Photophysical, spectroscopic, and computational studies of a series of Re(I) tricarbonyl complexes containing 2,6-dimethylphenylisocyanide and 5- and 6-derivatized phenanthroline ligands

Article abstract of DOI:10.1021/ic048786f

The ligand 2,6-dimethylphenylisocyanide (CNx) forms six complexes of the formula [Re(CO)₃(CNx)(L)]⁺, where L = 1,10-phenanthroline (1), 5-chloro-1,10-phenanthroline (2), 5-nitro-1,10-phenarithroline (3), 5-methyl-1,10-phenanthroline (4), 5,6-dimethyl-1,10-phenanthroline (5), and 1,10-phenanthrolinopyrrole (6). The lowest-energy absorption peaks of the complexes red-shift in the order 1 < 2 < 3 < 4 < 5 < 6. The time-dependent density functional theory (TDDFT) and conductor-like polarizable continuum model (CPCM) computed singlet excited states in ethanol deviate by 1000 cm^-1 or less from the experimental UV-vis peaks. The complexes undergo reversible reductions and irreversible oxidations. The electronic energy gap increases in the order 3< 2 < 1 < 4 < 5 < 6, which is the order of increasing electron-donating power of the phen substituents. The reduction potentials linearly correlate with the B3LYP calculated LUMO energies for 1-6. The complexes emit at room temperature and at 77 K except 3, which emits only at 77 K. The calculated ³MLLCT energies are within 1100 cm^-1 from the experimental emission energies at 77 K. The 77 K emission curve-fitting analysis results agree with the computational assignment of the emitting state as ³MLLCT for 1-5 and ³LC for 6. The experimental 77 K emission energies and the calculated ³MLLCT state energies increase in the order 6 < 5, 3 < 2 < 4, 1. The 77 K emission lifetimes increase upon addition of substituents from 65 μs for 1 to 171 μs for 2, to 230 μs for 4 and 5, and to 322 μs for 3. The emission quantum yields at room temperature in solution are 0.77, 0.78, 0.83, 0.56, and 0.11 for complexes 1, 2, 4, 5, and 6, respectively.

Full text of DOI:10.1021/ic048786f

Inorg. Chem. 2005, 44, 2297−2309
Photophysical, Spectroscopic, and Computational Studies of a Series of
Re(I) Tricarbonyl Complexes Containing 2,6-Dimethylphenylisocyanide
and 5- and 6-Derivatized Phenanthroline Ligands
John M. Villegas, Stanislav R. Stoyanov, Wei Huang, and D. Paul Rillema*
Department of Chemistry, Wichita State UniVersity, 1845 North Fairmount Street,
Wichita, Kansas 67260-0051
Received August 31, 2004
The ligand 2,6-dimethylphenylisocyanide (CNx) forms six complexes of the formula [Re(CO)3(CNx)(L)]+, where L
1,10-phenanthroline (1), 5-chloro-1,10-phenanthroline (2), 5-nitro-1,10-phenanthroline (3), 5-methyl-1,10-
)
phenanthroline (4), 5,6-dimethyl-1,10-phenanthroline (5), and 1,10-phenanthrolinopyrrole (6). The lowest-energy
absorption peaks of the complexes red-shift in the order 1 < 2 < 3 < 4 < 5 < 6. The time-dependent density
functional theory (TDDFT) and conductor-like polarizable continuum model (CPCM) computed singlet excited states
1
in ethanol deviate by 1000 cm- or less from the experimental UV
−vis peaks. The complexes undergo reversible
reductions and irreversible oxidations. The electronic energy gap increases in the order 3 < 2 < 1 < 4 < 5 < 6,
which is the order of increasing electron-donating power of the phen substituents. The reduction potentials linearly
correlate with the B3LYP calculated LUMO energies for 1
−
6. The complexes emit at room temperature and at 77
3
1
K except 3, which emits only at 77 K. The calculated MLLCT energies are within 1100 cm- from the experimental
emission energies at 77 K. The 77 K emission curve-fitting analysis results agree with the computational assignment
of the emitting state as 3MLLCT for 1 5 and 3LC for 6. The experimental 77 K emission energies and the calculated
−
3MLLCT state energies increase in the order 6 < 5, 3 < 2 < 4, 1. The 77 K emission lifetimes increase upon
addition of substituents from 65 s for 1 to 171 s for 2, to 230 s for 4 and 5, and to 322 s for 3. The emission
µ
µ
µ
µ
quantum yields at room temperature in solution are 0.77, 0.78, 0.83, 0.56, and 0.11 for complexes 1, 2, 4, 5, and
6, respectively.
Introduction
conversion.3,23-26 These complexes are ideally suited for such
use because they display intense luminescence in the visible
region of the spectrum and have long emission lifetimes.27
Rhenium(I) tricarbonyl complexes containing bidentate
heterocyclic ligands have been a source of interest for several
years,1-22 largely due to their potential use in solar energy
(11) Sahai, R.; Rillema, D. P.; Shaver, R. J.; Van Wallendael, S.; Jackman,
D. C.; Boldaji, M. Inorg. Chem. 1989, 28, 1022.
(12) Ruminski, R.; Cambron, R. T. Inorg. Chem. 1990, 29, 1574.
(13) Baiano, J. A.; Carlson, D. L.; Wolosh, G. M.; DeJesus, D. E.; Knowles,
C. F.; Szabo, E. G.; Murphy, W. R., Jr. Inorg. Chem. 1990, 29, 2327.
(14) Tapolsky, G.; Duesing, R.; Meyer, T. J. J. Phys. Chem. 1989, 93,
2885.
(15) Tapolsky, G.; Duesing, R.; Meyer, T. J. Inorg. Chem. 1990, 29, 2285.
(16) Lin, R.; Guarr, T. F. Inorg. Chim. Acta 1990, 167, 149.
(17) Lin, R.; Guarr, T. F.; Duesing, R. Inorg. Chem. 1990, 29, 4169.
(18) Kalyasundaram, K. J. Chem. Soc., Faraday Trans. 2 1986, 82, 2401.
(19) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.;
DeGraff, B. A. Inorg. Chem. 1990, 29, 4335.
(20) Leasure, R. M.; Sacksteder, L.; Nesselrodt, D.; Reitz, G. A.; Demas,
J. N.; DeGraff, B. A. Inorg. Chem. 1991, 30, 3722.
(21) Hino, J. K.; Della Ciana, L.; Dressick, W. J.; Sullivan, B. P. Inorg.
Chem. 1992, 31, 1072.
(22) Della Ciana, L.; Dressick, W. J.; Sandrini, D.; Maestri, M.; Ciano,
M. Inorg. Chem. 1990, 29, 2792.
(23) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193.
* Author to whom correspondence should be addressed. E-mail:
paul.rillema@wichita.edu.
(1) Wrighton, M.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998.
(2) Giordano, P. J.; Wrighton, M. S. J. Am. Chem. Soc. 1979, 101, 2888.
(3) Wrighton, M. S.; Geoffroy, G. L. Organometallic Photochemistry;
Academic Press: New York, 1979; Chapter 2.
(4) Lees, A. Chem. ReV. 1987, 87, 711.
(5) Giordano, P. J.; Fredericks, S. M.; Wrighton, M. S.; Morse, D. L. J.
Am. Chem. Soc. 1978, 100, 2257.
(6) Fredericks, S. M.; Luong, J. C.; Wrighton, M. S. J. Am. Chem. Soc.
1979, 101, 7415.
(7) Smothers, W. K.; Wrighton, M. S. J. Am. Chem. Soc. 1983, 105, 1067.
(8) Vogler, A.; Kisslinger, J. Inorg. Chim. Acta 1986, 115, 193.
(9) Juris, A.; Campagna, S.; Bidd, I.; Lehn, J. M.; Zeissel, R. Inorg. Chem.
1988, 27, 4007.
(10) Van Wallendael, S.; Shaver, R. J.; Rillema, D. P.; Yoblinski, B. J.;
Stathis, M.; Guarr, T. F. Inorg. Chem. 1990, 29, 1761.
10.1021/ic048786f CCC: $30.25
Published on Web 03/03/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 7, 2005 2297

