtain, using simple arithmetic,
both DGTso1lv and DGSso0lv values. In the case of the [Os(H)(CO)(bpy)-
where h is the Planck constant, kB the Boltzmann constant and T
the absolute temperature. The electron–vibration coupling con-
stant SH = (lH/hnH) is equal to the inner reorganization energy lH
expressed in units of the high-energy vibrational quanta hnH. The
inner reorganization energy lH corresponds to the high-frequency
motions (represented by a single ‘‘averaged’’ nH) associated with
the changes in the solute bond lengths and angles. The reorgani-
zation energy lO is related to the low-frequency motions such as
reorientation of the solvent shell (lS) as well as any other low- and
medium-frequency nuclear motions of the solute (lL and lM)
undergoing charge transfer. In the studied cases contributions
from the lS = DEsFoClv terms seem to be, in view of the DFT/TD-DFT
calculations, negligibly small. Thus lO can be approximated by the
sum of lL and lM.
In the performed band-shape analyses the relevant quan-
tities (i.e. EMLCT, lO, lH, and hnH) were varied as free fit
parameters. The obtained results are collected in Table 4.
Representative examples of the numerical fits of the MLCT
emission spectra (presented in Fig. 7 and 8) show that the
experimental emission profiles of all the complexes studied
could be quite well reproduced in both temperature regimes
of the performed measurements. The fittings have been done
0
1
(tpp)2]+ complex DGS0 ¼ ꢀ0:17 eV and DGTso1lv ¼ ꢀ0:01 eV from
solv
the solvent induced shift in energy of the excited 3*[Os(H)(CO)-
(bpy)(tpp)2]+ state (0.16 eV) can be obtained. In a similar way
DGsSo0lv ¼ ꢀ0:30 eV and DGsTo1lv ¼ ꢀ0:06 eV can be estimated for the
[Os(Cl)(CO)(bpy)(tpp)2]+ complex from DGsTo1lv ꢀ DGSso0lv ¼ 0:24 eV.
Taking into account the available DGSso1lv ꢀ DGsSo0lv differences
(0.18 and 0.27 eV for the excited 1*[Os(H)(CO)(bpy)(tpp)2]+ and
1*[Os(Cl)(CO)(bpy)(tpp)2]+ states, respectively) one can conclude that
dipole moment values mS of these 1*MLCT states are virtually equal
1
to their mT counterparts characterizing the excited 3*MLCT states.
1
The computed mT and mS values also allow the prediction of
1
0
solv
FC
solvent contributions to the Franck–Condon energies DE
in
the spectroscopic transitions under study. It can be done using
the Lippert–Mataga relationship63,64
ꢀ
ꢁ
2 ꢁ
ꢂ
ꢃ
mS ꢀ mT
e ꢀ 1
n2 ꢀ 1
solv
FC
0
1
DE
¼
ꢀ
(2)
3
a0
2e þ 1 2n2 þ 1
This journal is ©the Owner Societies 2016
Phys. Chem. Chem. Phys.

View Article Online
Paper
PCCP
Table
4
Summary of quantities (values in eV) describing the 3*MLCT
-
S0 emission from the excited 3*[Os(H)(CO)(N4N)(tpp)2]+ and
3*[Os(Cl)(CO)(N4N)(tpp)2]+ states as obtained from analysis of the S0
’ ,
3*MLCT emission band profiles. Fitted values of excited state energies EMLCT
inner reorganization energies lH, the vibrational quanta hnH, outer reorganization energies lO, and resulting energies of 0–0 transitions E00 = EMLCT ꢀ lO
298 K
77 K
Complex type
Ligand N4N
EMLCT
hcnH
lH
lO
E00
EMLCT
hcnH
lH
lO
E00
[Os(H)(CO)(N4N)(tpp)2]+
tmphen
47dmphen
56dmphen
phen
dpphen
dtbbpy
bpy
2.53
2.48
2.40
2.44
2.30
2.47
2.46
0.15
0.16
0.17
0.15
0.17
0.16
0.16
0.24
0.19
0.19
0.22
0.17
0.23
0.26
0.38
0.44
0.35
0.41
0.36
0.42
0.45
2.13
2.04
2.05
2.03
1.94
2.05
2.01
2.77
2.78
2.79
2.77
2.62
2.89
2.92
0.15
0.15
0.16
0.15
0.15
0.15
0.15
0.25
0.24
0.21
0.22
0.20
0.22
0.22
0.37
0.42
0.46
0.45
0.43
0.61
0.64
2.40
2.36
2.33
2.32
2.19
2.28
2.28
[Os(Cl)(CO)(N4N)(tpp)2]+
tmphen
47dmphen
56dmphen
phen
dpphen
dtbbpy
bpy
2.38
2.29
2.29
2.26
2.18
2.31
2.28
0.16
0.16
0.16
0.16
0.16
0.16
0.16
0.24
0.23
0.20
0.21
0.19
0.23
0.23
0.20
0.17
0.20
0.19
0.19
0.23
0.24
2.19
2.12
2.11
2.08
1.99
2.08
2.05
2.67
2.70
2.68
2.65
2.50
2.77
2.72
0.16
0.16
0.16
0.16
0.16
0.16
0.16
0.27
0.24
0.22
0.24
0.21
0.27
0.25
0.31
0.36
0.37
0.35
0.32
0.45
0.41
2.36
2.34
2.31
2.30
2.18
2.32
2.31
The fitted EMLCT and lO as well as lH and hnH quantities turn out to be somewhat correlated, leading to a numerical fit uncertainty (ꢂ0.02 eV) of
their values.
with an assumption that the electronic transition dipole
moment value Mem is independent of the emitted photon energy
but nearly the same values of the fitted parameters are obtained
in the analysis taking into account an appropriate dependence
of Mem vs. n~em (e.g. Mem B 1/n~em as expected from the Mulliken–
Hush formalism36–39). Because the performed fittings were done
using a single high-frequency mode approximation the resulting
EMLCT and lO may be overestimated (slightly at 298 K, markedly
at 77 K) due to the same classical treatment of the low- and
medium-frequency mode contributions. Principally the pro-
blem can be overcome using two-mode instead of one-mode
approximation65 but this approach is hardly applicable in the
Fig. 7 Emission spectra recorded for the [Os(H)(CO)(56dmphen)(tpp)2]+
(top) and [Os(Cl)(CO)(56dmphen)(tpp)2]+ (bottom) complexes in CH2Cl2
solutions at room temperature (yellow lines) and in 1 : 1 methanol/ethanol
matrices (cyan lines) at 77 K, respectively, and the corresponding numerical
fits (dashed black lines) according to eqn (3).
present cases because the used one-mode fitting procedure
quite adequately reproduces the recorded room-temperature
and 77 K emission spectra. In most cases the fittings done with
two-mode approximation did not improve the fits’ quality.
