A spectroscopic and computational study on the effects of methyl and phenyl substituted phenanthroline ligands on the electronic structure of Re(I) tricarbonyl complexes containing 2,6-dimethylphenylisocyanide

Article abstract of DOI:10.1039/b415079a

[Re(CO)₃(CNx)(L)]⁺, where CNx = 2,6- dimethylphenylisocyanide, forms complexes with L = 1,10-phenanthroline (1), 4-methyl-1,10-phenanthroline (2), 4,7-dimethyl-1,10-phenanthroline (3), 3,4,7,8-tetramethyl-1,10-phenanthroline (4), 2,9-dimethyl-1,10-phenanthroline (5) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (6). The metal-ligand-to-ligand charge transfer transition (MLLCT) absorption bands follow the series: 3 (27 800 cm^-1) > 1, 2,4 and 5 (27 500 cm ^-1) > 6 (26 600 cm^-1). Density functional theory (DFT) geometry optimizations reveal elongated Re-N (L) distances of 2.28 and 2.27 A for 5 and 6, respectively, compared to 2.23 A for 1-4. The reversible reduction potentials (E_1/2(red)) of 1-4 are linearly dependent on the B3LYP calculated LUMO energies. Time-dependent (TD) DFT and conductor-like polarizable continuum model (CPCM) calculated singlet excited states deviate by 700 cm^-1 or less from the experimental absorption maxima and aid in the spectral assignments. The ³MLLCT emitting state energies are within 900 cm^-1 of the experimental 77 K emission energies for 1-6. The 77 K emission energies, E_1/2(red), and the room temperature emission quantum yields (φ_LuMOem) decrease in the order 1 > 2 > 3 > 4 whereas E_LUMO and the room temperature emission energies follow the opposite trend. The emission lifetimes (τ_em) decrease in the order 3 > 4 > 2 > 1 > 5 with 3 having the highest emission lifetime values of 26.9 μs at room temperature and 384 μs at 77 K and complex 5 having the lowest emission lifetimes of 4.6 us at room temperature and 61 μs and 77 K. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b415079a

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F U L L P A P E R
A spectroscopic and computational study on the effects of methyl
and phenyl substituted phenanthroline ligands on the electronic
structure of Re(I) tricarbonyl complexes containing
2,6-dimethylphenylisocyanide†
John M. Villegas, Stanislav R. Stoyanov, Wei Huang and D. Paul Rillema*
Department of Chemistry, Wichita State University, 1845 N. Fairmount St., Wichita, KS,
67260-0051, USA. E-mail: paul.rillema@wichita.edu
Received 29th September 2004, Accepted 21st January 2005
First published as an Advance Article on the web 10th February 2005
[Re(CO)3(CNx)(L)]+, where CNx = 2,6-dimethylphenylisocyanide, forms complexes with L = 1,10-phenanthroline
(1), 4-methyl-1,10-phenanthroline (2), 4,7-dimethyl-1,10-phenanthroline (3), 3,4,7,8-tetramethyl-1,10-phenanthroline
(4), 2,9-dimethyl-1,10-phenanthroline (5) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (6). The
metal–ligand-to-ligand charge transfer transition (MLLCT) absorption bands follow the series: 3 (27 800 cm−1) > 1,
2, 4 and 5 (27 500 cm−1) > 6 (26 600 cm−1). Density functional theory (DFT) geometry optimizations reveal elongated
˚
˚
Re–N (L) distances of 2.28 and 2.27 A for 5 and 6, respectively, compared to 2.23 A for 1–4. The reversible reduction
potentials (E1/2(red)) of 1–4 are linearly dependent on the B3LYP calculated LUMO energies. Time-dependent (TD)
DFT and conductor-like polarizable continuum model (CPCM) calculated singlet excited states deviate by 700 cm−1
or less from the experimental absorption maxima and aid in the spectral assignments. The 3MLLCT emitting state
energies are within 900 cm−1 of the experimental 77 K emission energies for 1–6. The 77 K emission energies, E1/2(red)
,
and the room temperature emission quantum yields (φLUMO em ) decrease in the order 1 > 2 > 3 > 4 whereas ELUMO and
the room temperature emission energies follow the opposite trend. The emission lifetimes (sem) decrease in the order
3 > 4 > 2 > 1 > 5 with 3 having the highest emission lifetime values of 26.9 ls at room temperature and 384 ls at
77 K and complex 5 having the lowest emission lifetimes of 4.6 ls at room temperature and 61 ls and 77 K.
[Re(CO)3(4,4ꢀ/5,5ꢀ-bpy)Cl] complexes.16a Picosecond flash pho-
Introduction
tolysis measurements were used to assign the emissive state of
Studies of the photophysical and photochemical properties of
heterocyclic diimine complexes of the rhenium(I) tricarbonyl
moiety are of interest due to their photophysical properties
[Re(CO)3(dppz)(py)]+ (dppz = dipyrido[3,2-a:2ꢀ,3ꢀ-c]phenazine)
3
as an intraligand p → p* (phenazine) state which has been
supported by TDDFT results.16b Additionally, correlations
which are similar in some respects to ruthenium(II) polypyridyl
between TDDFT calculated MLCT states and UV-vis spectra
complexes.1 Such Re(I) complexes are ideally suited for solar
were reported for [Re(CO)3Cl(N–N)]+ (N–N = 7,8-diphenyl-
energy conversion dyes2,3 since they display intense luminescence
2,5-bis(phenylamino)-p-quinonediimine).16c
in the visible region of the spectrum with long emission lifetimes.4
The TDDFT method treats molecules in the gas phase
The origin of the emission was attributed to the metal-to-
and does not always give the right excited-state energies in
ligand charge transfer (MLCT) state based on broad and
solution.17,18 Hence, TDDFT and the conductor-like polarizable
structureless emission bands which were sensitive to changes
continuum model (CPCM) were combined and the UV-vis
in the nature of the environment.5–11 The photochemical and
absorption energies of [Ru(bpy)2(CNx)Cl]+ were calculated. The
photophysical properties of these complexes can be fine-tuned
computed singlet excited-state energies correlated linearly in a
by changing the chormophoric, bidentate ligand and/or the
series of seven solvents of varied polarity.17a The tandem use
ancillary “spectator” ligands.