Villegas et al.
Generally, Re(I) tricarbonyl complexes are MLCT emit-
ters, having broad and structureless emission bands that are
sensitive to changes in the nature of the environment.1,2,4-7,20
Thus, variations in the structure of the non-carbonyl bidentate
as well as the ancillary ligands produce considerable effects
on luminescence energies, lifetimes, and quantum yields.
Depending on the chromophoric bidentate ligand or the
ancillary “spectator” ligand used, the photochemical and
photophysical properties of the complex can be fine-tuned.
Os(II) polypyridyl complexes.33 It is the primary method used
in our study.
We have experimentally and computationally investigated
the effect of the 2,6-dimethylphenylisocyanide (CNx) ligand
on the complexes [Re(bpy)(CO)(CNx)3]+, [Re(CNx)5Cl], and
[Re(CNx)6]+.34 The highest emission quantum yield (φem)
and emission lifetime (τem) were observed for [Re(CO)3(bpy)-
(CNx)]+.34b Here, we extend the study by varying the
substituents attached on the 5- and 6-positions of 1,10-
phenantholine in the [Re(CO)3(CNx)(phen)]+ moiety. The
phenanthroline ligand has an enhancing effect on the φem
and τem of the system, allowing one to compare the effects
of the substituents on the photophysical properties of the
system.
Density functional theory (DFT) is a very useful method
for interpreting experimental results from electrochemistry
and electronic spectroscopy. Linear relationships of E1/2(ox)
versus the highest occupied molecular orbital (HOMO)
energies and E1/2(red) versus the lowest unoccupied molecular
orbital (LUMO) energies for a series of isoelectronic Ru(II)
diimine complexes were reported from our laboratory.28 DFT
calculations on the singlet ground and lowest-lying triplet-
states of a series of Re(I) tricarbonyl complexes were used
by others for investigating excited-state geometries and
electronic structures.29,30 Time-dependent density functional
theory (TDDFT) calculated MLCT states and UV-vis
spectra correlations were also reported for Re(I) complexes
containing the ligand azophenine.30c
Experimental Section
Materials. The ligand 2,4-dimethylphenylisocyanide was pur-
chased from Fluka. The ligands 1,10-phenanthroline, 5-chloro-1-
10-phenanthroline, 5-NO2-1,10-phenanthroline, and 5-methyl-1,10-
phenanthroline were purchased from GFS Chemicals. The 5,6-
dimethyl-1,10-phenanthroline ligand was purchased from Aldrich.
The 1,10-phenanthrolinopyrrole ligand (php) was prepared in our
laboratory.35 Optima grade methanol was purchased from Fischer
Scientific, while acetonitrile was purchased from Sigma-Aldrich.
AAPER Alcohol and Chemical Co. was the source of absolute
ethanol. The [Re(CO)5Cl] was purchased from Aldrich. Ethanol
and methanol were used in a 4:1 (v/v) mixture to prepare solutions
for the emission, and emission lifetime studies. Elemental analyses
were obtained from M-H-W Laboratories, Phoenix, AZ.
Instrumentation and Physical Measurements. UV-vis spectra
were obtained using a Hewlett-Packard model 8452A diode array
spectrophotometer. The IR spectra were acquired using 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 the cyclic voltammograms. The measurements were carried
out in a typical H-cell using a platinum disk working electrode, a
platinum wire counter electrode, and Ag/AgCl reference electrode
in acetonitrile. The supporting electrolyte used was 0.1 M tetrabu-
tylammonium hexafluorophosphate (TBAH). Ferrocene was added
for reference.
The TDDFT method treats molecules in the gas phase and
does not always give the right electronic transition energies
in solution.31,32 A model used with more success for
molecules in solution combines the TDDFT method with
the conductor-like polarizable continuum model (CPCM).
For example, we reported a correlation between UV-vis
absorption energies of [Ru(bpy)2(CNx)Cl]+ and singlet
excited-state energies computed in a series of seven solvents
of varied polarity using the TDDFT/CPCM model.32b Ac-
cording to other reports, the tandem use of TDDFT and
CPCM has produced dramatic changes in the assignments
and the energies of the singlet excited states for Ru(II) and
(24) Caspar, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23,
2098.
(25) Kober, E. M.; Marshall, J. L.; Dressick, W. J.; Sullivan, B. P.; Meyer,
T. J. Inorg. Chem. 1985, 24, 2755.
The sample preparation for emission studies involved dissolving
a small amount of sample (∼2 mg) in the appropriate solvent, and
the absorbance of the solution was measured. The concentration
of the solution for luminescence studies was altered to achieve an
absorbance of about 0.10 at the energy of excitation. Such a
concentration provides enough material for data acquisition but
excludes self-quenching processes. A 3-4 mL aliquot of the
solution was then placed in a 10 mm diameter Suprasil (Heraeus)
nonfluorescent 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 gases from
the sample. The manifold was then closed, and the sample was
allowed to equilibrate at room temperature. The solvent evaporation
(26) Kober, E. M.; Sullivan, B. P.; Dressick, W. J.; Caspar, J. V.; Mayer,
T. J. J. Am. Chem. Soc. 1980, 102, 1383.
(27) Luong, J. C.; Nadjo, L.; Wrighton, M. S. J. Am. Chem. Soc. 1978,
100, 5790.
(28) Stoyanov, S. R.; Villegas, J. M.; Rillema, D. P. Inorg. Chem. 2002,
41, 2941.
(29) (a) Dattelbaum, D. M.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J.
J. Phys. Chem. A 2004, 108, 3518-3526. (b) Dattelbaum, D. M.;
Omberg, K. M.; Hay, J. P.; Gebhart, N. L.; Martin, R. L.; Schoonover,
J. R.; Meyer, T. J. J. Phys. Chem. A 2004, 108, 3527-3536.
(30) (a) Yang, L.; Ren, A.-M.; Feng, J.-K.; Liu, X.-J.; Ma, Y.-G.; Zhang,
M.; Liu, X.-D.; Shen, J.-C.; Zhang, H.-X. J. Phys. Chem. A 2004,
108, 6797-6808. (b) Dyer, J.; Blau, W. J.; Coates, C. G.; Creely, C.
M.; Gavey, J. D.; George, M. W.; Grills, D. C.; Hudson, S.; Kelly, J.
M.; Matousek, P.; McGarvey, J. J.; McMaster, J.; Parker, A. W.;
Towrie, M.; Weinstein, J. A. Photochem. Photobiol. Sci. 2003, 2, 542-
554. (c) Frantz, S.; Rall, J.; Hartenbach, I.; Schleid, T.; Zalis, S. Kaim,
W. Chem.-Eur. J. 2004, 10, 149-154.
(31) (a) Monat, J. E.; Rodriguez, J. H.; McCusker, J. K. J. Phys. Chem. A
2002, 106, 7399. (b) Rodrigues, J. H.; Wheeler, D. E.; McCusker, J.
K. J. Am. Chem. Soc. 1998, 120, 12051.
(32) (a) Stoyanov, S. R.; Villegas, J. M.; Rillema, D. P. Inorg. Chem. 2003,
42, 7852. (b) Stoyanov, S. R.; Villegas, J. M.; Rillema, D. P. Inorg.
Chem. Commun. 2004, 7, 838-841. (c) Villegas, J. M.; Stoyanov, S.
R.; Huang, W.; Lockyear, L. L.; Reibenspies, J.; Rillema, D. P. Inorg.
Chem. 2004, 43, 6383-6396.
(33) Guillemoles, J.-F.; Barone, V.; Joubert, L.; Adamo, C. J. Phys. Chem.
A 2002, 106, 11345.
(34) (a) Villegas, J. M.; Stoyanov, S. R.; Reibenspies, J.; Rillema, D. P.
Organometallics 2005, 24 (3), 395-404. (b) Stoyanov, S. R.; Villegas,
J. M.; Cruz, A. J.; Lockyear, L. L.; Reibenspies, J.; Rillema, D. P. J.
Chem. Theory Comput. 2005, 1 (1), 95-106.
(35) Villegas, J. M.; Stoyanov, S. R.; Rillema, D. P. Inorg. Chem. 2002,
41, 6688.
2298 Inorganic Chemistry, Vol. 44, No. 7, 2005

