Synthesis, structure, and photophysical characterization of blue-green luminescent zinc complexes containing 2-iminophenanthropyrrolyl ligands

Article abstract of DOI:10.1021/ic901519s

New 2-iminophenanthro[9,10-c]pyrrole ligand precursors containing phenyl or 2,6-diisopropylphenyl groups at the imine nitrogen substituent, 2-arylformiminophenanthro[9,10-c]pyrroles (aryl = phenyl Na, 2,6-diisopropylphenyl lib) were synthesized and deprot

Full text of DOI:10.1021/ic901519s

11176 Inorg. Chem. 2009, 48, 11176–11186
DOI: 10.1021/ic901519s
Synthesis, Structure, and Photophysical Characterization of Blue-Green
Luminescent Zinc Complexes Containing 2-Iminophenanthropyrrolyl Ligands
†
†
†
Clara S. B. Gomes,† Pedro T. Gomes,*,† M. Teresa Duarte, Roberto E. Di Paolo, Antonio L. Mac-anita, and
ꢀ
‡
ꢀ
Maria Jose Calhorda
†
´
´
ꢀ
ꢀ
^
Centro de Quımica Estrutural, Departamento de Engenharia Quımica e Biologica, Instituto Superior Tecnico,
‡
´
´
Av. Rovisco Pais, 1049-001 Lisboa, Portugal, and Departamento de Quımica e Bioquımica, Faculdade de Ciencias,
Universidade de Lisboa, 1749-016 Lisboa, Portugal
Received July 30, 2009
New 2-iminophenanthro[9,10-c]pyrrole ligand precursors containing phenyl or 2,6-diisopropylphenyl groups at the
imine nitrogen substituent, 2-arylformiminophenanthro[9,10-c]pyrroles (aryl = phenyl IIa, 2,6-diisopropylphenyl IIb)
were synthesized and deprotonated in situ with NaH, originating solutions of the corresponding sodium salts (IVa,
IVb). The reaction of these salts with zinc chloride gave the homoleptic bis-ligand Zn(II) complexes [Zn(κ2N,N0-2-
arylformiminophenanthro[9,10-c]pyrrolyl)2] (aryl=phenyl 2a, 2,6-diisopropylphenyl 2b). The new ligand precursors
and complexes were characterized by NMR, elemental analysis, UV/vis spectroscopy, and X-ray crystallography,
when possible. The photophysical characterization was carried out using steady-state and picosecond time-resolved
luminescence techniques in solution. The influence of the π-extended conjugation of the condensed phenanthro group
on the deprotonated iminopyrrolyl ligands coordinated to Zn2þ greatly enhances fluorescence quantum yields of the
complexes (2a, 2b) in relation to those of their ligand precursors (IIa, IIb). Complex 2a shows emission in the green
spectral region (λmax=494 nm), presenting the highest fluorescence quantum yield (φf=8.8%). In the case of the
complex 2b (φf=3.9%), the bulkiness of the 2,6-diisopropyl substituents of the arylimino group highly restricts the aryl
ring rotation toward coplanarity with the ligand framework, inducing a shift in the emission to the blue region (λmax
459 nm). The values of the radiative (kf) and radiationless rate constants (knr) show that the fluorescence quantum
yield enhancement in the complexes results from a 50-fold increase in kf values, indicating much more allowed π-π*
transitions in complexes 2a and 2b than those occurring in the ligand precursors IIa and IIb, with an essentially n-π*
character. These assignments were confirmed by density-functional theory (DFT) and time-dependent DFT (TD-DFT)
molecular orbital calculations. Simple 2-aryliminopyrrole ligand precursors (Ia, Ib) and their Zn(II) complexes (1a, 1b)
were also prepared to compare their photophysical properties with those of the corresponding 2-aryliminophenanthro-
[9,10-c]pyrrolyl compounds.
=
Introduction
lighting applications.4 In 1963, Pope reported the first
example of electroluminescence observed in an organic
semiconductor, a single crystal of anthracene.5 Despite this
publication, the commercial value of this discovery was only
realized in 1987 when Tang and VanSlyke, from Eastman
Kodak, reported the observation of electroluminescence in
aluminum tris(quinoline-8-olate), Alq3, an organic thin-film
device prepared by vapor deposition.6 Several approaches
have been described to build full color displays, and this area
In recent years, great interest in the synthesis of coordina-
tion and organometallic compounds with luminescent prop-
erties has emerged, mainly for use in display technology,
because of their potential applications in LEDs (Light-
Emitting Diodes).1-3 These compounds contain chro-
mophores that emit visible light at wavelengths depending
on the composition and structure of the material. Research in
this area has the purpose of obtaining red, green, and blue
emissive materials, to be used together and be able to produce
the full color spectrum and white emitters for the use in
(4) Borchardt, J. K. Mater. Today 2004, 7, 42–46.