Our group has been systematically examining the electronic
of TDDFT and CPCM has also produced dramatic changes
in the singlet excited-state energies and assignments for other
and photophysical properties of Ru(II) and Re(I) complexes
ruthenium(II) and osmium(II) polypyridyl complexes19 and is
containing diimine and 2,6-dimethylphenylisocyanide (CNx)
the current method of choice for calculating energies of excited
ligands by comparing their properties to those calculated
states in similar complexes.
by density functional theory (DFT).12 Linear relationships
[Re(CO)3(phen)(py)]+ complexes have longer emission life-
of E1/2(ox) and E1/2(red) versus the B3LYP13 calculated highest
times (sem) and higher emission quantum yields (φLUMO em ) than
occupied molecular orbital (HOMO) and the lowest unoccupied
their bpy analogs.11,20 The CNx ligand was also found to enhance
molecular orbital (LUMO) energies, respectively, for a series
these important photophysical parameters when substituted for
of isoelectronic ruthenium(II) diimine complexes were reported
py in [Re(CO)3(bpy)(py)]+.12b Here we have combined the effects
from our laboratory.14 Others reported correlations of time-
of both the phen and the CNx ligands and report a series of
dependent (TD) DFT calculated singlet and triplet excited-
[Re(CO)3(CNx)(phen)]+ complexes where methyl groups were
state energies and spatial distributions with time-resolved
attached to the 2-, 3-, 4-, 7- and 9-positions and phenyl groups
IR studies for rhenium(I) tricarbonyl diimine complexes that
were attached to the 4- and 7-positions of the phen ligand.
contained ancillary py ligands15 and good agreement between
the computed and the experimental absorption bands of
Experimental
† Electronic supplementary information (ESI) available: The optimized
geometries (Table S1), the percent orbital contributions (Table S2)
Materials
The ligand 2,4-dimethylphenylisocyanide was purchased from
Fluka and ligands 1,10-phenanthroline, 4-methyl-1,10-phenan-
throline, 4,7-dimethyl-1,10-phenanthroline were purchased
and the calculated singlet excited-state energies of the six com-
plexes (Table S3), and additional comments. See http://www.rsc.org/
suppdata/dt/b4/b415079a/
1 0 4 2
D a l t o n T r a n s . , 2 0 0 5 , 1 0 4 2 – 1 0 5 1
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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from GFS Chemicals. The ligands 2,9-dimethyl-1,10-phenan-
throline (neocuproine), 3,4,7,8-tetramethyl-1-10-phenanthro-
line, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline were pur-
chased from Aldrich. Optima grade methanol was purchased
from Fischer Scientific while acetonitrile was purchased from
Sigma-Aldrich. AAPER Alcohol and Chemical Company was
the source of absolute ethanol. [Re(CO)5Cl] was purchased from
Aldrich. Ethanol and methanol were used in a 4 : 1 (v/v) mixture
to prepare solutions for emission and emission lifetime studies.
All purchased reagents were used without further purification.
Elemental analyses were obtained from M–H–W Laboratories,
Phoenix, AZ.
for 3–5 hours, about 0.20 mmole of the product was added to an
equimolar amount of AgCF3SO3 in a 125 mL round-bottomed
flask. Again, approximately 50 mL of absolute ethanol was
added and the mixture was refluxed for 4–6 hours. The solution
was cooled to room temperature and the AgCl precipitate was
removed by filtration. An equimolar amount of the CNx ligand
dissolved in 10 mL of ethanol was added to the filtrate and
the solution was refluxed again for another 3–5 hours. The
solvent was reduced in volume (about 5–10 mL) under vacuum.
A saturated NH4PF6 solution (15 mL) in water was then added
and the solution was diluted to 50 mL with water (or until
precipitation was completed). The precipitate was collected by
filtration, dried in a vacuum oven and weighed.
Instrumentation and physical measurements
(1) fac-[Re(CO)3(CNx)(phen)](PF6). Color: yellow. Yield:
97%. Anal. Calcd. for ReC24H17N3O3PF6: C, 39.67; H, 2.36; N,
5.78%. Found: C, 39.49; H, 2.20; N, 5.63%. (KBr pellet): 2170,
2037, 1937, 1632, 1605, 1521, 1431, 1227, 1151, 841, 779, 724,
UV-vis spectra were obtained using a Hewlett-Packard model
8452A diode array spectrophotometer and IR spectra were
acquired using a Nicolet Avatar 360 FT-IR spectrophotometer.
Proton NMR spectra were obtained using a Varian Mercury
300 FT NMR spectrometer. An EG & G PAR model 263A
potentiostat/galvanostat was used to obtain cyclic voltammo-
grams. Measurements were carried out in a typical H-cell using
a platinum disk working electrode, a platinum wire counter
electrode, and a Ag/AgCl reference electrode in acetonitrile.
The supporting electrolyte used was 0.1 M tetrabutylammo-
nium hexafluorophosphate (TBAH). Ferrocene was added as a
reference.
The sample preparation for emission studies involved dis-
solving a small amount of sample (≈2 mg) in the appropriate
solvent and the absorbance of the solution was measured. The
concentration of the solution was altered in order to achieve an
absorbance of about 0.10 at the lowest energy transition. Such
a concentration provided enough material for data acquisition
but excluded self-quenching processes. A 3–4 mL aliquot of
the solution was then placed in a 10 mm diameter Suprasil
(Heraeus) non-fluorescent quartz tube equipped with a tip-off
manifold. The sample was then freeze–pump–thaw degassed for
at least three cycles (to approximately 75 milliTorr) removing
any gasses from the sample. The manifold was then closed and
the sample was allowed to equilibrate at room temperature. The
solvent evaporation was assumed to be negligible, therefore the
concentrations were assumed to remain constant throughout
this procedure. The corrected emission spectra were collected
using a Spex Tau3 Fluorometer.
1
634, 558, 507, 471 cm−1. H NMR (DMSO): d (ppm) 1.72 (s,
6H), 7.06 (d, 2H, J = 7.5 Hz), 7.19 (dd, 1H, J = 0.9, 8.1 Hz),
8.18 (dd, 2H, J = 3.3, 5.1 Hz), 8.38 (s, 2H), 9.06 (dd, 2H, J =
1.2, 8.4 Hz), 9.57 (dd, 2H, J = 1.5, 5.1 Hz).
(2) fac-[Re(CO)3(CNx)(4-Me-phen)](PF6). Color:
light
yellow. Yield: 89%. Anal. Calcd. for ReC25H19N3O3PF6: C,
40.54; H, 2.59; N, 5.67%. Found: C, 39.54; H, 2.40; N, 5.70%.
(KBr pellet): 2171, 2036, 1964, 1935, 1653, 1635, 1576, 1522,
1473, 1431, 1384, 1261, 1168, 1032, 842, 782, 727, 635, 612, 558,
508, 421 cm−1. 1H NMR (DMSO): d (ppm) 1.72 (s, 6H), 2.97 (s,
3H), 7.05 (d, 2H, J = 7.5 Hz), 7.19 (dd, 1H, J = 0.9, 7.2 Hz),
8.03 (dd, 1H, J = 0.6, 5.4 Hz), 8.17 (dd, 1H, J = 3.0, 5.4 Hz),
8.43 (dd, 2H, J = 9.3, 13.8), 9.06 (dd, 1H, J = 1.5, 8.4 Hz), 9.41
(d, 1H, J = 5.4 Hz), 9.56 (dd, 1H, J = 1.5, 5.1 Hz).