Studies of a Series of Re(I) Tricarbonyl Complexes
was assumed to be negligible; therefore, the concentrations were
assumed to remain constant throughout this procedure. The cor-
rected emission spectra were collected using a Spex Tau3 fluo-
rometer.
The emission quantum yields were then calculated using eq 1,
where φx is the emission quantum yield of the sample and φstd is
the emission quantum yield for the standard [Ru(bpy)3]2+, Astd and
Ax represent the absorbance after degassing the standard and the
sample, respectively, while Istd and Ix are the integrals of the
emission envelope of the standard and the sample, respectively.36
Figure 1. Schematic diagram of the synthesis of the complexes.
8.4 Hz), 9.16 (dd, 1H, J ) 1.2, 8.4 Hz), 9.57 (dd, 1H, J ) 1.5, 5.1
Hz), 9.65 (dd, 1H, J ) 0.9, 5.1 Hz).
(3) fac-[Re(CO)3(CNx)(5-NO2-phen)](PF6). Color: yellow.
Yield: 80%. Anal. Calcd for ReC24H16N4O5PF6: C, 37.36; H, 2.09;
N, 7.26. Found: C, 37.50; H, 2.16; N, 7.22. (KBr pellet): 2174,
2041, 1973, 1939, 1684, 1653, 1541, 1521, 1473, 1457, 1345, 1159,
φx ) (Astd/Ax)(Ix/Istd)φstd
(1)
The excited-state lifetimes were determined by exciting the
sample at 355 nm using an OPOTEK optical parametric oscillator
pumped by a frequency tripled Continuum Surlite Nd:YAG laser
run at ∼20 mJ/10 ns pulse. The oscilloscope control and data curve-
fitting analysis were accomplished using the Origin 6.1 program
by OriginLab Corp. The excited-state lifetime experiments were
conducted as previously published.35
1
844, 756, 663, 635, 558, 420 cm-1. H NMR (DMSO): δ ppm
1.80 (s, 6H), 7.07 (d, 2H, J ) 7.5 Hz), 7.20 (dd, 1H, J ) 1.2, 8.1
Hz), 8.29 (m, 2H), 9.25 (dd, 1H, J ) 1.2, 8.4 Hz), 9.34 (dd, 1H, J
) 1.2, 8.7 Hz), 9.44 (s, 1H), 9.69 (dd, 2H, J ) 1.2, 5.4 Hz).
(4) fac-[Re(CO)3(CNx)(5-Me-phen)](PF6). Color: light yellow.
Yield: 80%. Anal. Calcd for ReC25H19N3O3PF6: C, 40.54; H, 2.59;
N, 5.67. Found: C, 40.30; H, 2.77; N, 5.57. (KBr pellet): 2171,
2037, 1968, 1937, 1653, 1630, 1559, 1522, 1429, 1389, 1159, 1032,
Preparation of fac-[Re(CO)3(CNx)(L)](PF6). The complexes
were synthesized according to previously published procedures,37,38
which were modified as follows: A 0.55 mmol sample of
[Re(CO)5Cl] was added to an equimolar amount of the phenan-
throline-based ligand in a 125 mL round-bottomed flask. Ap-
proximately 50 mL of absolute ethanol was added, and the mixture
was refluxed for 2-4 h. A colored precipitate formed in the
solution, which was cooled to room temperature and filtered. After
drying in a vacuum oven for 3-5 h, about 0.20 mmol 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 h. 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 again refluxed for another 3-5 h. 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.
1
842, 728, 635, 611, 558, 478 cm-1. H NMR (DMSO): δ ppm
1.73 (s, 6H), 2.87 (s, 3H), 7.06 (d, 2H, J ) 7.5 Hz), 7.20 (dd, 1H,
J ) 1.2, 8.4 Hz), 8.14 (m, 2H), 8.20 (s, 1H), 8.93 (dd, 1H, J )
1.5, 8.4 Hz), 9.09 (dd, 1H, J ) 1.5, 8.4 Hz), 9.48 (dd, 1H, J ) 1.5,
5.1 Hz), 9.57 (dd, 1H, J ) 1.2, 5.1 Hz).
(5) fac-[Re(CO)3(CNx)(5,6-Me2-phen)](PF6). Color: light yel-
low. Yield: 77%. Anal. Calcd for ReC26H21N3O3PF6 (containing
0.5 mol of SiO2): C, 39.80; H, 2.70; N, 5.36. Found: C, 39.86; H,
2.66; N, 5.54. (KBr pellet): 2170, 2037, 1968, 1935, 1653, 1614,
1
1602, 1475, 1432, 1174, 1034, 842, 727, 635, 558, 478 cm-1. H
NMR (DMSO): δ ppm 1.75 (s, 6H), 2.83 (s, 6H), 7.07 (d, 2H, J
) 7.5 Hz), 7.21 (dd, 1H, J ) 1.2, 8.1 Hz), 8.16 (dd, 2H, J ) 3.3,
5.1 Hz), 9.16 (dd, 2H, J ) 1.5, 8.7 Hz), 9.51 (dd, 2H, J ) 1.2, 5.1
Hz).
(6) fac-[Re(CO)3(CNx)(php)](PF6). Color: yellow. Yield: 90%.
Anal. Calcd for ReC26H18N4O3PF6: C, 40.79; H, 2.37; N, 7.32.
Found: C, 41.00; H, 2.14; N, 7.15. (KBr pellet): 2169, 2035, 1966,
1934, 1602, 1524, 1472, 1441, 1381, 1281, 1252, 1160, 1102, 1072,
1030, 846, 779, 730, 637, 608, 558, 541, 478, 429 cm-1. 1H NMR
(DMSO): δ ppm 1.80 (s, 6H), 7.07 (d, 2H, J ) 7.5 Hz), 7.22 (dd,
1H, J ) 1.2, 8.1 Hz), 8.27 (dd, 2H, J ) 0.9, 6.5 Hz), 9.11 (s, 2H),
9.51 (dd, 2H, J ) 0.9, 10.4 Hz), 9.81 (d, 2H, J ) 10.4 Hz), 13.60
(s, 1H).
(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, 634,
558, 507, 471 cm-1. 1H NMR (DMSO): δ 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).
Results
Synthesis. The synthesis of the complexes was carried out
according to the scheme presented in Figure 1. The [Re-
(CO)5Cl] was first reacted with the phenanthroline-based
ligand (L) to form a neutral complex with the general formula
[Re(CO)3(L)Cl]. The product then was allowed to react with
AgCF3SO3 removing the chloro ligand from the coordination
(2) fac-[Re(CO)3(CNx)(5-Cl-phen)](PF6). Color: light yellow.
Yield: 81%. Anal. Calcd for ReC24H16N3O3ClPF6 (containing 0.8
mol of SiO2): C, 35.63; H, 2.12; N, 5.52. Found: C, 35.50; H,
2.18; N, 5.46. (KBr pellet): 2172, 2040, 1971, 1938, 1653, 1603,
1559, 1519, 1426, 1209, 972, 842, 727, 635, 610, 558 cm-1. H
1
-
NMR (DMSO): δ ppm 1.78 (s, 6H), 7.07 (d, 2H, J ) 7.8 Hz),
7.21 (dd, 1H, J ) 1.2, 8.1 Hz), 8.19 (dd, 1H, J ) 3.3, 5.1 Hz),
8.28 (dd, 1H, J ) 3.3, 5.1 Hz), 8.72 (s, 1H), 8.98 (dd, 1H, J ) 1.2,
sphere by precipitating AgCl and replacing it with CF3SO3 .
The CNx ligand then 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 3 h 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 formed in relatively high yield.
(36) (a) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (b)
Cook, M. J.; Lewis, A. B.; McAuliffe, G. S. G.; Skarda, V.; Thomson,
A. J.; Glasper, A. L.; Robbins, D. J. J. Chem. Soc., Perkin Trans. 2
1984, 1293.
(37) Wallace, L.; Rillema, D. P. Inorg. Chem. 1993, 32, 3836-3843.
(38) Shaver, R. J.; Rillema, D. P. Inorg. Chem. 1992, 31, 4101.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2299