Another approach has been proposed by Cortes, Heitele, and
Jortner.44 Their approach, based on the semi-classical treatment
of the medium-frequency modes together with the classical and
quantum treatment of the low-frequency and high-frequency
modes, leads to the reformulation of eqn (3) through simple
modification of the denominator in the exponential parts of this
equation. Due to the contribution of the solute medium-
frequency modes hnM (associated with the reorganization energy
lM) the original 4kBTlO denominator should be replaced by the
4kBT(lL + lS) + 2hnMlM coth(hnM/2kBT) term. The introduced
modification of the denominator allows corrections of the fitted
EMLCT and lO values according to the following relationships44
lO(fit) = lO(true) + lM[(hnM/2kBT)coth(hnM/2kBT) ꢀ 1]
(4)
Fig. 8 Emission spectra recorded for the [Os(H)(CO)(bpy)(tpp)2]+ (top)
and [Os(H)(CO)(bpy)(tpp)2]+ (bottom) complexes in CH2Cl2 solutions at
room temperature (yellow lines) and in 1 : 1 methanol/ethanol matrices
(cyan lines) at 77 K, respectively and the corresponding numerical fits
(dashed black lines) according to eqn (3).
and
EMLCT(fit) = EMLCT(true) + lM[(hnM2/kBT)coth(hnM/2kBT)]
(5)
Phys. Chem. Chem. Phys.
This journal is ©the Owner Societies 2016

View Article Online
PCCP
Paper
At room temperature, however, the required corrections are where the values of EMLCT, lO, hnH and SH = lH/hnH are derived
usually negligibly small for the typical values of hnM quanta, with from the band-shape analysis of the S0 ’ 3*MLCT luminescence
n
M E 350 cmꢀ1 values, as reported for the homoleptic [Os(bpy)3]2+ spectra. ‘‘An effective’’ V30 value in eqn (6) describes the electronic
3
and heteroleptic [Os(H)(CO)(bpy)2]+ or [Os(Cl)(CO)(bpy)2]+ interactions between the excited *MLCT and ground S0 states.
complexes.66 For example the required lO corrections are as small According to the Mulliken–Hush formalism, connecting the opti-
as ca. 25% of lM and correspondingly less for the overall lO value cal and thermal processes, the V30 values are available from the
3
because lO 4 lM. This means that the room temperature data from investigations of the radiative properties of the *MLCT states.
the band shape analyses can be directly used in the further
Within the Mulliken–Hush approach the transition dipole
discussion. The fitted EMLCT values, 2.46 eV for [Os(H)(CO)(bpy)- moment M for the optical electron transfer is related to the
(tpp)2]+ and 2.28 eV for [Os(H)(CO)(bpy)(tpp)2]+, are distinctly larger electronic coupling matrix element for the thermal electron
compared to their TD-DFT values. Most probably the observed transfer, V, via the change in dipole moment Dm associated with
discrepancies can be attributed to the underestimation of the the electron transfer and the energy gap DE between the states
computed EMLCT values. This is the well-known tendency of the involved in these processes.36–39
applied DFT/TD-DFT calculations.70,71 In a similar way the experi-
V
M ¼
Dm
(7)
mentally found values of lO + lH terms, equal to 0.71 and 0.46 eV,
DE
em
FC
are distinctly larger than the DFT estimates of the DE energies,
In the case of the S0 - 1*MLCT transition the observable Mabs can
be related to the energy of MLCT absorption hcn~mabasx according to
0.47 and 0.33 eV for the [Os(Cl)(CO)(bpy)(tpp)2]+ and [Os(H)(CO)-
(bpy)(tpp)2]+ complexes, respectively. Thus, one can state for these
complexes only semi-quantitative congruency between the experi-
ment and theory. The crucial energetic parameters associated with
V10
Mabs
¼
maxDm
(8)
hcn~abs
the S0
’
3*MLCT emission, as available from the band shape
where the electronic coupling element V10 describes interactions
analyses and the DFT/TD-DFT computations, remain in only semi-
quantitative agreement with discrepancy up to 0.3 eV. A similar lack
of self-consistency is also expected for remaining investigated
chelates but the final confirmation requires more systematic DFT/
TD-DFT computations that go beyond the framework of this paper.
Basically eqn (4) and (5) allow us to introduce appropriate
corrections to the fitted EMLCT and lO values in both temperature
regimes but it is hardly possible without detailed knowledge of
hnM and an additional assumption of relative contributions of lL
and lM to lO. At 77 K, the (hnM/2kBT)coth(hnM/2kBT) correction
factor becomes as large as 3.3. It leads to serious overestimations
of both lO and EMLCT terms. On the other hand the 4kBT(lS + lL)
values must be definitely smaller than 2lMhnM coth(hnM/2kBT)
because of 2kBT { hnM and suppressed (as compared to 298 K)
lS + lL values.67–69 Thus the lM values can be estimated with quite
between the ground S0 and the excited 1*MLCT states.