(3) fac-[Re(CO)3(CNx)(4,7-Me2-phen)](PF6). Color: off-
white. Yield: 92%. Anal. Calcd. for ReC26H21N3O3PF6: C, 41.38;
H, 2.81; N, 5.57%. Found: C, 41.19; H, 2.74; N, 5.70%. (KBr
pellet): 2171, 2035, 1695, 1933, 1653, 1608, 1577, 1525, 1425,
1386, 1233, 1171, 1035, 839, 781, 726, 634, 612, 558, 483,
1
419 cm−1. H NMR (DMSO): d (ppm) 1.72 (s, 6H), 2.98 (s,
6H), 7.05 (d, 2H, J = 7.5 Hz), 7.19 (dd, 1H, J = 0.9, 8.1 Hz),
8.02 (dd, 2H, J = 0.6, 5.4 Hz), 8.46 (s, 2H), 9.40 (d, 2H, J =
5.1 Hz).
The emission quantum yields were then calculated using
eqn. (1), where φLUMO x is the emission quantum yield of the
sample and φLUMO std is the emission quantum yield for the
standard [Ru(bpy)3]2+, Astd and Ax represent the absorbance of
the standard and the sample, respectively, while Istd and Ix are
the integrals of the emission envelope of the standard and the
sample, respectively.21
(4) fac-[Re(CO)3(CNx)(3,4,7,8-Me4 -phen)](PF6). Color:
light yellow. Yield: 92%. Anal. Calcd. for ReC28H25N3O3PF6: C,
42.97; H, 3.22; N, 5.37%. Found: C, 42.17; H, 3.42; N, 5.29%.
(KBr pellet): 2170, 2036, 1967, 1929, 1653, 1624, 1531, 1431,
1
1388, 1248, 1177, 843, 781, 723, 634, 614, 579, 558 cm−1. H
NMR (DMSO): d (ppm) 1.68 (s, 6H), 2.63 (s, 6H), 2.86 (s, 6H),
7.04 (d, 2H, J = 7.5 Hz), 7.19 (dd, 1H, J = 0.9, 7.8 Hz), 8.46 (s,
2H), 9.29 (s, 2H).
φLUMO x = (Astd/Ax)(Ix/Istd)φLUMO std
(1)
The excited state lifetimes were determined by exciting the
sample at 355 nm using the third harmonic of a Continuum
Surlite Nd:YAG laser run at ≈20 mJ (10 ns pulse)−1. The oscil-
loscope control and data curve fitting analysis was accomplished
using the Origin 6.1 program by OriginLab Corporation. The
excited state lifetime experiments were conducted as previously
published.22
(5) fac-[Re(CO)3(CNx)(2,9-Me2-phen](PF6). Color: light
yellow. Yield: 66%. Anal. Calcd. for ReC26H21N3O3PF6: C, 41.38;
H, 2.80; N, 5.57%. Found: C, 41.25; H, 2.79; N, 5.36%. (KBr
pellet): 2175, 2052, 1966, 1926, 1902, 1628, 1592, 1506, 1439,
1
1383, 1167, 1037, 838, 774, 730, 662, 646, 616, 558 cm−1. H
NMR (DMSO): d (ppm) 1.72 (s, 6H), 3.30 (s, 6H), 7.06 (d, 2H,
J = 7.5 Hz), 7.20 (dd, 1H, J = 0.9, 8.4 Hz), 8.18 (d, 2H, J =
8.4 Hz), 8.24 (s, 2H), 8.86 (d, 2H, J = 8.4 Hz).
Preparation of fac-[Re(CO)3(CNx)(L)](PF6)
The complexes were synthesized according to previously
published procedures23,24 which were modified as follows: a
0.55 mmole sample of [Re(CO)5Cl] was added to an equimolar
amount of the phenanthroline-based ligand in a 125 mL round-
bottomed flask. Approximately 50 mL of absolute ethanol was
added and the mixture was refluxed for 2–4 hours. A yellowish-
colored precipitate formed in the solution which was cooled to
room temperature and filtered. After drying in a vacuum oven
(6) fac-[Re(CO)3(CNx)(2,9-Me2-4,7-Ph2-phen)](PF6). Color:
yellow. Yield: 97%. Anal. Calcd. for ReC38H29N3O3PF6
(containing 2 moles H2O): C, 48.41; H, 3.52; N, 4.46%. Found:
C, 48.20; H, 3.40; N, 4.60%. (KBr pellet): 2172, 2034, 1966,
1927, 1653, 1627, 1570, 1490, 1446, 1387, 1031, 841, 775, 710,
637, 617, 558, 484, 419 cm−1. 1H NMR (DMSO): d (ppm) 1.79
(s, 6H), 3.35 (s, 6H), 7.10 (d, 2H, J = 7.2 Hz), 7.23 (dd, 1H, J =
1.2, 7.2 Hz), 7.65 (m, 10H), 8.04 (s, 2H), 8.23 (s, 2H).
D a l t o n T r a n s . , 2 0 0 5 , 1 0 4 2 – 1 0 5 1
1 0 4 3

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Table 1 Experimentala electronic transitions and calculatedb singlet
excited states of Re(I) complexes 1–6
Results
Synthesis
E
exp/103 cm−1
The synthesis of the complexes was carried out according to
the scheme presented in Fig. 1. [Re(CO)5Cl] was first allowed to
react with the phenanthroline-based ligand (L) to form a neutral
complex with the general formula [Re(CO)3(L)Cl]. The product
was next allowed to react with AgCF3SO3 thereby removing
the chloro ligand from the coordination sphere by precipitating
AgCl and replacing it with CF3SO3−. The CNx ligand then
Complex
(e/M−1 cm−1
)
E
calc/103 cm−1
Assignment
1
27.5 (3 300)
28.2
33.5
37.1
38.4
MLCT
MCDCT
CDLCT
LLCT
33.3 (15 000)
37.0 (40 000)
38.5 (38 000)
2
27.5 (3 000)
36.8 (42 000)
38.8 (39 000)
43.9 (43 000)
27.1
36.8
39.2
—
MLLCT
LC
MLLCT
LC
−
replaced CF3SO3 by reaction of [Re(CO)3(L)(CF3SO3)] with
a slight excess of CNx added to the filtrate. After refluxing
the solution for about three hours and reducing the volume by
rotary evaporation, the final product was precipitated by adding
a saturated solution of NH4PF6 and diluting it with more water
until precipitation was complete. The products were obtained in
relatively high yield.