Villegas et al.
band of 3 was red-shifted by 500 cm-1 relative to 1. When
electron-donating methyl groups were attached in 4 and 5
positions, the MLLCT band was red-shifted to 26 900 cm-1
in 4 and 26 500 cm-1 in 5. Upon extension of the phen
aromatic system in 6, the lowest energy peak was observed
at 24 400 cm-1 and assigned as a LC state (vide infra).
Electrochemical Studies. The redox potentials of the
complexes in the series were determined by cyclic voltam-
metry and are listed in Table 2. All complexes in the series
showed irreversible oxidation waves in the range of 1.97-
2.05 V.
The reduction potentials increased from -1.18 V for 1 to
-1.08 for 2 and -0.53 V for 3 when the hydrogen atom in
the 5-position of the parent phen ligand was replaced with
an electron-withdrawing substituent. However, the potentials
shifted in the opposite direction from -1.18 V for 1 to -1.24
V for 4, to -1.26 V for 5 and to -1.51 V for 6 when the
hydrogen atom(s) in 5 and/or 6 position of the parent phen
ligand were replaced with electron-donating methyl group-
(s) in complexes 4 and 5 or with a pyrrole group in complex
6.
Emission Properties and Excited-State Lifetimes. The
emission properties and excited-state lifetimes (τem) of the
complexes were determined both at room temperature and
at 77 K in 4:1 (v/v) ethanol/methanol. The values of the
emission lifetimes at room temperature and 77 K were
determined by curve-fitting analysis. The data are listed in
Table 3. Temperature-dependent studies of the emission
lifetimes were not conducted because the complexes under-
went photodecomposition upon continuous exposure to laser
radiation.
Figure 2. Structures of the complexes used in the study.
Figure 2 shows a schematic diagram of the Re(I) tricar-
bonyl isocyanide complexes with the phenanthroline-based
ligands. As noted in this series, the complexes are facial and
only the 5- and/or 6-positions of the parent phenanthroline
ligand were modified by substituting the hydrogen atoms
with electron-withdrawing or electron-donating groups.
Electronic Absorption Studies. The electronic absorption
properties of the complexes were studied at room temperature
using 4:1 (v/v) ethanol/methanol as solvent, and spectra are
shown in Figure 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 types of
complexes.1-3,27,39 The results are listed in Table 1.
The lowest energy transitions of the complexes were
assigned as metal-ligand-to-ligand-charge transfer (MLLCT)
for 1-5 and as a ligand-centered (LC) transition for 6.
Several πfπ* transitions were reported for the complexes
at higher energies. It is important to note that because the
MLLCT band occurs as a broad shoulder, the exact position
of the band as well as the extinction coefficient were subject
to error.
The emission maxima of the complexes were shifted to
higher energies at 77 K as compared to room temperature
as shown in Figure 4. The 77 K emission maxima of
complexes 2-6 were red-shifted relative to complex 1
regardless of the nature or the number of the substituents
attached. The emission energies ranged from 21 800 cm-1
for 1 to 21 100 cm-1 for 2 and 20 400 cm-1 for 3, or
underwent a red-shift of 700 and 1400 cm-1, respectively.
Relative to 1, the emission maxima red-shifted by 200 cm-1
when an electron-donating methyl group was attached in
complex 4 and by 1100 cm-1 when a second methyl group
was attached in complex 5. The largest red shift of 1800
cm-1 was recorded for complex 6 relative to 1 when the
conjugated pyrrole π-system was attached.
After converting the wavelength (abscissa) values to
energy, the emission spectral data were fit to eq 2, where
the summation was carried out over the two sets of six
vibrational levels.40
I(E) ) I + A[
n2[(E0 - n1pω1 - n2pω2)/E0]4
∑n1∑
0
The MLLCT peak did not significantly shift from 1 for 2
and 3 as the electron-withdrawing groups Cl and NO2 were
attached in the 5-position of the parent phen ligand. The
bands were located at 27 500 cm-1 for 1 and 2. The MLLCT
(S1n1/n1!)(S2n2/n2!) exp{-4 log 2[(E - E0 + n1pω1 +
n2pω2)/υ1/2]2}] (2)
(40) (a) Caspar, J. V. Ph.D. Thesis, University of North Carolina, Chapel
Hill, NC, 1982. (b) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer,
T. J. J. Am. Chem. Soc. 1984, 106, 2613.
(39) Juris, A.; Belser, P.; Barigelletti, F.; von Zelewsky, A.; Balzani, V.
Inorg. Chem. 1986, 25, 256.
2300 Inorganic Chemistry, Vol. 44, No. 7, 2005

Studies of a Series of Re(I) Tricarbonyl Complexes
Figure 3. Experimental absorption spectra of 1-6 and calculated singlet excited states. The excited states are shown as vertical bars with height equal to
the extinction coefficient.31a Black ) MLLCT, green ) LLCT, blue ) πfπ*, red ) MCDCT, cyan ) LCDCT (ligand f delocalized), and O‚‚‚‚‚‚‚‚O )
mixed excited state.
The parameters were as follows: I0 was equal to 0, A was
the peak area, n1, n2 ) 0-5, E0 was the zero-zero energy,
pω1 and pω2 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 modes,41 and υ1/2 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.
ligand ring breathing modes. For complexes 1-5, the low-
frequency modes were attributed to metal-ligand vibrations.
For complex 6, curve-fitting analysis using one set of six
vibrational levels produced more reasonable values than for
a two-set curve-fitting analysis as was used for the others.
(41) 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.
The results of the emission spectral curve-fitting at 77 K
(Figure 4) are listed in Table 4. The values of the high-
frequency modes at about 1400 cm-1 correspond to the phen
Inorganic Chemistry, Vol. 44, No. 7, 2005 2301

Villegas et al.
3
Table 1. Experimentala Electronic Transitions and Calculatedb Singlet
Excited States of Re(I) Complexes 1-6
Table 3. Calculated MLLCT State Energiesa and Emission Properties
of the Complexes at 77 K and Room Temperatureb (Energies in ×103
cm-1, s ) shoulder)
E
exp, ×103 cm-1
E
calc, ×103
c
complex
ꢀ (M-1 cm-1
)
cm-1
assignment
Eexp
Eexp
τ
em, µs
τ
em, µs
æem
complexes Ecalc 77 K room temp 77 K room temp room temp
1
27.5 (3300)
28.2
33.5
37.1
38.4
27.6
35.9
38.3
27.5
38.5
MLLCT
MCDCT
CDLCT
33.3 (15 000)
37.0 (39 700)
38.5 (37 500)
27.5 (4800)
36.2 (49 600)
38.5 (50 900)
27.0 (3500)
38.5 (38 600)
42.4 (36 900)
26.9 (3000)
36.2 (39 300)
38.5 (36 000)
42.7 (39 900)
26.5 (3400)
35.5 (40 600)
38.5 (34 500)
41.7 (43 100)
24.4 (3700)
30.3 (17 600)
38.5 (51 900)
44.2 (49 300)
1
22.6 21.8
20.4
19.1
17.7 (s)
22.2 21.1
19.7
18.3
17.1 (s)
20.9 20.4
19.0
17.8 (s)
22.6 21.6
20.2
18.8
17.5 (s)
20.7 20.7
19.3
18.0
16.6 (s)
19.1 20.0
18.6
19.7
65
8.6
0.77
LLCT
2
3
4
MLLCT
MCDCT
MLLCT
MLLCT
MLLCT
LC (πfπ*)
MLLCT
LCDCT
MCDCT
LC (πfπ*)
MLLCT
MCDCT
MLLCT
LC (πfπ*)
LC (πfπ*)
MLLCT
MCDCT
LC (πfπ*)
2
19.1
171
1.5
0.78
3
4
322
231
28.1
36.3
38.5
19.6
20.2
30.9
6.2
0.83
0.56
0.11
5
6
26.9
35.7
39.1
5
6
20.3
19.3 (s)
229
268
23.1
30.7
38.8
18.5
17.3
16.0 (s)
a In 4:1 (v/v) ethanol:methanol. b In ethanol.
a In the gas phase. b In 4:1 (v/v) EtOH:MeOH. c Relative to [Ru(bpy)3]2+
Table 2. Electrochemical Properties of the Complexes in CH3CN at
(ref 35).
Room Temperature
complex
E
1/2(ox), Va
E
1,2(red), Va (L)
Nonequilibrium TDDFT46/CPCM47 calculations were em-
ployed to produce a number of singlet excited states48 of
complexes 1-6 in ethanol based on the singlet ground-state
geometry optimized in the gas phase.49 The TDDFT/CPCM
calculations are nonequilibrium calculations with respect to
the polarization 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.50 The 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 wave function coefficients of the
excitations. The singlet excited states of the six comp-
1
2
3
2.01b
2.03b
2.05b
-1.18
-1.08
-0.53
-1.06
-1.24
-1.26
-1.51
4
5
6
2.00b
1.98b
1.97b
a Potential in volts vs SSCE (scan rate ) 250 mV/s). b Irreversible
oxidation wave.
Computational Section. The singlet ground-state geom-
etries of complexes 1-6 were optimized in the gas phase
using the B3LYP42 functional of the Gaussian 0343 program
package. The Stuttgart-Dresden (SDD) ECP44 was used for
the Re core potentials. The {(8s7p6d)/[6s5p3d]}-GTO was
applied for the valence shell of Re together with the all-
electron 6-311G* basis set45 for Cl, O, N, C, and H atoms.
The optimized geometries of the complexes are listed in the
Supporting Information Table S1.
(44) Andrae, D.; Hauessermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor.
Chim. Acta 1990, 77, 123.
(45) (a) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639.
(b) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem.
Phys. 1980, 72, 650.
(46) (a) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys.
1998, 109, 8218. (b) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett.
1996, 256, 454. (c) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub,
D. R. J. Chem. Phys. 1998, 108, 4439.
(47) (a) Cossi, M.; Barone, V. J. Chem. Phys. 2001, 115, 4708. (b) Barone,
V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995. (c) Cossi, M.; Rega,
N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669.
(48) The CPCM is designed to account for the bulk physical properties of
the solvent. It does not account for specific solvent-solute interactions.
The TDDFT is known to perform well for the computing of charge-
transfer excited states between closely spaced moieties. The tandem
use of CPCM and TDDFT is currently the most suitable computational
approach for the treatment of the solvent effects to the transition metal
complexes excited-state energies.
(49) Geometry optimization in solvents was not achieved. Partial optimiza-
tions (change in distance of less than 0.001 Å and change in angles
of less than 0.01°) followed by TDDFT/CPCM calculation produced
excited-state energies that were not in better agreement with the
experimental excited-state energies than the excited-state energies
based on the gas-phase optimized geometry.
(42) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Vosko, S. H.; Wilk,
L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(43) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P.
Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J.
B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
B.03; Gaussian, Inc.; Pittsburgh, PA, 2003.
(50) Frisch, Æ.; Frisch, M. J.; Trucks, G. W. Gaussian 03 User’s Reference,
Version 7.0; Gaussian, Inc.: Carnegie, PA, 2003; p 206.
2302 Inorganic Chemistry, Vol. 44, No. 7, 2005