In the case of the S0
’
3*MLCT transition the respective
relation is somewhat more complicated due to the spin for-
bidden nature of this process. For the transition metal com-
plexes, however, the presence of the heavy metal ion allows (due
to strong metal-induced spin–orbit couplings) mixing between
the states of different multiplicity. This results in the contribu-
1
tion of the *MLCT electronic configuration to the wave func-
tion describing the excited 3*MLCT state. The admixture of the
1*MLCT to 3*MLCT state is proportional to the spin–orbit
coupling VSOC and inversely proportional to the energy gap
DE33 between these states. Thus, in the case of the S0 ’ 3*MLCT
transition the observable Mem can be related to the energy of
max
em
MLCT emission hcn~
according to72,73
good accuracy from the fitted lO values according to lO
E
VSOC V10
DEST hcn~emmax
Mem
¼
Dm
(9)
lM(hnM/2kBT)coth(hnM/2kBT) approximation. For the [Os(H)(CO)-
(bpy)(tpp)2]+ and [Os(Cl)(CO)(bpy)(tpp)2]+ complexes one can
obtain lM E 0.19 eV and lM E 0.12 eV, respectively. Thus for The term V10VSOC/DEST that appears in eqn (9) can be inter-
these complexes one conclude similar contribution (at 298 K) of preted as ‘‘an effective’’ V30 value describing the electronic
3
the lM and lL components to the overall lO values.
interaction between the excited *MLCT and ground S0 states.
The V30 value, required for the calculation of knr values applying
eqn (6), can be straightforwardly estimated from the experi-
mentally available Mem and hcn~memax values. Both eqn (8) and (9)
are strictly valid only when the Mabs and Mem are determined
exclusively by the direct interactions between the 1*MLCT state
and the S0 ground state without any significant contributions
3
Kinetics of the non-radiative *MLCT - S0 processes
Pursuing the analogy between the radiative and non-radiative
electron transfer rates in the inverted Marcus region one can
additionally test the band shape analysis data. It can be done by
comparing the experimentally found rates of the non-radiative
1
3
from the locally excited *IL configurations. To a large extent,
however, this seems to be justified because Dm values are
distinctly larger than the transition dipole moments MIL char-
S0 ’ *MLCT transitions with those obtained from the calcula-
tions performed using the following equation42,43
2
4p2
h
V30
1
acteristic of the S0 - *IL transitions. The Dm values (typically
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
knr
¼
4plOkBT
in the range of 10–20 D) are larger than the difference between
mgs and mes vectors because rarely the excited 1*MLCT and
3*MLCT states are possessing 100% MLCT character. The V30
values (data in Table 5) estimated with Dm = 15 D seem to be
"
#
(6)
j
2
ꢀSH
X
e
S
ðEMLCT þ lO þ jhnHÞ
4lOkBT
ꢁ
H exp ꢀ
j!
j¼0
This journal is ©the Owner Societies 2016
Phys. Chem. Chem. Phys.

View Article Online
Paper
PCCP
Table 5 Summary of the kinetic data for the investigated [Os(H)(CO)(N4N)(tpp)2]+ and [Os(Cl)(CO)(N4N)(tpp)2]+ complexes. Experimental and
calculated values of the non-radiative knr rate constants, and experimental values of the radiative kr rate constants. Estimated values of the electronic
coupling elements V30 (at 298 K) and the medium-frequency mode reorganization energies lM (at 77 K)
298 K
77 K
Complex type
Ligand N4N
V30a/eV
knr(calc)b/sꢀ1
knr(exp)c/sꢀ1
krc/sꢀ1
lM(true)d/eV
knr(calc)e/sꢀ1
knr(exp)f/sꢀ1
[Os(H)(CO)(N4N)(tpp)2]+
tmphen
47dmphen
56dmphen
phen
dpphen
dtbbpy
bpy
0.0045
0.0056
0.0082
0.0064
0.0062
0.0094
0.0077
9.7 ꢁ 103
3.8 ꢁ 104
1.0 ꢁ 105
8.7 ꢁ 104
1.0 ꢁ 105
5.2 ꢁ 105
2.1 ꢁ 106
3.7 ꢁ 104
7.8 ꢁ 104
1.3 ꢁ 105
2.0 ꢁ 105
1.2 ꢁ 105
8.9 ꢁ 105
2.0 ꢁ 106
4.9 ꢁ 103
6.5 ꢁ 103
1.0 ꢁ 104
8.6 ꢁ 103
7.7 ꢁ 103
1.8 ꢁ 104
1.2 ꢁ 104
0.11
0.13
0.14
0.14
0.13
0.18
0.19
6.9 ꢁ 102
1.8 ꢁ 103
3.9 ꢁ 103
1.4 ꢁ 103
3.1 ꢁ 103
1.2 ꢁ 104
9.8 ꢁ 103
6.3 ꢁ 103
1.2 ꢁ 104
1.8 ꢁ 104
1.8 ꢁ 104
2.0 ꢁ 104
5.3 ꢁ 104
3.4 ꢁ 104
[Os(Cl)(CO)(N4N)(tpp)2]+
tmphen
47dmphen
56dmphen
phen
dpphen
dtbbpy
bpy
0.0059
0.0071
0.0081
0.0092
0.0082
0.0136
0.0129
2.2 ꢁ 104
3.8 ꢁ 104
1.9 ꢁ 104
5.2 ꢁ 104
4.6 ꢁ 104
3.2 ꢁ 105
5.1 ꢁ 105
8.7 ꢁ 104
6.9 ꢁ 104
8.0 ꢁ 104
8.5 ꢁ 104
5.8 ꢁ 104
4.2 ꢁ 105
6.8 ꢁ 105
7.6 ꢁ 103
1.1 ꢁ 104
1.4 ꢁ 104
1.8 ꢁ 104
1.4 ꢁ 104
3.9 ꢁ 104
3.4 ꢁ 104
0.09
0.11
0.11
0.11
0.10
0.14
0.12
1.0 ꢁ 104
4.6 ꢁ 103
4.5 ꢁ 103
1.6 ꢁ 104
1.9 ꢁ 104
1.8 ꢁ 105
6.2 ꢁ 104
1.6 ꢁ 104
9.2 ꢁ 103
1.9 ꢁ 104
1.8 ꢁ 104
1.4 ꢁ 104
5.7 ꢁ 104
7.1 ꢁ 104
a
b
Estimated according to eqn (9) using the experimental Mem and n~memax values. Calculated according to eqn (6) with the EMLCT, lO, lH and hnH data
c
summarized in Table 4. knr and kr values obtained from the experimental fem and tem values according to the knr = (1 ꢀ fem)/tem and kr = fem/tem
relationship. Estimated from the fitted lO values (77 K data) using the lO E lM(hnM/2kBT)coth(hnM/2kBT) relationship with hnM = 350 cmꢀ1
.
d
e
f
Calculated according to eqn (10) with the E00, lH and hnH data summarized in Table 4. Estimated according to eqn (11).
directly applicable in the calculation of the knr rate constants nature of the investigated 3*MLCT states but for better quantita-
according to eqn (6). The obtained knr values are collected in tive description any more (than applied in this work) sophisti-
Table 5 and the relation of the calculated values to the experi- cated theory level can be recommended.
mentally found ones is shown in Fig. 9.