(5) Pope, M.; Kallmann, H. P.; Magnante, P. J. Chem. Phys. 1963, 38,
2042–2043.
*To whom correspondence should be addressed. E-mail: pedro.t.gomes@
ist.utl.pt. Phone: þ351 218419612. Fax: þ351 218419612.
(1) Holder, E.; Langeveld, B. M. W.; Schubert, U. S. Adv. Mater. 2005, 17,
1109–1121.
(2) Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. Rev. 2006,
250, 2093–2126.
(3) Geffroy, B.; le Roy, P.; Prat, C. Polym. Int. 2006, 55, 572–582.
(6) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913–915.
(7) For example: (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-
Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.;
Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304–4312. (b) Pandya, S.;
Yu, J.; Parker, D. Dalton Trans. 2006, 2757–2766. (c) Wang, S. Coord. Chem.
Rev. 2001, 215, 79–98.
r
pubs.acs.org/IC
Published on Web 10/28/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11177
Chart 1. Homoleptic Bis(aryliminopyrrolyl) Metal Complexes: Square
Planar A (M=Ni; R=R2=H; R1=iPr.15,18 M=Ni; R=R1=R2=Me.17
reported in the 1960s.13 More recently, these ligands have
been employed in the syntheses of metal complexes that
behave as precatalysts for olefin polymerization.14
We have been interested in the chemistry of iminopyrrolyl
ligands, and recently our group and other authors have
reported the synthesis and characterization of several homo-
leptic square planar or tetrahedral complexes containing
arylimino derivatives of these ligands with metals such as
Co(II),15,16 Ni(II),15,17,18 and Zn(II)19 (Chart 1).
i
M=Ni; R=R2=H; R1=Me.18 M=Co; R=Me; R1=Pr, R2=H16a) and
tetrahedral B (M=Co; R=H; R1=iPr.16a M=Co; R=H, Me; R1=H,
Me.16a M=Zn; R=H; R1=iPr19
)
The closed-shell electronic configuration of the latter ion
(d10) turnsZn(II) by itselfspectroscopicallyinactive, and thus
the luminescent properties of its complexes can be tuned by
the modification of the corresponding ligands.9 In this paper,
wereportthe synthesisof[Zn(2-aryliminopyrrolyl)2] contain-
ing simple iminopyrrolyl ligands, such as those represented in
Chart 1 (aryl=Ph; 2,6-iPr2Ph), and the characterization of
their absorption and emission properties. We further describe
the modification of the pyrrolyl ligand backbone, to improve
its chromophore properties, by the addition of a fused
aromatic ring system, such as phenanthrene, to the pyrrole
C3-C4 bond, allowing the synthesis of new 2-iminophenan-
thropyrrolyl ligand precursors. Subsequently, we describe the
synthesis and molecular characterization of new [Zn(2-
aryliminophenanthro[9,10-c]pyrrolyl)2] and compare their
luminescence features with those of the corresponding simple
[Zn(2-aryliminopyrrolyl)2]. We have characterized the
photophysical properties of these compounds using steady-
state and time-resolved luminescence techniques in solution.
Density-functional theory (DFT) and time-dependent DFT
(TD-DFT) calculations (ADF program) were also carried
out for these new ligand precursors and for their Zn(II)
complexes to assign the electronic transitions, to determine
the geometry of the first excited state, and to try to rationalize
the luminescence behavior exhibited.