Studies of a Series of Re(I) Tricarbonyl Complexes
Figure 4. Emission spectra of the complexes at 77 K (-), curve-fitted (red ‚‚‚), and at room temperature (- - -) in 4:1 (v/v) ethanol/methanol.
lexes are presented in Figure 3 as vertical bars with height
equal to the extinction coefficient calculated from the oscil-
lator strength.31
The lowest-lying triplet-state geometries of the six com-
plexes were calculated using unrestricted B3LYP in the gas
phase. The spin contamination from states of higher multi-
Inorganic Chemistry, Vol. 44, No. 7, 2005 2303

Villegas et al.
Table 4. Emission Spectral Curve-Fitting Parameters of Complexes
1-6 in 4:1 (v/v) EtOH/MeOH at 77 Ka
Table 5. Calculated Triplet Excited States of Complexes 1-6 in
Ethanol Based on the Lowest-Lying Triplet State Geometrya
parameter
1
2
3
4
5
6
state
f
ψofψv
type
EVER
E
em, cm-1
21 800 21 100
21 800 21 120
20 400
20 440
1458
0.88
661
0.56
21 600 20 700 20 000
21 550 20 710 20 000
Complex 1
E0, cm-1
1
0.01 H-2 f H (0.7) Red, CO f Red, CNx
H-1 f H (0.6) Red f Red, CNx
0.00 H-4 f H (1.0) Phen, CNx f Red, CNx
0.01 L f L+1 (0.8) LC πfπ*
0.10 H-1 f H (0.7) Red f Red, CNx
H-2 f H (0.5) Red, CO f Red, CNx
27.6
pω1, cm-1
1395
0.89
545
1401
0.75
505
1414
0.88
525
1404
0.72
570
1460
1.40
S1
2
3
4
28.6
28.9
30.4
pω2, cm-1
S2
υ1/2
A
0.63
510
0.55
480
0.80
520
0.91
0.51
610
0.94
800
0.93
1430
0.90
0.90
0.94
Complex 2
a Error limits are as follows: E0, (6.0 cm-1, pω, (15 cm-1, S, (0.02,
1/2, (20 cm-1, A, (0.02.
1
2
0.02 H-1 f H (0.7) Red f Red, CNx
L f L+1 (0.7) LC πfπ*
0.00 L f L+1 (0.6) LC πfπ*
H-2 f H (0.5) Red, CO f Red, CNx
0.00 H-4 f H (0.9) 5-Cl-phen f Red, CNx
26.9
27.9
υ
3
4
28.2
29.6
0.10
H-1f H (0.6) Red f Red, CNx
Complex 3
1
2
3
4
0.00 H-8 f H (0.8) 5-NO2-phen f Red, CNx
0.00 H-1 f H (1.0) Red f Red, CNx
0.00 H-2 f H (1.0) CNx f Red, CNx
0.00 H-4 f H (0.8) CNx, Red f Red, CNx
24.5
29.6
31.6
32.6
Complex 4
1
2
0.00 H-1 f H (1.0) Red, 5-Me-phen f Red, CNx
0.01 L f L+1 (0.6) LC πfπ*
26.8
28.4
H-4 f H (0.6) 5-Me-phen, Red f Red, CNx
H-2 f H (0.5) Red, CO f Red, CNx
3
4
0.00 H-4 f H (0.8) 5-Me-phen, Red f Red, CNx
0.08 H-2 f H (0.7) Red, CO f Red, CNx
28.7
30.0
Complex 5
1
2
3
4
0.02 L f L+1 (1.0) LC πfπ*
25.8
0.02 H-1 f H (1.0) Red, 5,6-Me2-phen f Red, CNx 30.9
0.00 H-2 f H (1.0) Red, CO f Red, CNx
0.00 H-3 f H (1.0) 5,6-Me2-phen, Red f Red, CNx 33.1
32.0
Complex 6
Figure 5. Triplet excited-state energy diagram for complexes 1-6.
1
2
3
4
0.00 H-1 f H (0.8) Del f php
0.02 H-2 f H (0.8) Red, php f php
0.01 L f L+1 (1.0) php f CNx, CO
0.00 H-3 f H (0.5) Red, CO f php
H-4 f H (0.5) Red, php f php
26.0
26.7
27.9
28.9
plicity was low. The value of S2 was 2.017 for 1 and 2,
2.009 for 3, 2.015 for 4, 2.041 for 5, and 2.024 for 6. The
energies of the lowest-lying triplet states were higher than
those of the corresponding ground states by 22 600 cm-1
for 1 and 4, 22 200 cm-1 for 2, 20 900 cm-1 for 3, 20 700
cm-1 for 5, and 19 100 cm-1 for 6 (Figure 5). The lowest-
lying triplet states for complexes 1-5 were 3MLLCT states,
L f L+2 (0.5) LC πfπ*
a EVER is the energy of the vertical transition in ×103 cm-1, f is the
oscillator strength, and ψo and ψv are the occupied and the virtual orbitals
that define the transition. The transition type is determined on the basis of
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.)
3
but for complex 6 it was a LC state. These states featured
single occupancy of the HOMO and the LUMO.
geometry. Triplet excited states calculated on the basis of
the singlet ground-state geometry are multiplicity forbidden
(f ) 0).
Four triplet excited states were computed in ethanol using
the TDDFT/CPCM method based on the lowest-lying triplet-
state geometries for each of the complexes and are listed in
Table 5, even if the f values were low. The energies of these
triplet excited states are given relative to the singlet ground
state. The energies and the assignments of the lowest-lying
triplet states and the four higher-lying triplet excited states
from Table 5 are shown in Figure 5. These excited states
were used for the interpretation of temperature-dependent
emission properties of the complexes. The higher-lying triplet
states were obtained via single electron vertical excitations
from the lowest-lying 3MLLCT states, and the excited-state
energies reported were not the minima. The triplet excited
states were calculated on the basis of 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 on the basis of the most stable triplet
Discussion
Molecular Orbitals. The 12-frontier molecular orbital
energy diagram for complexes 1-6 in ethanol is shown in
Figure 6. Schematic diagrams of the HOMOs and the
LUMOs of complexes 1-6 are shown in Figure 7. The
HOMOs of complexes 1-5 (Figure 7) contained 45% or
higher Red character and 27% or higher CNx ligand
character. The HOMOs-1 of complexes 1, 2, and 3
contained 61% or higher Red contributions. Additionally,
there were 16% diimine ligand contributions for each of
complexes 1 and 2 as well as 13% for complex 3. The
HOMOs-1 contained 43% or higher Red and 27% or higher
diimine ligand contributions for complexes 4 and 5. For
complexes 1, 2, 4, and 5, the HOMOs-2 contained 66% or
2304 Inorganic Chemistry, Vol. 44, No. 7, 2005