3
4
27.8 (4 000)
32.3 (15 000)
36.8 (51 000)
28.4
32.9
37.1
MLLCT
MLLCT
LC
27.5 (3 000)
32.5 (18 000)
36.0 (42 000)
40.3 (37 000)
27.7
32.3
36.1
40.4
MLLCT
LC
MCDCT
LCDCT
5
6
27.5 (2 000)
32.9 (19 000)
35.7 (29 000)
38.8 (37 000)
43.5 (35 000)
27.4
32.8
35.4/36.0
39.4
—
MLLCT
MLLCT
MLLCT/MCDCT
LCDCT
LC
Fig. 1 Schematic diagram of the synthesis of the complexes.
Fig. 2 shows a schematic diagram of the Re(I) tricarbonyl
isocyanide complexes with the phenanthroline-based ligands.
As noted for this series of complexes, the 2-, 3-, 4-, 7-, 8- and/or
9-positions (peripheral) of the parent phenanthroline ligand
were modified by replacing the hydrogen atoms with electron-
donating methyl and/or electron withdrawing phenyl groups.
26.6 (4 800)
30.7 (19 000)
33.8 (48 000)
38.5 (42 000)
26.6
31.2
33.8
38.6
LMLCT
LMLCT
LC
MLLCT
a In 4 : 1 (v/v) ethanol–methanol. b In ethanol.
as solvent and spectra are shown in Fig. 3. The absorption
coefficients of the transitions involved were determined from
Beer’s Law studies using at least five dilution points. The
probable assignments of these bands were made on the basis
of the computational assignment of the singlet excited states
and the documented optical transitions of similar type of
complexes.3–6,25 The results are listed in Table 1.
The lowest energy transitions of the complexes were assigned
as metal–ligand-to-ligand-charge transfer (MLLCT) while those
at higher energies were assigned as ligand p → p* transitions.
It is important to note that since the MLLCT bands occur as
broad shoulders, the exact positions of the bands as well as the
extinction coefficients were subject to error.
The MLLCT maxima for complexes 1 to 5 were located in the
narrow range of 27 500–27 800 cm−1. For 6, where two phenyl
groups were located in the 4- and 7-positions and two methyl
groups were attached in the 2- and 9-positions of the phen ligand,
the MLLCT band red-shifted by 900 cm−1 relative to 1. The
positions of the intraligand p → p* transitions for the series of
complexes studied occurred over a broader range from 32 000 to
44 000 cm−1.
Electrochemical studies
The redox potentials of the complexes in the series were
determined by cyclic voltammetry and are listed in Table 2. All
complexes in the series showed irreversible oxidation waves in
the range of 1.90–2.03 V.
The reduction potentials decreased in the following order: 1
(−1.18 V) > 2 (−1.25 V) > 3 (−1.33 V) > 4 (−1.42 V) as the
number of methyl substituents in the 3, 4, 7 and 8-positions
of the parent ligand was increased. The reduction potentials
for the two complexes 5 and 6 containing methyl groups in the
2,9-positions were the same within experimental error (−1.25
0.01 V).
Fig. 2 Structures of the complexes used in the study.
Electronic absorption studies
The electronic absorption properties of the complexes were
studied at room temperature using 4 : 1 (v/v) ethanol–methanol
1 0 4 4
D a l t o n T r a n s . , 2 0 0 5 , 1 0 4 2 – 1 0 5 1

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Fig. 3 Experimental absorption spectra of complexes 1–6 and calculated singlet excited- states. The excited states are shown as vertical bars with
height equal to the extinction coefficient.18a ꢀ = MLLCT or MLCT, ꢀ = LLCT, ꢁ= p → p*, ᭹ = MCDCT, ᭜ = LCDCT (ligand → delocalized).
Table 2 Electrochemical properties of the complexes in CH3CN at
room temperature
Emission properties
The emission properties and excited state lifetimes of the
complexes were determined both at room temperature and at
77 K in 4 : 1 (v/v) ethanol–methanol. The results are listed in
Table 3. Overlays of the room temperature and 77 K emission
spectra of the six complexes are shown in Fig. 4. The emission
maxima of the complexes were shifted to higher energies at
77 K compared to room temperature. Temperature-dependent
emission studies were not conducted because the complexes
underwent photodecomposition upon continuous exposure to
laser light.
Complex
E
1/2(ox)/Va
E
1/2(red)/Va (L)
1
2
3
4
5
6
2.01b
1.93b
1.90b
1.92b
2.02b
2.03b
−1.18
−1.25
−1.33
−1.42
−1.26
−1.25
a Potential in V vs. SSCE (scan rate = 250 mV s−1). b Irreversible oxidation
wave.
Table 3 Calculated 3MLLCT state energiesa and emission properties of the complexes at 77 K and room temperatureb
exp/103 cm−1
sem/ls
φLUMO em
c
E
Complexes
E
calc/103 cm−1
77 K
RT
77 K
RT
8.6
RT
1
22.6
22.6
22.0
21.7
21.9
21.4
21.8
20.4
19.1
17.7 (s)
19.7
65
148
384
254
61
0.77
0.70
0.61
0.52
0.22
0.39
2
3
4
5
6
21.7
20.2
18.9
17.6 (s)
19.8
20.7 (s)
10.1
26.9
20.8
4.6
21.5
20.1
18.7
17.5 (s)
20.8
19.8
21.5
20.0
18.7
17.4 (s)
21.0
19.8
22.1
20.7
19.5
18.1 (s)
20.1
18.9
20.2
19.1
101
40.7
17.8 (s)
a In ethanol. b In 4 : 1 (v/v) EtOH–MeOH; s = shoulder. c Relative to [Ru(bpy)3]2+ (ref. 22).
D a l t o n T r a n s . , 2 0 0 5 , 1 0 4 2 – 1 0 5 1
1 0 4 5

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Fig. 4 Emission spectra of the complexes at 77 K (—), curve-fitted (· · ·) and at room temperature (---) in 4 : 1 (v/v) ethanol–methanol.
After converting the wavelength (abscissa) values into energy,
the emission spectral data were fit to eqn. (2), where the
summation was carried out over the two sets of six vibrational
levels.26 The parameters were as follows: I0 was equal to 0, A was
the peak area, n1, n2 = 0 to 5,
of the phenanthroline ligands. The low frequency modes for the
six complexes were assigned as metal–ligand vibrations.