Nominally eqn (6) allows also very crude estimations of the
The discrepancies between the experimental rate constants knr knr rate constants using the data from the band shape analyses
and the values calculated from eqn (6) are smaller than one order performed for the 77 K emission spectra. The resulting values
of magnitude which may be regarded as more than satisfactory. are two/three orders of magnitude smaller than their 298 K
The obtained agreement allows us to conclude that the intrinsic counterparts. It could suggest that at 77 K the non-radiative
parameters (from the band shape analysis data) associated with S0 ’ 3*MLCT transitions are totally suppressed. It seems to be
the S0 ’ 3*MLCT transitions are more or less correctly estimated. not a case because the experimentally found tem values are
This is also true for the EMLCT energies, otherwise the observed several times shorter than expected from the kr values at 298 K.
agreement between the calculated and experimentally found knr An alternative explanation of this finding, based on the tem-
is hardly expected. Thus, one can assume the EMLCT values from perature induced changes in the kr values, can be most likely
the emission band shape analysis as more reliable than those ruled out, otherwise the transition dipole moments character-
from the performed DFT/TD-DFT computations. Without any izing the radiative S0 ’ 3*MLCT transition at 77 K must be ca.
doubts the DFT/TD-DFT results give additional insight into the two times larger than that at 298 K. This is potentially possible
but very hard to rationalize.
Much more plausible explanation of the observed disagree-
ment assumes the temperature induced changes in the mecha-
nism of the non-radiative electron transfer, from temperature
independent tunnelling between nuclear potential surfaces at
low temperatures to an activated rate expression at high
temperatures. Based on the above assumption one can, according
to the Jortner considerations,74 propose following expression as
more appropriate for calculations of the knr values at low (77 K
and below) temperatures
pð jÞ
ꢀSM
SM
j
SH e
2
ꢀSH
4p2 V30
e
X
knr
¼
(10)
h hnM
j!
pð jÞ!
j¼0
where SM = (lM/hnM) is equal to the medium-frequency mode
reorganization energy lM expressed in units of the medium-
frequency vibrational quanta hnM and p( j) is taken as an integer
closest to (E00 ꢀ jhnH)/hnM.75 According to the proposed approach
Fig. 9 Relationship between the calculated and experimental knr rate
constants for the investigated [Os(H)(CO)(N4N)(tpp)2]+ (cyan symbols)
and [Os(Cl)(CO)(N4N)(tpp)2]+ (yellow symbols) complexes. Data for
CH2Cl2 solutions at room temperature are also given.
Phys. Chem. Chem. Phys.
This journal is ©the Owner Societies 2016

View Article Online
PCCP
Paper
3
the electron transfer takes place during tunnelling from the Kinetics of the radiative *MLCT - S0 processes
nuclear configuration of the initial 3*MLCT state to the vibronically
Similar to the rates of the non-radiative deactivation of the excited
3*MLCT states, the radiative kr rate constants depend on the
nature of the N4N ligand present in structures of the investigated
complexes. For both the studied series the experimentally found kr
values are in an opposite sequence (cf. data in Tables 1 and 5) as it
could be expected from positions of their S0 ’ 3*MLCT emission
bands. This arises from different values of the transition dipole
excited states of the final S0 products. For effective tunnelling the
energies of the initial and final states must be nearly degenerated
that is fulfilled for an appropriate combination of the high- and
medium-frequency quanta (E00 ꢀ jhnH ꢀ p( j)hnM) E 0.
Due to the lack of the quantum yields characterizing the 77 K
emission it is not possible to obtain the radiative kr and non-
radiative knr rate constant characterizing the S0
’
3*MLCT
3
moments Mem characterizing the given *MLCT emitter. In parti-
transitions in the low temperature matrices. However, an approx-
imate estimation of the knr values is possible assuming that the
transition dipole moment Mem remains the same in both fluid
and solid media. Then ‘‘the experimental’’ knr values can be
obtained as follows
cular large differences are observed between the Mem values for the
complexes with the tmphen ligand and their bpy analogues.
Comparing the Mem values for both series of the investigated
complexes it can be clearly seen that these from the
[Os(Cl)(CO)(N4N)(tpp)2]+ series are better emissive than their
[Os(H)(CO)(N4N)(tpp)2]+ analogues. According to eqn (9) it can
be tentatively attributed to different values of the VSOC/DE13 term
1
knrð77 KÞ ꢃ
ꢀ krð298 KÞ
(11)
tem
1
3
describing the mixing between the excited *MLCT and *MLCT
states. This statement is only valid if the V10Dm terms have similar
values for the given [Os(H)(CO)(N4N)(tpp)2]+ and [Os(Cl)(CO)-
(N4N)(tpp)2]+ pair. The latter assumption arises from the compar-
ison of the Mabs values characterizing the S0 - 1*MLCT absorption
This is obviously only approximate solution but the resulting knr
values, expected to be not far away from ‘‘the true’’ ones, can be
used to test theoretical predictions.