Studies of a Series of Re(I) Tricarbonyl Complexes
for 4, 32 000 cm-1 for 5, and 28 200 cm-1 for 6. The
HOMO-LUMO energy gaps for complexes 1, 2, 4, and 5
were within 700 cm-1 of the 32 000 cm-1 value for [Re-
(CO)3(bpy)(CNx)]+.34b
Electrochemical Behavior. The redox potentials of im-
portance in discussing the electrochemistry of the complexes
in the series are those that are derived from the processes
involving the HOMO and the LUMO.37 The HOMOs of
complexes 1-5 consist of dπ orbitals located on the metal
center and CNx ligand π orbitals. The LUMOs are predomi-
nantly located on the π* orbital of the phen-based ligand.
The irreversible oxidations of 1-5 that involve the removal
of an electron from the HOMO could be due to the 27% or
higher CNx ligand contribution. The one-electron reduction,
on the other hand, involves addition of an electron to the π*
orbital localized on the diimine moiety.51-53
The addition of an electron-withdrawing group to the phen
moiety shifted the reduction potentials to higher values in
the order 1 < 2 < 3. As expected on the basis of the
electrochemical behavior of nitro-organic compounds, com-
plex 3, which contains the NO2-group attached to phenan-
throline, was easily reduced at -0.53 V. However, according
to the computational results (Figure 7), the LUMO is
delocalized onto the 5-NO2-phen ligand, indicating the whole
unit is reduced, not just the nitro group. The electron-
donating methyl groups, on the other hand, shifted the
reduction potentials to lower values in the order 5 < 4 < 1.
The extended conjugation of the php ligand in 6 resulted in
the lowest reduction potential and the highest electronic
energy gap (∆E1/2 ) E1/2(ox) - E1/2(red)). The oxidation
potentials of 1-6 did not vary significantly. As a conse-
quence, the electronic energy gap increased in the order 3
< 2 < 1 < 4 < 5 < 6. The linear dependence (R2 ) 0.97 (
0.10) between the reduction potentials and the LUMO
energies for complexes 1-6 is shown in Figure 8. The slope
of -0.81 ( 0.07 was less than -2.32 reported for a series
of 10 isoelectronic Ru(II) diimine complexes.28 A linear
correlation (R2 ) 0.95 ( 0.15) of the ordinary Hammett
substituent constants (σp) for 1-5 versus E1/2(red) had a slope
of 0.72 ( 0.09 and an intercept of -1.14 ( 0.04.54 This
result was similar to the slope of 0.97 and the intercept of
-1.16 reported for [Re(CO)3(Etpy)(bpy)]+ complexes (R2
) 0.98).21 Similarly, a linear plot (R2 ) 0.91 ( 0.16) of
ELUMO versus σp for 1-5 had a slope of -0.63 ( 0.11 and
an intercept of -2.91 ( 0.04.
Figure 6. Molecular orbital energy diagram for six occupied (H) and six
virtual (L) frontier orbitals of 1-6 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).
higher Red character and 20% or higher carbonyl character.
The HOMO-2 of complex 3 contained 98% CNx ligand
contribution. The character of the HOMOs and HOMOs-2
for complexes 1, 2, 4, and 5 closely resembled that of the
[Re(CO)3(bpy)(CNx)]+ complex.34b The HOMOs-1, how-
ever, contained significant diimine ligand contributions for
complexes 1-5 that were not found in [Re(CO)3(bpy)-
(CNx)]+.34b For complex 6, the HOMO contained ∼100%
php ligand character, whereas HOMO-1 contained 42% Red,
24% CNx, and 21% php ligand contributions; thus HO-
MO-1 was assigned as complex-delocalized. The HOMO-2
contained 43% Red and 34% php ligand contributions. The
ligand-centered nature of the HOMO of complex 6 (Figure
7) did not correlate with the metal-containing nature of the
HOMOs of previously reported rhenium diimine tricarbonyl
complexes.29,34
The LUMOs of complexes 1-6 (Figure 4) contained 81%
or higher diimine ligand contribution like the LUMO of [Re-
(CO)3(bpy)(CNx)]+.34b According to Figure 7, the LUMO
of complex 6 contained primarily contributions from the phen
portion of the php ligand. The LUMOs+1 of complexes 1-5
contained 82% or higher diimine ligand contribution, dif-
ferent from [Re(CO)3(bpy)(CNx)]+ where the LUMO+1
contained mostly CNx and CO character.34b The LUMO+1
of complex 6 contained 49% CNx and 19% CO character
and was assigned as CNx, CO. The LUMOs+2 of complexes
1, 2, and 5 contained CNx and CO character, while those of
complexes 3 and 6 contained 5-NO2-phen and php characters,
respectively. The LUMO+2 of complex 4 contained 16%
Rep and Red as well as 21% carbonyl, 48% CNx, and 16%
5-Me-phen contributions; it was assigned as complex-
delocalized. The percent molecular orbital contributions for
complexes 1-6 are listed in the Supporting Information
Table S2.
Reversible oxidations of the pyridine (py) analogues of
complexes 1 and 5, [Re(CO)3(phen)(py)]+ and [Re(CO)3-
(5,6-Me2-phen)(py)]+, occurred at 1.69 and 1.66 V, respec-
tively.37 Replacement of py with CNx results in an increase
in the oxidation potential by approximately 0.32 V. The
(51) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.; Vining, W. J.; Meyer,
T. J. J. Chem Soc., Chem. Commun. 1985, 1414.
(52) Abruna, H. D.; Breiks, A. I. J. Electroanal. Chem. Interfacial
Electrochem. 1986, 201, 347.
(53) O’Toole, T. R.; Sullivan, B. P.; Bruce, M. R. M.; Margerum, L. D.;
Murray, R. W.; Meyer, T. J. J. Electroanal. Chem. Interfacial
Electrochem. 1989, 259, 217.
(54) Gordon, A. J.; Ford, R. A. The Chemists Companion; John Wiley &
Sons: New York, 1972; pp 145-146.
The HOMO-LUMO energy differences were 32 100 cm-1
for 1, 31 300 cm-1 for 2, 27 400 cm-1 for 3, 31 900 cm-1
Inorganic Chemistry, Vol. 44, No. 7, 2005 2305

Villegas et al.
Figure 7. Schematic diagram of the HOMO and the LUMO of complexes 1-6.
reduction potentials of these py analogues at -1.22 and
-1.27 V37 were slightly higher by 0.04 and 0.01 V as
compared to the reduction potentials of 1 and 5, respectively.
Singlet Excited States and Electronic Absorption Spec-
tra. The singlet excited states of complexes 1-6 with f >
0.01 are shown in Figure 3 as vertical bars with height equal
to the molar absorptivity coefficient (ꢀ). The excited states
expressed with the higher bars have higher contributions to
the experimental peaks. Because calculated excited-state ꢀ
values have not correlated very well with the experimental
values,32b,c we based our assignment of the experimental
electronic transitions on the correlation of the calculated
excited-state energies with the experimental peak energies.
The bars are presented in colors that correspond to the type
of the singlet excited state as follows: black ) MLLCT,
blue ) πfπ*, green ) LLCT, red ) MCDCT (metal to
complex-delocalized charge transfer), and cyan ) both
LCDCT (ligand to complex-delocalized charge transfer) and
CDLCT (complex-delocalized to ligand charge transfer).
Mixed excited states are denoted with O‚‚‚‚‚‚‚‚O.55
Excited-states 1 and 2 of complex 1 at 26 300 and 28 200
cm-1, respectively, were associated with MLLCT transitions
and correlated with the experimental UV-vis peak at 27 500
cm-1. However, only the singlet excited-state 2 that is the
closest in energy to the UV-vis peak is listed in Table 1.
Excited-state 11 at 33 500 cm-1 was associated with a
transition from the HOMO (Red and CNx) to the LUMO+3
(complex-delocalized) with a transition coefficient of 0.4 and
2306 Inorganic Chemistry, Vol. 44, No. 7, 2005