Computational technique
The singlet ground-state geometries of the complexes 1–6 were
optimized in the gas phase using the B3LYP13 functional of
the Gaussian ’0328 program package. For the Re valence shell
a (8s7p6d)/[6s5p3d] Gaussian-type orbital (GTO) was used
whereas the Stuttgart–Dresden (SDD) effective core potential
(ECP)29 was used for the Re core. The all-electron 6-311G*
basis set30 was applied for O, N, C, and H atoms. Selected metal–
ligand bond lengths are listed in Table 5. The singlet ground state
optimized geometries of the complexes are listed in Table S1.†
The singlet excited-states31 of complexes 1–6 in ethanol
were calculated using the non-equilibrium TDDFT32/CPCM33
method based on the singlet ground-state geometry optimized
in the gas phase.34 The TDDFT/CPCM calculations are
non-equilibrium calculations with respect to the polarization
ꢀ
ꢀ
n
I(E) = I0 + A[
[(E0 − n1x1 − n2x2)/E0]4(S1 1/n1!)
n
1
n
2
(S2 2/n2!) exp{−4 log2 [(E − E0 + n1x1 + n2x2)/t1/2]2}] (2)
n
E0 was the zero–zero energy, x1 and x2 represented the energies
of the high and low vibrational frequency acceptor modes, S1
and S2 were the measures of the distortion in the high and low
frequency acceptor modes27 and t1/2 was the full-width at half-
maximum of the zero-zero vibronic component in the emission
spectra. The maximum intensity was adjusted to 1 for the curve-
fitting analysis.
The results of the emission spectral curve fitting at 77 K
(Fig. 4) are listed in Table 4. The values of the high frequency
modes at about 1400 cm−1 corresponded to ring breathing modes
Table 4 Emission spectral curve fitting parameters of complexes 1–6 in 4 : 1 (v/v) EtOH–MeOH at 77 Ka
Parameter
1
2
3
4
5
6
E
em/cm−1
21 800
21 800
1395
0.89
545
0.63
510
0.90
21 700
21,654
1415
0.89
544
0.68
554
0.86
21 500
21,502
1402
0.94
502
0.57
500
0.93
21 500
21,456
1420
0.94
558
0.58
569
0.94
22 100
22,094
1443
1.10
593
0.90
637
0.84
20 200
20,695
1464
0.91
502
1.67
730
0.48
E0/cm−1
x1/cm−1
S1
x2/cm−1
S2
t1/2
A
a Error limits are as follows: E0, 6.0 cm−1, x, 15 cm−1, S, 0.02, t1/2
,
20 cm−1, A, 002.
˚
Table 5 Calculated Re–ligand distances in A for complexes 1–6
Complex
Re–C (CNx)
Re–C (CO trans to CNx)
Re–C (CO cis to CNx)
Re–N (phen)
1
2
3
4
5
6
2.10
2.10
2.10
2.10
2.10
2.10
2.01
2.01
2.00
2.00
2.00
2.00
1.96
1.96
1.96
1.95
1.95
1.95
2.23
2.23
2.23
2.23
2.28
2.27
1 0 4 6
D a l t o n T r a n s . , 2 0 0 5 , 1 0 4 2 – 1 0 5 1

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process between the solvent reaction field and the charge
density of the electronic state indicated in the input. For singlet
excited states this is the singlet ground state.35 The TDDFT
output contained information for the excited-state energies and
oscillator strengths (f ) and a list of the excitations that give
rise to each excited state, the orbitals involved as well as the
wavefunction coefficients of the excitations. The singlet excited
states of the six complexes are presented in Fig. 3 as vertical
bars with its height equal to the extinction coefficient calculated
from the oscillator strength.18
The lowest-lying triplet state geometries of the six complexes
were calculated using unrestricted B3LYP in the gas phase. The
spin contamination from states of higher multiplicity was low.
The value of <S2> was 2.017 for 1 and 5, 2.016 for 2, 2.036
for 3 and 4, 2.019 for 6. The energies of the lowest-lying triplet
states were higher than these of the corresponding ground states
by 22 600 cm−1 for 1 and 2, 22 000 cm−1 for 3, 21 700 cm−1 for
4, 21 900 cm−1 for 5, 20 400 cm−1 for 6 (Fig. 5). The lowest-
lying triplet states for complexes 1–6 were 3MLLCT states. These
states featured single occupancy of the HOMO and the LUMO.
Fig. 6 Molecular orbital energy diagram for six occupied (H) and six
virtual (L) frontier orbitals of the complexes in the singlet ground state
in ethanol. a = Red, CNx, b = Red, CO, c = CO, d = delocalize, m =
Red, p = phen-based ligand and x = CNx. For example, orbitals H-3
and H-4 of complex 5 are assigned as p, m & x where H-3 is on the
phen-based ligand (p) and Red (m) but H-4 is on the CNx ligand (x).
Fig. 5 Triplet excited-state energy diagram for complexes 1–6.
A number of triplet excited states were computed based on the
lowest-lying triplet-state geometry for complexes 1–6. The four
low-lying triplet excited states are listed in Table 6 and shown in
Fig. 5, even if the f value was low.
Discussion
Geometry optimization
The distances between the Re atom and the ligand atoms bound
to it are listed in Table 5. The Re–N and Re–C (CO cis to CNx)
varied across the series. The Re–N distance for complexes 5 and
˚
˚
6 of 2.28 and 2.27 A, respectively, was longer than 2.23 A for
complexes 1–4, due to the steric hindrance caused by the methyl
groups in the 2- and 9-positions. The Re–N bond elongation in 5
˚
and 6 was accompanied by a 0.01 A bond shortening of the Re–
C (CO cis to CNx) distance relative to 1–4. In the acetonitrile
analog of complex 6, fac-[Re(CO)3(2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline)(CH3CN)]+, the Re–N (2,9-diemthyl-4,7-
Fig. 7 Schematic diagram of the HOMO and the LUMO of complexes
1–6.