In the case of the studied complexes, the necessary quan-
tities, E00, lH and hnH, are directly available from the performed
with the Mem values attributed to the S0
Whereas for the given [Os(H)(CO)(N4N)(tpp)2]+ and [Os(Cl)-
’
3*MLCT emission.
band shape analyses of the 77 K S0
spectra, whereas the remaining lM values can be estimated (CO)(N4N)(tpp)2]+ pair the Mabs values are quite similar to one
’
3*MLCT emission
from the fitted lO terms appropriately corrected with the nM
=
another that allows us to assume comparable V10Dm values, the
Mem values exhibit larger variability. In particular changes in the
Mabs/Mem ratios for the [Os(Cl)(CO)(N4N)(tpp)2]+ series are up to
two times larger than those for their [Os(H)(CO)(N4N)(tpp)2]+
counterparts. The observed difference is not extremely large but
surely larger than any possible errors (ca. 10%) in the estimation of
the discussed Mabs and Mem quantities.
350 cmꢀ1. The knr values computed according to eqn (10) are
summarized in Table 5. The results of the calculations, done
with the same V30 values as applied in the calculation of the
room temperature knr rate constants, are in reasonable agree-
ment with the 77 K estimates. Similarly as it was found for the
298 K data, deviations are smaller than one order of magnitude,
although the presence of some systematic error (cf. Fig. 10)
seems to be evident, especially for the complexes from
[Os(H)(CO)(N4N)(tpp)2]+ series. There are many reasons that
can be responsible for this outcome, including somewhat improper
nM or V30 values, but, at the present stage of investigations, the
problem must remain unsolved due to the lack of necessary reliable
knr and nM data.
More detailed discussion of the observed changes in the VSOC
/
DEST term requires additional information that could allow the
separation of their values into the individual VSOC and DE13
components. Unfortunately, the already available data do not
allow us to make it without any additional assumptions. The
energy differences between the excited 1*MLCT and 3*MLCT
states obtained from the DFT/TD-DFT calculations are similar
(ca. 0.50–0.60 eV in the equilibrated triplet conformations) for the
[Os(H)(CO)(bpy)(tpp)2]+ and [Os(Cl)(CO)(bpy)(tpp)2]+ complexes.
This may suggest that different VSOC values are mostly respon-
sible for the observed difference in the Mem values for these
complexes.
The DFT/TD-DFT values of the energy gap between the excited
T1 and S1 states as large as 0.5–0.6 eV cannot be a priori excluded
from considerations but so large energy splittings DEST are more
characteristic of the transition metal complexes exhibiting the
intra-ligand S0
’
3*IL emission.76 Usually for this type of
3
1
transition metal complex the perturbed *IL and *IL states are
less energetically separated as compared to the isolated ligand
molecules. This is caused by small admixtures of the metal
d-orbital or MLCT character of the ligand-centred states which
increases the spatial extension of the wave-functions. This
reduces the exchange integral and the energy gap DEST between
the perturbed states. This tendency should be even more distinct
Fig. 10 Relationship between the calculated and experimental knr rate
constants for the investigated [Os(H)(CO)(N4N)(tpp)2]+ (cyan symbols) and
[Os(Cl)(CO)(N4N)(tpp)2]+ (yellow symbols) complexes. Data for 1 : 1
methanol/ethanol matrices at 77 K are also given.
This journal is ©the Owner Societies 2016
Phys. Chem. Chem. Phys.

View Article Online
Paper
PCCP
with increasing MLCT character of the given emitter. More
delocalized charge transfer states should show distinctly reduced
singlet–triplet splitting. The DEST value as small as 0.04 eV has
been recently reported for the cyclometalated Ir(dpbic)3 complex
with 1,3-diphenyl-imidazolin-2-ylidene ligand dpbic.77 Taking
into account the semi-empirical relation between kr and DE13
values presented by Da Como et al.77 one can anticipate that in
the case of the studied complexes the DEST values are in the range
of 0.05–0.15 eV.
Generally speaking, an accurate estimation of the DE13 energy
splitting between the excited 1*MLCT and 3*MLCT states is rather
a challenging task. Experimentally it can be done by the detection
of singlet emission in phosphorescent complexes but, due to the
large spin–orbit coupling and consequently the fast intersystem
crossing rates, it is rarely possible. On the other hand, the
Fig. 11 Relationship between Mem and (EIL ꢀ EMLCT + lO)/hcn~memax quantities.
Room temperature data for the [Os(Cl)(CO)(N4N)(tpp)2]+ (cyan symbols) and
[Os(H)(CO)(N4N)(tpp)2]+ (yellow symbols) in CH2Cl2 solutions. Data for the
previously investigated [Os(Cl)(CO)(N4N)(P4P)]+ chelates (grey symbols) in
quantum-mechanical computations are usually not enough pre- CH3CN solutions34 are presented for comparison. The dotted lines represent
1
3
linear fits with the intercept values equal to zero.
cise, especially when the excited *MLCT and *MLCT states are
close in energy. In the case of ‘‘pure’’ 1*MLCT and 3*MLCT states
one can expect very small values of the energy splitting between
evident despite the fact that the experimental points are
somewhat scattered. The observed differences in the Mem values
for the three series can be attributed to differences in the
1
them. When these states are interacting with the excited *IL or
3*IL states the energy gap between the resulting 1*MLCT and
3*MLCT configurations becomes larger. In the first approximation
one can consider the electronic interactions between the states
that are closest in energy, between the lowest 3*IL and 3*MLCT
and the lowest 1*IL and 1*MLCT, respectively. This approach
has been applied in our previous work34,35 for interpretation of
the luminescence properties of [Os(Cl)(CO)(N4N)(P4P)]+ as well
as [Re(CO)3(N4N)(CH3CN)]+ or [Re(Cl)(CO)3(N4N)] chelates.
In the framework of this model one can estimate many
different parameters related to the MLCT excitation. In particu-
lar, one can relate the energy gap between the resulting 1*MLCT
and 3*MLCT configurations to experimentally available quantities.