Studies of a Series of Re(I) Tricarbonyl Complexes
The excited states below 30 000 cm-1 of complexes 3 and
4 were assigned as MLLCT. Four primarily LC πfπ*
excited states were calculated in the range of 35 000-37 000
cm-1 for complex 3. The highest contribution to the peak at
38 500 cm-1 was from the MLLCT state 28 (ꢀ ) 17 800
M-1 cm-1) at the same energy. For complex 4, the LCDCT
states 16 and 17 at 36 300 and 36 500 cm-1 involve electronic
transitions between ligand and complex-delocalized orbitals
and are presented as cyan-colored bars. These states were
blue-shifted by 100 and 300 cm-1, respectively, relative to
the experimental peak at 36 200 cm-1. The highest contribu-
tion to the latter peak would be expected from the 5-Me-
phen πfπ* state 18 (ꢀ ) 33 600 M-1 cm-1). The MCDCT
excited-state 22 (ꢀ ) 38 700 M-1 cm-1) was at the same
energy as the experimental peak at 38 500 cm-1.
The excited states below 30 000 cm-1 for complex 5 were
of MLLCT character. Singlet excite state 16 at 35 700 cm-1
based on a MCDCT transition was 200 cm-1 blue-shifted
relative to the experimental peak at 35 500 cm-1. The largest
contribution to the latter peak can be attributed to exited state
18 (ꢀ ) 29 400 M-1 cm-1) at 36 500 cm-1, based on 5,6-
Me2-phen πfπ* and MCDCT transitions. Excited-states 23
and 24 at 39 000 and 39 100 cm-1 were assigned as MCDCT
and MLLCT, respectively. The former was 500 cm-1 blue-
shifted relative to the 38 500 cm-1 experimental peak.
In complex 6, the excited-state 1 at 23 100 cm-1 was based
on the HOMO f LUMO transition (Figure 7) and was
assigned as a LC πfπ* state. If php is considered in terms
of phen and pyrrole moieties, this state can be described as
π(php)fπ*(phen) as shown in Figure 7. Excited-state 1 was
300 cm-1 red-shifted relative to the broad experimental UV-
vis absorption peak at 24 400 cm-1. The MLLCT excited-
state 7 at 30 700 cm-1 was 400 cm-1 blue-shifted relative
to the experimental peak at 30 300 cm-1. Excited-state 22
at 36 700 cm-1 had the highest ꢀ value of 24 600 M-1 cm-1,
and it was highly mixed. For state 22, the highest transition
coefficient of 0.4 was from an MCDCT transition, but there
were two more MLLCT transitions with coefficients of 0.3
and 0.2.
The experimental UV-vis peak at 38 500 cm-1 that
appeared in the spectra of complexes 1-6 was assigned as
LLCT for complex 1, MLLCT for complexes 2, 3, and 5,
but as MCDCT for complexes 4 and 6. The results from
simulating Gaussian line shapes from the singlet excited
states by integration following a procedure previously
reported31a (not shown) did not correlate well with the UV-
vis spectrum curvature. The use of the TDDFT/CPCM
method is known to produce optical energies in good
agreement with the experimental absorption spectra.32c
However, the ꢀ values produced from this method are found
to deviate from the experiment.32b The singlet excited states
are listed in the Supporting Information Table S3.
Figure 8. Linear dependence of E1/2(red) and the LUMO energies for
complexes 1-6.
was assigned as the MCDCT state. However, there were two
more transitions, one from the HOMO-1 (Red) to LUMO+3
(complex-delocalized) assigned also as MCDCT and the
other from HOMO-2 (Red and CO) to LUMO+2 (CNx,
CO) assigned as MLLCT, each with transition coefficients
of 0.3. Therefore, excited-state 11 is shown in Figure 3 as a
mixed MCDCT state. This state was only 200 cm-1 higher
in energy than the shoulder at 33 300 cm-1 in the experi-
mental UV-vis spectrum. Excited-state 18 at 36 800 cm-1
was based on CDLCT and MCDCT transitions but was
assigned as CDLCT on the basis of the higher transition
coefficient. A similar approach was taken for excited-state
19 at 37 100 cm-1 Singlet excited-states 18 and 19 were
correlated with the experimental peak at 37 000 cm-1, but
only state 19 that is 100 cm-1 blue-shifted relative to the
experimental peak was listed in Table 1. The contribution
of the phen πfπ* state 20 at 37 300 cm-1 to the latter
experimental peak would be the highest because state 20 had
the highest ꢀ value of 29 100 M-1 cm-1 for complex 1.
Excited-states 1 and 2 for complex 2 were of MLLCT
type, and state 2 was 100 cm-1 blue-shifted relative to the
experimental peak at 27 500 cm-1. Several singlet excited
states had energies near the experimental peak at 36 200
cm-1. The MCDCT excited-state 17 at 35 900 cm-1 was 300
cm-1 red-shifted as compared to the latter experimental peak.
Singlet excited-states 18, 19, and 20 assigned as 5-Cl-phen
πfπ* states were in the range of 36 700-36 900 cm-1.
States 20 (ꢀ ) 29 400 M-1 cm-1) and 21 (ꢀ ) 29 500 M-1
cm-1) would have the highest contributions to the peak at
36 200 cm-1.
(55) These assignments were made on the basis of 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
excitations 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 the symbol O‚‚‚‚‚‚‚‚O.
Triplet Excited States. Five low-lying triplet excited states
of complexes 1-6 calculated relative to the ground state are
presented in Figure 5. The lowest-lying triplet excited states
of complexes 1-5 were 3MLLCT states, but for complex 6
3
a LC state was found. This computational assignment is
supported by the results of the 77 K emission curve-fitting
Inorganic Chemistry, Vol. 44, No. 7, 2005 2307