diphenyl-1,10-phenanthroline) distances were 2.211(3) and
are shown. The HOMOs of complexes 1–5 contained 45%
or more Red contributions and 28% or more CNx ligand
contributions. The HOMO of complex 6, however, contained a
47% contribution from 2,9-Me2-4,7-Ph2-phen ligand and only a
31% Red contribution (Fig. 7). The character of the HOMO and
HOMO-2 of complexes 1–5 as well as of HOMO-1 of complexes
1 and 2 was similar to the character of the corresponding orbitals
for the [Re(bpy)(CO)3(CNx)]+ complex.12b The LUMOs and
LUMOs + 1 of complexes 1–6 contained 77 and 98% diimine
36
˚
˚
2.215(3) A, shorter by ≈0.05 A than our calculated value of
˚
˚
2.27 A. The 0.05 A overestimation of Re–N (diimine) distance
is the same as reported earlier when B3LYP theory was used.12
Molecular orbital analysis
The molecular orbital energy diagram in Fig. 6 shows the
twelve frontier orbitals of complexes 1–6. In Fig. 7 schematic
diagrams of the HOMOs and the LUMOs of the six complexes
D a l t o n T r a n s . , 2 0 0 5 , 1 0 4 2 – 1 0 5 1
1 0 4 7

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Table 6 Calculated triplet excited-states of complexes 1–6 in ethanol based on the lowest-lying triplet state geometry. EVER is the energy of the
vertical transition, f is the oscillator strength, wo and wv are the occupied and the virtual orbitals that define the transition. The transition type is
determined based on the change in the spatial distribution from occupied to virtual orbital. The absolute value of the transition coefficient for each
transition is given in parentheses. H = HOMO and L = LUMO (see text for calculation details)
State
f
wo → wv
Type
E
VER/103 cm−1
Complex 1
1
0.01
H-2 → H (0.7)
H-1 → H (0.6)
H-4 → H (1.0)
L → L + 1 (0.8)
H-1 → H (0.7)
H-2 → H (0.5)
Red, CO → Red, CNx
Red → Red, CNx
27.6
2
3
4
0.00
0.01
0.10
phen, CNx → Red, CNx
LC p → p*
28.6
28.9
30.4
Red → Red, CNx
Red, CO → Red, CNx
Complex 2
1
2
3
4
0.00
0.01
0.00
0.09
H-1 → H (1.0)
L → L + 1 (0.9)
H-4 → H (1.0)
H-2 → H (0.8)
Red → Red, CNx
27.2
28.5
29.0
30.8
LC p → p*
4-Me-phen, CNx → Red, CNx
Red, CO → Red, CNx
Complex 3
1
2
3
4
0.02
0.02
0.02
0.00
L → L + 1 (1.0)
H-1 → H (0.9)
H-2 → H (1.0)
H-3 → H (1.0)
LC p → p*
26.8
29.4
30.6
31.4
Red, 4,7-Me2-phen → Red, CNx
Red, CO → Red, CNx
CNx → Red, CNx
Complex 4
1
2
3
4
0.02
0.02
0.03
0.00
L → L + 1 (1.0)
H-1 → H (0.9)
H-2 → H (0.9)
H-3 → H (1.0)
LC p → p*
26.8
29.4
30.6
31.6
Red, Me4-phen → Red, CNx
Red, CO → Red, CNx
CNx → Red, CNx
Complex 5
1
0.01
H-1 → H (0.7)
H-2 → H (0.6)
L → L + 1 (0.9)
H-4 → H (0.7)
H-2 → H (0.6)
H-4 → H (0.6)
H-5 → H (0.5)
H-2 → H (0.4)
H-1 → H (0.4)
Red, 2,9-Me2-phen → Red, CNx
Red, CO → Red, CNx
27.3
2
3
0.01
0.01
LC p → p*
28.8
29.1
2,9-Me2-phen, Red → Red, CNx
Red, CO → Red, CNx
4
0.05
2,9-Me2-phen, Red → Red, CNx
2,9-Me2-phen → Red, CNx
Red, CO → Red, CNx
30.3
Red, 2,9-Me2-phen → Red, CNx
Complex 6
1
2
3
4
0.02
0.00
0.10
0.03
H-1 → H (0.9)
L → L + 1 (1.0)
H-2 → H (0.8)
H-3 → H (0.9)
Red, Me2–Ph2-phen → Me2–Ph2-phen, Red
LC p → p*
27.4
28.0
29.6
30.1
Red, Me2–Ph2-phen → Me2–Ph2-phen, Red
Me2–Ph2-phen, Red → Me2–Ph2-phen, Red
ligand contributions, respectively. The LUMOs + 2 of complexes
1–6 contained 46% CNx character and 20% CO character. The
LUMO, LUMO + 2 and LUMO + 1 of [Re(bpy)(CO)3(CNx)]+
contained a 82% bpy ligand contribution12b similar to the
LUMOs, LUMOs + 1 and LUMOs + 2 of complexes 1–6,
respectively. The percentages of molecular orbital contributions
for complexes 1–6 are listed in Table S2.†
The HOMO–LUMO gap increased in the order: 32 100 (1) <
32 300 (2) < 32 700 (3) < 33 000 cm−1 (4) as the number of
methyl substituents increased. Upon addition of the sterically
hindering methyl groups in the 2- and 9-positions, the HOMO–
LUMO gap decreased by 200 cm−1 for 5 (32 500 cm−1) relative
to isomer 3 (32 700 cm−1). The smallest HOMO–LUMO gap of
31 500 cm−1 was calculated for 6 due to the sterically hindering
methyl groups located in the 2- and 9-positions and the two
phenyl groups located in the 4,7-positions.
removal of an electron from a mixed heritage orbital consisting
of considerable dp character with a rather large contribution of
CNx (28%) character which perhaps accounts for the irreversible
electrochemical behavior. The one-electron reduction on the
other hand, involved addition of an electron to the p* orbital
localized on the diimine moiety consistent with the observation
of emission decay involving the ring breathing vibronic mode
obtained from the emission curve fitting analysis.37
The irreversible oxidation potentials of the methyl substituted
complexes 2, 3 and 4 were less than 1 but the values did not
change proportionally to the number of methyl groups. The
increase in the number of methyl substituents attached to the
parent phen ligand caused a decrease in the reduction potentials
in the order 1 > 2 > 3 > 4. The linear relationship (R2
0.98 0.01) between the reduction potentials and the LUMO
=
energies for complexes 1–4 is shown in Fig. 8. A slope of
−0.51
0.05 was obtained. Both E1/2(red) (R2 = 0.98
0.03)
and ELUMO (R2 = 1.00
0.01) were linearly dependent on
Electrochemical behavior
the Hammett substituent constants rT (rt = rp + rm)38 for
1–4. The slope of the line for E1/2(red) versus rT (Fig. 9) was
0.48 0.04 with an intercept of −1.18 0.01, similar to −1.16
reported for [Re(CO)3(Etpy)(bpy)]+ complexes with meta and
para substituents located on the bpy ring.39 The slope of ELUMO
The redox potentials of importance in discussing the electro-
chemistry of the complexes are those derived from the processes
involving the HOMO and the LUMO.23 The HOMO consisted
mainly of the dp orbitals located on the metal center while the
dominant contribution to the LUMO was from the p* orbital
of the phen-based ligand. The oxidations, then, involved the
versus rT (Fig. 10) was −0.25
0.01 and the intercept was
−2.82 0.01.
1 0 4 8
D a l t o n T r a n s . , 2 0 0 5 , 1 0 4 2 – 1 0 5 1

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molar absorptivity coefficient (e). The excited states expressed
with the taller bars have higher contributions to the experimental
peaks. The symbol on the bar corresponds to the type of the
singlet excited state as follows: ꢀ = MLLCT, ꢀ = LLCT, ꢁ =
p → p*, ᭹ = MCDCT (metal to complex-delocalized charge
transfer) and ᭜ = both LCDCT (ligand to complex-delocalized
charge transfer) and CDLCT (complex-delocalized to ligand
charge transfer).43 Here we use several abbreviations for the
computational assignment of the singlet excited states for a
more appropriate description of the excited states that had large
contributions from excitations involving delocalized molecular
orbitals. For example, the MLLCT states involve transfer of
electronic charge from both the Re atom and the CNx ligand to
the diimine ligand. The major contributing excitation in these
states is from an occupied orbital that contains mainly Red
and CNx ligand contributions to a virtual orbital that contains
primarily a diimine ligand contribution, e.g. HOMO → LUMO
for complexes 1–5. In the MCDCT singlet excited states the
major contributing excitation is from an occupied molecular
orbitalthatcontainsmainlyRed contributionstoavirtualorbital
that contains high contributions from three or more different
moieties, e.g. HOMO-2 → LUMO + 3 of complex 1. Since
calculated excited state e values do not correlate very well with
the experimental ones,17 we based our assignment primarily on
the correlation of the calculated excited state energies with the
experimental peak energies.