Assuming that the electronic interactions between two states
closest in their energies are mainly responsible for the DE13
value, the following relationship can be easily obtained34,35
2
V10VSOCDm/V33 parameters. Taking into account comparable
values of the transition dipole moment Mabs characterizing the
S0 - 1*MLCT absorption one can anticipate similar V10Dm values
for all three investigated series. Thus the observed variability in
2
the V10VSOCDm/V33 terms seems to arise from differences in the
VSOC or V10 and V33 values. The latter option seems to be less
likely because one can expect (according to the formalism
proposed by Dogonadze et al.78) similar values V33 and V10 for
the given central metal ion. If this holds, the observed variability
in the Mem values can be mainly ascribed to the changes in values
of the spin–orbit coupling VSOC elements. Comparing reported
types of osmium(II) complexes, namely [Os(Cl)(CO)(N4N)(P4P)]+,
[Os(Cl)(CO)(N4N)(tpp)2]+ and [Os(H)(CO)(N4N)(tpp)2]+ series,
one can conclude that ligand orientation around the central
metal ion as well as the ligand composition affects the observed
VSOC values, despite the fact that the investigated complexes are
pretty similar. Taking into account structural similarity between
the [Os(Cl)(CO)(N4N)(tpp)2]+ and [Os(H)(CO)(N4N)(tpp)2]+ series
one can judge that the latter is more pronounced. Conclusion
that this is solely caused by simple exchange of the heavy chloride
into the light hydrogen is, however, extremely speculative.
2
2
V33
V33
DEST
ꢃ
¼
(12)
E3 ꢀ EMLCT þ lO E3 ꢀ E00
3
where E3 and V33 are the energy of the lowest excited *IL state
and the coupling element describing the electronic interactions
between the initial 3*IL and ‘‘pure’’ 3*MLCT states. The previously
defined EMLCT, lO and E00 quantities are available from the band
shape analysis. Combining eqn (9) with (12) one obtains
V10VSOCDm ðE3 ꢀ EMLCT þ lOÞ
Mem
ꢃ
¼
2
max
Concluding remarks
V33
hcn~
em
V10VSOCDm ðE3 ꢀ E00Þ
(13)
Comparative studies of the luminescence properties of the
[Os(H)(CO)(N4N)(tpp)2]+ and [Os(Cl)(CO)(N4N)(tpp)2]+ chelates
2
max
V33
hcn~
em
The obtained equation predicts a linear relationship between the point to very similar nature of their lowest excited states. In both
3
values of the transition dipole moments Mem characterizing S0 ’ series emission takes place from *T1 states possessing distinct
3*MLCT emission with experimentally available (E3 ꢀ EMLCT
+
MLCT character. For the given a-diimine N4N ligand MLCT
max
em
lO)/hcn~
quantities, of course if the remaining parameters emission is observed at the nearly same wavelengths. Despite
substituted into eqn (13) are similar to one another. For the all similarities, the investigated 3*MLCT states exhibit some
investigated complexes the expected relationships are presented important differences like values of the reorganization energies
in Fig. 11. Similarly as it was previously observed for [Os(Cl)- accompanying the electron transfer from the Os2+ centre to the
(CO)(N4N)(P4P)]+ chelates,34 the expected trends seem to be coordinated N4N moiety. Another well pronounced difference
Phys. Chem. Chem. Phys.
This journal is ©the Owner Societies 2016

View Article Online
PCCP
Paper
between both series of the investigated complexes is different exhibiting the MLCT phenomena. To which extent the model
values of the transition dipole moments Mem characterizing the will be applicable in any other cases remains an open question.
radiative S0
values describing S0
’
3*MLCT transitions in contrast to similar Mabs Further work in this direction is necessary to give any decisive
-
1*MLCT absorption. The observed answer. Studies of any class of transition metal complexes with
phenomenon can be attributed to different values of the spin– the same ligand environment but different central metal ions
orbit coupling elements VSOC that are responsible for the mixing would be particularly interesting. For example, comparative
between the excited 1*MLCT and 3*MLCT states. The Mem values studies done with the Ru2+ analogues of the Os2+ complexes
typical for two series of the reported complexes with those Mem reported in this work could provide more detailed insight into
characteristic of the previously investigated 3*[Os(Cl)(CO)(N4N)- the factors determining the VSOC and V33 values. Because the
(P4P)]+ chelates have been compared. Based on the observed V33 quantity is expected to be dependent mostly on the nature
differences one can conclude that the nature of the ligands of the ligand(s) accepting electron from the central metal ion,
coordinating to the central Os2+ core as well as their mutual the experimentally found V33 values should be similar for all
arrangement plays an important role in their luminescence complexes with the a-diimines N4N as it was already found for
the Os2+ and Re(CO)3 chelates. In contrast to that much more
+
properties.
The luminescence properties of the studied complexes have pronounced differences are expected in the VSOC values. Systematic
been examined using results from the performed band shape studies on the luminescence properties of the excited 3*MLCT
analyses of the S0 ’ 3*MLCT emissions. The obtained quantities states may clarify the role of the metal and ligand(s) present in the
have been applied in the interpretation of the experimentally given MLCT emitter.
determined rates of the non-radiative deactivations of the excited
3*MLCT species. The found agreement between the theoretically
predicted knr values with those found experimentally (in both
Acknowledgements
298 K and 77 K temperature regimes) allows us to regard the
´
Our students, Paulina Wo´zniak, Izabela Tuszynska and Wioletta
emission band shape analyses as very useful tools in more detailed
characterization of the excited 3*MLCT states. In particular the
data from band shape analyses can be applied to test results from
the quantum-mechanical computation.
´
Muszynska, are deeply appreciated for their technical assistance
in the preparation of the studied complexes.
The latter opportunity seems to be crucial for more adequate
description of the nature of the excited 3*MLCT species. Whereas
the quantum-mechanical computations (usually done using the
DFT/TD-DFT method) quite well represent the electronic nature
Notes and references
¨
1 K. Kalyanasundaram and M. Gratzel, Coord. Chem. Rev.,
1998, 177, 347–414.
2 M. K. Nazeeruddin, C. Klein, P. Liska and M. Gratzel, Coord.
3
¨
of the excited *MLCT states providing credible information on
the charge and spin redistribution or the molecular geometry
changes accompanying the electron transfer between the metal
and the ligand(s) some of the computed quantities are under- or
overestimated. In particular the computed energy splittings
Chem. Rev., 2005, 249, 1460–1467.