Villegas et al.
analysis for 1-6. For complexes 1-5, the high and the low
acceptor modes were related to ligand and metal-ligand
vibrations, respectively. For complex 6, only the high
acceptor modes related to ligand vibrations were identified
(vide supra). The emitting state for 6 can be described as
π(php)fπ*(phen). This assignment is analogous to the one
reported for fac-[Re(CO)3(dppz)(py)]+, where dppz ) dipy-
rido[3,2-a:2′,3′-c]phenazine.30b If we consider dppz in terms
of phen and phenazine moieties, the emitting state was
assigned as π (dppz) f π* (phenazine) based on picosecond
and nanosecond visible, IR, and Raman spectroscopic studies
supported by TDDFT calculations.30b Both the calculated 3-
MLLCT and the experimental emission energies at 77 K
decrease in the order 1 > 2 > 3 when electron-withdrawing
substituents were added to the phen ligand. The calculated
3MLLCT energies were higher than the experimental emis-
sion energies at 77 K by 800, 1100, 500, and 1000 cm-1 for
complexes 1, 2, 3, and 4, respectively. The calculated
3MLLCT and the experimental 77 K emission energies were
the same for complex 5. For complex 6, the calculated 3LC
energy was lower than the experimental 77 K emission
energy by 900 cm-1.
through interactions with higher lying singlet and triplet states
by 1700-2500 cm-1.56 The triplet excited-states 1 of
complex 3 would then be thermally more accessible as
compared to the excited-states 1 of the other five complexes,
thus providing nonradiative relaxation pathways for 3.
Because the energy differences between the states given in
Table 5 are for vertical transitions, the actual thermal
activation energies could be lower.
Emission Properties and Excited-State Lifetimes. The
emission spectra of the complexes are shown in Figure 4.
Selected emission properties are listed in Table 3. The
complexes exhibit broad emission spectra at room temper-
3
ature, a typical behavior for MLCT emitters.37 Except for
3, the complexes were emissive at room temperature;
however, all complexes were highly emissive at 77 K.
The room-temperature emission peaks of the series showed
a red-shift from 19 700 cm-1 for 1 to 19 100 cm-1 for 2
when the electron-withdrawing Cl was attached in the
5-position of the parent ligand. This shift in the emission
peak is consistent with the decrease in the emission lifetime
from 8.6 µs in complex 1 to 1.5 µs in 2 in accord with the
energy gap law. The emission peak of complex 4 was red-
shifted by 100 cm-1 relative to 1 when the electron-donating
methyl group was attached. A more observable change was
evident in 5, when two methyl groups were added to the
parent phen ligand. The emission peak of 5 was blue-shifted
from 19 700 cm-1 for complex 1 to 20 300 cm-1. The
increase of the emission energy of 5 relative to 4 was
accompanied by an increase of τem in agreement with the
energy gap law. The room-temperature emission lifetimes
in the series increased proportionally to the number of
electron-donating methyl groups attached in the 5- and
6-positions of the parent ligand from 8.6 µs for 1 to 20.2
and 30.9 µs for complexes 4 and 5, respectively. Increasing
of the conjugation of the phen ligand in 6 resulted in a red-
shift of the emission peak (18 500 cm-1) and a decrease in
τem (6.2 µs). Generally, the room-temperature emission
lifetimes decreased as electron-withdrawing substituents were
attached and increased when electron-donating groups were
attached.
Four low-lying triplet excited states calculated on the basis
3
of the MLLCT geometry are listed in Table 5. Triplet
excited-state 1 (f ) 0.01) of complex 1 (Table 5) was based
3
on two d-d excitations with transition coefficients of 0.7
and 0.6, respectively. The triplet excited-state 1 of complex
2 was a 3d-d, 3LC state, while for 3 it was a 3LMLCT state.
The triplet excited-state 1 (f ) 0.00) of complex 4 was based
on a Red, 5-Me-phen f Red, CNx transition and was assigned
as a 3MLLCT state. Excited-state 1 (f ) 0.02) of complex 5
was of 5,6-Me2-phen πfπ* character. For complex 6,
excited-state 1 (f ) 0.00) was associated with a transition
between orbitals that contained complex-delocalized and php
ligand contributions. It was assigned as a CDLCT state.
Excited-state 1 of complex 3 was 3600 cm-1 higher in energy
3
than the emitting MLLCT state. This was the smallest
energy difference between the emitting and the next higher
triplet excited state, as compared to 5000 cm-1 for 1, 4500
cm-1 for 2, 4200 cm-1 for 4, 5100 cm-1 for 5, and 6900
cm-1 for 6.
By definition, the decay rate constant, k, is related to the
sum of the radiative and nonradiative decay rate, kr and knr,
respectively, (k ) kr + knr). Thus, the decrease in the emission
lifetime of 1 as compared to 4 could be attributed to the
decrease in the kr term. In complex 6, the increase of the
conjugated system size from the parent phen ligand to php
increased the nonradiative decay rate term, resulting in the
decrease of the emission lifetime.
Triplet excited-state 2 (f ) 0.00) of complex 1 located at
27 600 cm-1 was based on a phen, CNx f Red, CNx
transition with a coefficient of 1.0 and was assigned as a
3LMLCT state. Triplet excited-state 2 (f ) 0.00) of complex
3
3
2 located at 17 900 cm-1 was a d-d, LC state similar to
excited-state 1 of the same complex. Excited-state 2 (f )
3
0.01) of complex 4 was based on three transitions: 3LC, -
3
LMLCT, and d-d with transition coefficients of 0.6, 0.6,
3
The emitting MLCT state energy is essentially temper-
and 0.5, respectively, and was assigned as a 3LMLCT, 3d-d
state. Excited-state 2 of 5 was similar to state 1 of complex
4. Excited-state 2 (f ) 0.02) at 26 700 cm-1 of complex 6
ature dependent, as reported elsewhere.34b,37 This dependency
becomes more evident at low temperature when the emission
maxima of the complexes in the series are shifted to higher
energies. As a consequence of this shift, the emission spectra
3
was a MLLCT state.
The TDDFT calculations for the triplet excited states
presented in Figure 5 were performed relative to the 3MLLCT
states and did not account for spin-orbit coupling. For third-
row transition metal complexes, the treatment with spin-
orbit coupling could lower the predicted triplet-state energies
3
become structured typical for LC emission. This could be
due to the fact that at 77 K the 3MLCT state shifts to a much
(56) Makedonas, C.; Mitsopoulou, C. A.; Lahoz, F. J.; Balana, A. I. Inorg.
Chem. 2003, 42, 8853-8865.
2308 Inorganic Chemistry, Vol. 44, No. 7, 2005

Studies of a Series of Re(I) Tricarbonyl Complexes
higher energy than the 3LC state, suggesting that under these
conditions, the emission would most likely occur from a state
that has a substantial ligand-centered character. The increase
of the 3LC character of emitting 3MLCT states due to mixing
of bpy πfπ* character into the MLCT state has been
reported for fac-[Re(CO)3(bpy)(4-ethylpyridine)]+ based on
ligand with varying substituents in the 5- and 6-positions,
was synthesized and investigated using spectroscopic and
computational methods. The correlated results revealed the
following: (1) The lowest energy transitions and the corre-
sponding singlet excited states were MLLCT for complexes
1-5 and LC for complex 6. (2) The electrochemical studies
revealed that the irreversible ReII/I oxidation and reversible
diimine ligand reduction potentials increased when electron-
withdrawing groups (in 2 and 3) were attached to the parent
phenanthroline ligand and decreased when electron-donating
groups were attached (in 4, 5, and 6). (3) E1/2(red) was linearly
dependent with ELUMO and the Hammett constant σp. (4)
ELUMO and the Hammett constant σp were linearly related.
(5) The complexes were highly emissive both at room
temperature and at 77 K except 3 that was emissive only at
77 K. (6) The computational assignment of the emitting states
as 3MLLCT for 1-5 and 3LC [π(php)fπ*(phen)] for 6 was
supported by the emission curve-fitting analysis results. (7)
The room-temperature lifetimes decreased as electron-
withdrawing substituents were attached and increased when
electron-withdrawing groups were attached. (8) The 77 K
lifetimes increased in the order 1 < 2 < 4, 5 < 6 < 3. (9)
The quantum yields for the complexes were significantly
higher than the standard, [Ru(bpy)3]2+, making the complexes
potentially very good candidates for bimolecular excited-
state electron-transfer reactions in solar energy conversion.
3
B3LYP calculations.29a Emissions arising from LC states
are relatively insensitive to solvent and temperature effects,
as these transitions involve minimal redistribution of electron
density with respect to the solvent dielectric field.37
The 77 K emission lifetimes showed that when electron-
withdrawing groups were attached, the emission lifetimes
increased from 65 µs for 1 to 171 and 322 µs for 2 and 3,
respectively. The τem also increased to ∼230 µs when
electron-donating methyl groups were attached in complexes
4 and 5. The emissions lifetime of complex 6 increased to
268 µs.
The room-temperature emission lifetimes for the com-
plexes were not directly related with their emission quantum
yields (φem). All complexes except 3 had higher φem than
the 0.089 value of standard complex [Ru(bpy)3]Cl2 (Table
3) under the same experimental conditions. Complexes 1 and
2 had quantum yields of 0.77 and 0.78, respectively.
Complex 4 had a φem of 0.83, which decreased to 0.56 when
the second methyl group was added in complex 5. Complex
6 featured the lowest φem value of 0.11.
The emission maxima of [Re(CO)3(phen)(py)]+ and [Re-
(CO)3(5,6-Me2-phen)(py)]+ were located at 20 100 and
16 400 cm-1 at 77 K in the same solvent.37 Substitution of
the CNx ligand for py37 in complexes 1 and 5 caused a 1700
and 4300 cm-1 blue-shift for 1 and 5, respectively, relative
to their py analogues (at 77 K in the same solvent). This
shift was accompanied by an increase of the 77 K emission
lifetimes from 11.7 and 23 µs for the py analogues37 to 65
and 229 µs for complexes 1 and 5, respectively, in accord
with the energy gap law.
Acknowledgment. We thank the support of the National
Science Foundation under Grant No. EIA-0216178 and Grant
No. EPS-0236913, matching support from the State of
Kansas and the Wichita State University High Performance
Computing Center, the Wichita State University Office of
Research Administration, the Department of Energy, and
Parker Fellowships (J.M.V. and S.R.S.).
Supporting Information Available: The optimized geometries
(Table S1), the percent orbital contributions (Table S2), and the
calculated singlet excited-state energies of the six complexes (Table
S3). This material is available free of charge via the Internet at
http://pubs.acs.org.
Conclusions
A series of six complexes with the general formula [Re-
(CO)3(CNx)(L)](PF6), where L is a phenanthroline-based
IC048786F
Inorganic Chemistry, Vol. 44, No. 7, 2005 2309