Fig. 8 Linear dependence of E1/2(red) and the LUMO energies for
complexes 1–4.
The singlet excited states in the range of 25 000–30 000 cm−1
were assigned as MLLCT states for complexes 1–5. The lowest-
lying excited states with f > 0.01 were within 700 cm−1 from the
experimental lowest-energy absorption peaks and were assigned
based on the singlet excited states for the six complexes. Several
excited states were calculated for complexes 1–6 that were related
to the transitions in the range 30 000–40 000 cm−1 and assigned
as phen ligand p → p*, MCDCT or LCDCT. The absorption
profile of complex 5 differed from the profiles of complexes 1–
4 because the most intense peak was at 38 800 cm−1 and there
were pronounced shoulders at 32 900 and 35 700 cm−1. Singlet
excited states 17 (e = 28 000 M−1 cm−1) at 36 500 cm−1 and
21 (e = 26 100 M−1 cm−1) at 37 300 cm−1 for complex 5 were
associated with LMLCT and 2,9-phen ligand p → p* transitions
and assigned as LMLCT (black) based on the higher transition
coefficient.43 Singlet excited states 14 at 35 400 cm−1and 15 at
36 000 cm−1 were computed for 5 and assigned as MLLCT
and MCDCT states, respectively. State 14 was red-shifted by
300 cm−1 and state 15 was blue-shifted by 300 cm−1 relative to
the experimental shoulder at 35 700 cm−1 (Table 1). For complex
6, excited state 17 (e = 21 700 M−1 cm−1) at 33 800 cm−1 was
primarily a 2,9-Me2-4,7-Ph2-phen ligand p → p* transition and
had the same energy as the most intense absorption peak. The
MLLCT excited state 32 at 38 600 cm−1 was only blue-shifted by
100 cm−1 relative to the experimental peak at 38 500 cm−1. The
agreement between singlet excited-state energies and absorption
maxima presented in Table 1 is very good. The singlet excited-
states are listed in Table S3.†
Fig. 9 Linear dependence of E1/2(red) and rT for complexes 1–4.
Triplet excited states
Fig. 10 Linear dependence of ELUMO and rT for complexes 1–4.
The lowest-lying triplet states for the six complexes were of
3MLLCT origin. These states featured single-electron occu-
pancy of both the HOMO and the LUMO. The calculated
3MLLCT state energies generally decreased in the order 1 ≈
2 > 3 > 4 > 6 similar to the 77 K experimental emission
energies. The calculated 3MLLCT energies were 500–1200 cm−1
higher than the corresponding experimental emission energies
for complexes 1–3 and 6. Four upper lying triplet excited states
The addition of the methyl groups in the 2- and 9-positions
in complex 5 produced reduction and oxidation potentials that
were higher by 0.07 and 0.12 V than the corresponding values
for isomer 3. Addition of phenyl groups in 6 did not alter the
redox potentials compared to 5. The energy difference DE1/2
,
where DE1/2 = E1/2(ox)–E1/2(red), and the HOMO–LUMO energy
gap increased in the order 1, 2 < 3 < 4 indicating the parallel
relationship between the thermodynamic and electronic energy
spacings.25,40–42
3
were computed based on these MLLCT states and listed in
3
Table 6. The d–d excited states 1 for complexes 1, 2 and 5
were based on transitions from HOMO-1 and HOMO-2 to
the HOMO, where the three orbitals contained 43% or higher
Red character. The LC excited states 1 for complexes 3 and 4
were based on LUMO to LUMO + 1 transitions, where the
Singlet excited states and UV-Vis absorption spectra
3
In Fig. 3 the calculated singlet excited states of complexes 1–6
with f > 0.01 are shown as vertical bars with height equal to the
D a l t o n T r a n s . , 2 0 0 5 , 1 0 4 2 – 1 0 5 1
1 0 4 9

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two orbitals contained 81% or higher contributions from the
respective phen derivative ligands. For complex 6 however, triplet
excited state 1 was based on a transition from the HOMO-1 to
HOMO, where the HOMO-1 contained 43% Red and 26% 2,9-
Me2-4,7-Ph2-phen character whereas the HOMO contained 47%
2,9-Me2-4,7-Ph2-phen and 31% Red character. Excited state 1 of
complex 6 was assigned as a mixed 3d–d and 3LC state as shown
in Fig. 5. Most of the higher energy states listed in Table 6 were
either 3d–d or 3LC states.
By definition, the decay rate constant, k, is related to the
sum of the radiative and non-radiative decay rate, kr and knr,
respectively (k = kr + knr). Thus, the decrease in the emission
lifetime of 5 compared to 1 could be attributed to the increase
in the knr term. This decrease could also be attributed to the
steric effects of the methyl groups in the 2- and 9-positions. The
emission lifetime was enhanced in 6 due to the added phenyl
groups in the parent phen ligand. However, sem of 6 did not
dramatically increase at 77 K compared to room temperature
as exhibited by the other five complexes. This is most likely due
to the geometry of 6 in the glass where the phenyl groups are
not coplanar with the phen moiety. According to the geometry
optimization, the angles between the approximate planes of the
phenyl rings and the phen ligand were in the range 52–56◦.
The emission quantum yields (φLUMO em ) were not directly
related with the room temperature emission lifetimes for the
complexes. φLUMO em decreased gradually from 0.77 for complex
1 to 0.70 for 2, 0.61 for 3, and 0.52 for 4. Complex 5 had the
lowest φLUMO em value of 0.22. The addition of phenyl groups in
6 caused φLUMO em to increase to 0.39. The φLUMO em of complexes
1, 3, 5 and 6 were significantly higher than 0.18, 0.29, 0.17 and
0.17 reported for their respective py analogs in acetonitrile.23
The excited states 1 for complexes 1–6 were located more than
3
4 600 cm−1 above the MLLCT states (Fig. 5). The energies of
the higher-lying triplet states were determined by single electron
vertical excitations from the lowest lying 3MLLCT state; hence
the reported excited state energies were not the minima.44
Emission properties
The emission spectra of the complexes are shown in Fig. 4.