3 D. Kumaresan, K. Shankar, S. Vaidya and R. H. Schmehl,
Top. Curr. Chem., 2007, 281, 101–142.
4 S. Ardo and G. J. Meyer, Chem. Soc. Rev., 2009, 38, 115–164.
5 K. Govardhan Reddy, T. G. Deepak, G. S. Anjusree,
S. Thomas, S. Vadukumpully, K. R. V. Subramanian, S. V.
Nair and A. S. Nair, Phys. Chem. Chem. Phys., 2014, 16,
6838–6858.
6 R. C. Evans, P. Douglas and C. J. Winscom, Coord. Chem.
Rev., 2006, 250, 2093–2126.
7 P.-T. Chou and Y. Chi, Chem. – Eur. J., 2007, 13, 380–395.
8 L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong and J. Kido,
Adv. Mater., 2011, 23, 926–952.
1
between the excited 3*MLCT and *MLCT states are markedly
larger than those expected for the excited species possessing
dominant MLCT character. One can expect that more advanced
quantum-mechanical computations (more sophisticated than an
ordinary B3LYP/LANL2DZ/3-21G+ combination applied in this
work) should bring results with definitely better agreement
between theory and experiment. This is, however, another story
requiring further investigations that may confirm the raised
anticipation.
The results obtained from the presented studies on the
luminescence properties of reported [Os(H)(CO)(N4N)(tpp)2]+
9 R. D. Costa, E. Orti, H. J. Bolink, F. Monti, G. Accorsi and
N. Armaroli, Angew. Chem., Int. Ed., 2012, 51, 8178–8211.
and [Os(Cl)(CO)(N4N)(tpp)2]+ chelates also allow us to conclude 10 S. Fantacci and F. De Angelis, Coord. Chem. Rev., 2011, 255,
that the simple model applied in our previously published
work34,35 is quite satisfactory in description of the excited 11 Q. Zhao, F. Li and C. Huang, Chem. Soc. Rev., 2010, 39,
3*MLCT state properties. The model is based on the electronic
3007–3030.
interactions between the lowest excited *IL and *MLCT states 12 V. W.-W. Yam and K. M.-C. Wong, Chem. Commun., 2011,
that may be treated as oversimplification. Despite the fact that 47, 11579–11592.
the model quite well predicts the changes in the properties of the 13 Z. Liu, W. He and Z. Guo, Chem. Soc. Rev., 2013, 42,
excited 3*MLCT states using necessary quantities that are
1568–1600.
experimentally accessible, until yet the model has been tested 14 H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck and
2704–2726.
3
3
successfully only for several types of transition metal complexes
T. Fischer, Coord. Chem. Rev., 2011, 255, 2622–2652.
This journal is ©the Owner Societies 2016
Phys. Chem. Chem. Phys.

View Article Online
Paper
PCCP
´
15 G. Accorsi, A. Listorti, K. Yoosaf and N. Armaroli, Chem. Soc. 45 M. Arias, J. Concepcion, I. Crivelli, A. Delgadillo, R. Diaz,
´
Rev., 2009, 38, 1690–1700.
A. Francois, R. Lopez, A. M. Leiva and B. Loeb, Chem. Phys.,
16 Y. Chi, B. Tong and P.-T. Chou, Coord. Chem. Rev., 2014,
281, 1–25.
17 W.-K. Chu, S.-M. Yiu and C.-C. Ko, Organometallics, 2014, 33,
6771–6777.
18 E. M. Kober, B. P. Sullivan, W. J. Dressick, J. V. Caspar and
T. J. Meyer, J. Am. Chem. Soc., 1980, 104, 630–632.
19 J. V. Caspar, E. M. Kober, B. P. Sullivan and T. J. Meyer,
J. Am. Chem. Soc., 1982, 102, 7383–7385.
2006, 326, 54–70.
46 B. J. Coe and S. J. Glenwright, Coord. Chem. Rev., 2000, 203,
5–80.
47 B. L. Foley, C. A. Smith and N. C. Thomas, Inorg. Chim. Acta,
1988, 150, 163.
48 A. Wo´zna, A. Kapturkiewicz and G. Angulo, Inorg. Chem.
Commun., 2013, 27, 26–29.
49 T.-Y. Luh, Coord. Chem. Rev., 1984, 60, 255–276.
20 J. V. Caspar, B. P. Sullivan, E. M. Kober and T. J. Meyer, 50 H. D. Kaesz and R. B. Saillant, Chem. Rev., 1972, 72,
Chem. Phys. Lett., 1982, 91–95. 231–281.
21 J. V. Caspar, B. P. Sullivan and T. J. Meyer, Organometallics, 51 L. Pazderski, L. Pawlak, J. Sitkowski, L. Kozerski and
1983, 2, 551–554. E. Szłyk, Magn. Reson. Chem., 2010, 48, 450–457.
22 B. P. Sullivan, J. V. Caspar, S. R. Johnson and T. J. Meyer, 52 B. P. Sullivan, R. S. Lumpkin and T. J. Meyer, Inorg. Chem.,
Organometallics, 1984, 3, 1241–1251. 1987, 26, 1247–1252.
23 E. M. Kober, J. L. Marshall, W. J. Dressick, B. P. Sullivan, 53 A. L. Mu¨ler, H. Wadepohl and l. H. Gade, Organometallics,
J. V. Caspar and T. J. Meyer, Inorg. Chem., 1985, 24,
2755–2763.
24 E. M. Kober, J. V. Caspar, B. P. Sullivan and T. J. Meyer,
Inorg. Chem., 1988, 27, 4587–4598.
2015, 34, 2810–2818.
54 P. Datta, D. Sardar, U. Panda, A. Halder, N. B. Manik,
C.-J. Chen and C. Sinha, Appl. Organomet. Chem., 2016, 30,
323–334.
25 R. S. Lumpkin, E. M. Kober, L. A. Worl, Z. Murtaza and 55 M. Bixon, J. Jortner and J. W. Verhoeven, J. Am. Chem. Soc.,
T. J. Meyer, J. Phys. Chem., 1990, 94, 239–243. 1994, 116, 1349–7355.
26 B. Carlson, G. D. Phelan, W. Kaminsky, L. Dalton, X. Jiang, 56 J. W. Verhoeven, T. Scherer, B. Wegewijs, R. M. Hermant,
S. Liu and A. K.-Y. Jen, J. Am. Chem. Soc., 2002, 124,
J. Jortner, M. Bixon, S. Depaemelaere and F. C. De Schryver,
14162–14172.
Recl. Trav. Chim. Pays-Bas, 1995, 114, 443–448.
27 B. Carlson, G. D. Phelan, J. Benedict, W. Kaminsky and 57 J. B. Birks, Photophysics of Aromatic Molecules, Wiley, New
L. Dalton, Inorg. Chim. Acta, 2004, 357, 3967–3974. York, 1978.
28 B. Carlson, G. D. Phelan, J. Benedict, W. Kaminsky and 58 J. Michl and E. W. Thulstrup, Spectroscopy with Polarized
L. Dalton, Inorg. Chim. Acta, 2006, 359, 1093–1102. Light, VCH, New York, 1986.
29 B. Carlson, B. E. Eichinger, W. Kaminsky and G. D. Phelan, 59 M. Senoue, T. Iwaki, K. Seki and M. Yagi, J. Photochem.
J. Phys. Chem. C, 2008, 112, 7858–7865. Photobiol., A, 1996, 101, 257–263.
30 B. Carlson, B. E. Eichinger, W. Kaminsky, J. P. 60 M. A. A. Rashid, B. Saad, E.-S. M. Negim and M. I. Saleh,
Bullock and G. D. Phelan, Inorg. Chim. Acta, 2009, 362,
World Appl. Sci. J., 2012, 17, 958–963.
1611–1618.
61 Z.-Z. Xie and W.-H. Fang, THEOCHEM, 2005, 717, 179–187.
31 M. Schnuriger, M. M. Richter and B. Carlson, Inorg. Chim. 62 L. Onsager, J. Am. Chem. Soc., 1936, 58, 1486–1493.
Acta, 2011, 378, 202–205. 63 E. Lippert, Z. Naturforsch., 1955, A10, 541–545.
32 S. Anastasova, A. Kapturkiewicz and J. Nowacki, Inorg. 64 N. Mataga, Y. Kaifu and M. Koizumi, Bull. Chem. Soc. Jpn.,
Chem. Commun., 2005, 8, 1177–1180. 1955, 28, 690–691.
33 A. Kamecka, A. Kapturkiewicz, Z. Karczmarzyk and 65 A. Ito and T. J. Meyer, Phys. Chem. Chem. Phys., 2012, 14,
W. Wysocki, Inorg. Chem. Commun., 2013, 27, 138–141. 13731–13745.
34 A. Kamecka and A. Kapturkiewicz, Phys. Chem. Chem. Phys., 66 E. G Megehee and T. J. Meyer, Inorg. Chem., 1989, 28,
2015, 17, 23332–23345. 4084–4091.
35 A. Wo´zna and A. Kapturkiewicz, Phys. Chem. Chem. Phys., 67 R. A. Marcus, J. Phys. Chem., 1990, 94, 4963–4966.
2015, 17, 30468–30480.
68 P. Chen and T. J. Meyer, Inorg. Chem., 1996, 35,
5520–5524.
69 A. Ito, T. E. Knight, D. J. Stewart, M. K. Brennaman and
T. J. Meyer, J. Phys. Chem. A, 2014, 118, 10326–10332.
70 T. A. Niehaus, T. Hofbeck and H. Yersin, RSC Adv., 2015, 5,
63318–63329.
36 R. S. Mulliken, J. Am. Chem. Soc., 1952, 74, 811–824.
37 R. S. Mulliken, J. Phys. Chem., 1952, 56, 801–822.
38 N. S. Hush, Prog. Inorg. Chem., 1967, 8, 391–444.
39 N. S. Hush, Electrochim. Acta, 1968, 13, 1005–1023.
40 R. A. Marcus, J. Phys. Chem., 1989, 93, 3078–3086.
41 R. A. Marcus and N. Sutin, Biochim. Biophys. Acta, 1985, 811, 71 C. Adamo and D. Jacquemin, Chem. Soc. Rev., 2013, 42,
265–322. 845–856.
42 J. Ulstrup and J. Jortner, J. Chem. Phys., 1975, 63, 4358–4368. 72 S. Haneder, E. D. Como, J. Feldmann, J. M. Lupton,
43 M. Bixon and J. Jortner, J. Phys. Chem., 1991, 95, 1941–1944.
44 J. Cortes, H. Heitele and J. Jortner, J. Phys. Chem., 1994, 98,
2527–2536.
C. Lennartz, P. Erk, E. Fuchs, O. Molt, I. Mu¨nster,
C. Schildknecht and G. Wagenblast, Adv. Mater., 2008,
20, 3325–3330.
Phys. Chem. Chem. Phys.
This journal is ©the Owner Societies 2016

View Article Online
PCCP
Paper
73 N. Turro, Modern Molecular Photochemistry, University 77 S. Haneder, E. Da Como, J. Feldmann, J. M. Lupton,
Science Books, Palo Alto, CA, 1991.
74 J. Jortner, J. Chem. Phys., 1976, 64, 4860–4867.
75 J. J. Markham, Rev. Mod. Phys., 1959, 31, 956–989.
C. Lennartz, P. Erk, E. Fuchs, O. Molt, I. Mu¨nster,
C. Schildknecht and G. Wagenblast, Adv. Mater., 2008,
20, 3325–3330.
76 H. Yersin and D. Donges, Top. Curr. Chem., 2001, 214, 78 R. R. Dogonadze, A. M. Kuznetsov and T. A. Marsagishvili,
81–184.
Electrochim. Acta, 1980, 25, 1–28.
This journal is ©the Owner Societies 2016
Phys. Chem. Chem. Phys.