The complexes were emissive both at room temperature and at
77 K. Selected emission properties, including the excited state
lifetimes of the six complexes are listed in Table 3. The room
temperature emission spectrum of complex 1 featured a bell
3
shape envelope typical for MLCT emitters with a maximum
at 19 700 cm−1. For 2, a shoulder appeared at higher energy in
addition to the maximum at 19 800 cm−1. For 3 and 4 containing
additional methyl groups, the low energy absorption remained
at the same energy as for 1 and 2, but the emission profile
now featured two well defined peaks located at higher energy
that were 1000 cm−1 apart in 3 and 1200 cm−1 apart in 4. The
appearance of the fine structure in the room temperature spectra
Conclusions
A
series of six complexes with the general formula
[Re(CO)3(CNx)(L)](PF6) where L is a 1,10-phenanthroline
derivative with one, two and four methyl groups in the peripheral
3-, 4-, 7- and 8-positions or in the sterically hindering 2- and
9-positions, the latter coupled with phenyl substituents on 4-
and 7-positions, were synthesized and investigated using both
computational and spectroscopic methods. The conclusions
follow: (1) The complexes exhibited typical MLLCT electronic
transitions that were located near 27 500 cm−1 for the methyl
derivatives with one exception, the MLLCT electronic transition
for 6 was red-shifted relative to 27 500 cm−1. (2) E1/2(red) was
linearly dependent on ELUMO . (3) The complexes were highly
emissive at both room temperature and 77 K. (4) The emission
energies at room temperature increased in the order 1 < 2 <
3 < 4 but at 77 K followed the opposite trend. (5) The emission
quantum yields ranged from 0.77 for 1 to 0.22 for 5 and were
significantly higher than 0.089 for the standard, [Ru(bpy)3]2+
and their py analogs. (6) The emission lifetimes both at room
temperature and at 77 K increased in the order 5 < 1 < 2 < 4 <
3 and were in the microsecond time scale.
3
of 3 and 4 could be due to the mixing between the MLLCT
and 3LC states (Fig. 5). An alternative explanation used to
3
3
explain the increased LC character of the emitting MLCT
state based on B3LYP calculations for fac-[Re(CO)3(bpy)(4-
ethylpyridine)]+ was mixing of diimine ligand p → p* character
into the emitting 3MLCT state.15a Overall, the room temperature
emission energies of the complexes increased in the order 1 <
2 < 3 < 4. The room temperature emission profiles of complexes
5 and 6 were bell-shaped curves and the maxima were located at
20 100 and 18 900 cm−1, respectively.
The 77 K emission spectra of the complexes were more
structured (Fig. 4) and shifted to higher energies due to the
temperature dependence of the 3MLLCT state energy.12,23b The
peak maxima were located at 21 800 cm−1 for complex 1,
21 700 cm−1 for complex 2, at 21 500 cm−1 for 3, at 21 500 cm−1
for 4, at 22 100 cm−1 for 5 and at 20 200 cm−1 for 6. The 77 K
emission energies of complexes 1, 5 and 6 were 700, 1000 and
500 cm−1 greater than the energies of their respective py analogs
in the same solvent.23
The values of the sem at room temperature and at 77 K
were determined by exponential decay curve-fitting analysis. The
room temperature sem values increased sequentially from 8.6 ls
for 1 to 10.1 ls for 2, 20.8 ls for 4 and 26.9 ls for 3. However,
for 5 and 6 containing the sterically hindered methyl groups, sem
dropped to 4.6 ls in 5 but increased to 40.4 ls for 6 which was
greater than for any of the others. In like manner, an increase
of the emission lifetime from 15.3 ls for [Re(CO)3(py)(2,9-Me2-
phen)]+ to 22.1 ls for [Re(CO)3(py)(2,9-Me2-4,7-Ph2-phen)]+ in
the same solvent has been reported.23
A parallel trend for sem values was also observed at 77 K for
the complexes in this series. The sem value of 148 ls for complex 2
was more than twice as large as the sem value of 65 ls for complex
1. The sem value increased even more to 384 ls and 254 ls in 3 and
4, respectively. As expected, a decrease to 61 ls was noted in 5
relative to 1. When the two phenyl substituents were attached in
6, the sem value increased to 101 ls. The 77 K emission lifetimes
for complexes 1, 5 and 6 were several times higher than the
reported 11.7, 15.3 and 22.1 ls for their respective py analogs in
the same solvent.23 Overall, the emission lifetimes increased in
the order 5 < 1 < 2 < 4 < 3 both at room temperature and at 77 K.
Acknowledgements
We thank the support of National Science Foundation under
Grant No. EIA-0216178 and Grant No. EPS-0236913, matching
support from the State of Kansas and the Wichita State Uni-
versity High Performance Computing Center, the Wichita State
University Office of Research Administration, the Department
of Energy, and Parker Fellowships (J. M. V., H. W. and S. R. S.).
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D a l t o n T r a n s . , 2 0 0 5 , 1 0 4 2 – 1 0 5 1

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43 These assignments were made based on the major contributing
excitation. The singlet excited-states had contributions from several
excitations. For those presented with solid bars there was a major
contributing excitation (with a transition coefficient for the major
excitation being higher than the transition coefficient of the other ex-
citations by more than 0.2). For some singlet excited-states, however,
there was more than one contributing excitation with high transition
coefficient. Singlet excited-states that contained contributions from
several excitations with transition coefficients that were within 0.2
of the major excitation transition coefficient are assigned as mixed
singlet excited-states and are presented with dotted vertical bars.
44 The triplet excited states were calculated based on the lowest-lying
triplet state geometry because according to Kasha’s rule this state
would be the emitting state. Thus the triplet excited states were
determined based on the most stable triplet geometry. The TDDFT
calculations did not account for spin–orbit coupling. For third-row
transition metal complexes the treatment of spin–orbit coupling
could lower the predicted triplet state energies by 1700–2500 cm−1
as outlined in C. Makedonas, C. A. Mitsopoulou, F. J. Lahoz and
A. I. Balana, Inorg. Chem., 2003, 42, 8853–8865.
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27 The spatial distribution of the singly occupied orbitals in the excited
state can be determined from the vibrational frequency acceptor
modes. Example: for the MLCT state the high frequency acceptor
mode would be the ligand ring breathing mode and the low frequency
acceptor mode would be related to the vibrations of metal–ligand
bonds. The coefficients S1 and S2 are indicative of the relative
contributions of the high and the low vibrational modes to the fine
vibronic structure of the emission.
28 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, J. A. Montgomery, T. Vreven, Jr., K. N.
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B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H.
Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima , T. Honda, O. Kitao, H. Nakai, M. Klene, X.
Li, J. E. Knox, H. P. Hratchian, J. B. Cross, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,
D a l t o n T r a n s . , 2 0 0 5 , 1 0 4 2 – 1 0 5 1
1 0 5 1