of investigation was rapidly extended to a wide range of metal
complexes, which include for example, Ir(III),7a lanthanides
such as Eu(III) and Tb(III),7b but also Al(III), Zn(II), and
B(III)7c among others. In fact, luminescent Zn(II) complexes
containing chelating ligands, such as benzothiazolates,8a
quinolinolates,8b salicylaldiminates,8c dipyridylamino,7c 7-azain-
dolyl,7c 2-(2-pyridyl)indolyl,7c and anilido-imine,9 have been
widely studied in recent years, not only for their use in LEDs
but also for application in imaging techniques10 and as
fluorescent sensors for Zn2þ in biological systems.11
Iminopyrrolyl bidentate chelating ligands, which contain
pyrrolyl anionic ring and neutral imine donor moieties, may
be considered structurally similar to salicylaldiminate, anilido-
imine or 2-(2-pyridyl)indolyl ligands and have been
employed in the synthesis of several transition-metal com-
pounds.12 The first syntheses of homoleptic metal complexes
of Co(II), Ni(II), Pd(II), Cu(II), and Zn(II) containing these
kind of ligands, although only with alkylimino groups, were
Experimental Section
General Procedures. All experiments dealing with air- and/or
moisture-sensitive materials were carried out under inert atmo-
sphere using a dual vacuum/nitrogen line and standard Schlenk
techniques. Nitrogen gas was supplied in cylinders by specia-
lized companies (e.g., Air Liquide, etc.) and purified by passage
˚
through 4 A molecular sieves. Unless otherwise stated, all
€
reagents were purchased from commercial suppliers (e.g., Acros,
Aldrich, Fluka) and used without further purification. All
solvents to be used under inert atmosphere were thoroughly
deoxygenated and dehydrated before use. They were dried and
purified by refluxing over a suitable drying agent followed by
distillation under nitrogen. The following drying agents were
used: sodium (for toluene, diethyl ether, and tetrahydrofuran
(THF)), calcium hydride (for hexane and dichloromethane).
(8) For example: (a) Yu, G.; Yin, S.; Liu, Y.; Shuai, Z.; Zhu, D. J. Am.
Chem. Soc. 2003, 125, 14816–14824. (b) Ghedini, M.; La Deda, M.; Aiello, I.;
Grisolia, A. Inorg. Chim. Acta 2004, 357, 33–40. (c) Chang, K.-H.; Huang,
C.-C.; Liu, Y.-H.; Hu, Y.-H.; Chou, P.-T.; Lin, Y.-C. Dalton Trans. 2004, 1731–
1738.
(9) Su, Q.; Gao, W.; Wu, Q.-L.; Ye, L.; Li, G.-H.; Mu, Y. Eur. J. Inorg.
Chem. 2007, 4168–4175.
(10) (a) Pascu, S. I.; Waghorn, P. A.; Conry, T. D.; Betts, H. M.;
Dilworth, J. R.; Churchill, G. C.; Pokrovska, T.; Christlieb, M.; Aigbirhio,
F. I.; Warren, J. E. Dalton Trans. 2007, 4988–4997. (b) Descalzo, A. B.; Xu,
H.-J.; Xue, Z.-L.; Hoffmann, K.; Shen, Z.; Weller, M. G.; You, X.-Z.; Rurack, K.
Org. Lett. 2008, 10, 1581–1584.
˚
Deuterated solvents were dried by storage over 4 A molecular
(15) Dawson, D. M.; Walker, D. A.; Thornton-Pett, M.; Bochmann, M.
J. Chem. Soc., Dalton Trans. 2000, 459–466.
ꢀ
(11) (a) Bereau, V. Inorg. Chem. Commun. 2004, 7, 829–833. (b) Wu, Z.;
(16) (a) Carabineiro, S. A.; Silva, L. C.; Gomes, P. T.; Pereira, L. C. J.;
Veiros, L. F.; Pascu, S. I.; Duarte, M. T.; Namorado, S.; Henriques,
R. T. Inorg. Chem. 2007, 46, 6880–6890. (b) Carabineiro, S. A.; Gomes, P.
T.; Veiros, L. F.; Freire, C.; Pereira, L. C. J.; Henriques, R. T.; Warren, J. E.; Pascu,
S. I. Dalton Trans. 2007, 46, 5460–5470. (c) Carabineiro, S. A.; Bellabarba, R.
M.; Gomes, P. T.; Pascu, S. I.; Veiros, L. F.; Freire, C.; Pereira, L. C. J.; Henriques,
R. T.; Oliveira, M. C.; Warren, J. E. Inorg. Chem. 2008, 47, 8896–8911.
(17) Bellabarba, R. M.; Gomes, P. T.; Pascu, S. I. Dalton Trans. 2003,
4431–4436.
Chen, Q.; Yang, G.; Xiao, C.; Liu, J.; Yang, S.; Ma, J. S. Sens. Actuators, B 2004,
99, 511–515. (c) Zhang, G.-Q.; Yang, G.-Q.; Zhu, L.-N.; Chen, Q.-Q.; Ma, J.-S.
Sens. Actuators, B 2006, 114, 995–1000.
(12) For a recent review see: Mashima, K.; Tsurugi, H.; J. Organomet.
Chem. 2005, 690, 4414-4423, and references cited therein.
(13) Holm, R. H.; Chakravorty, A.; Theriot, L. J. Inorg. Chem. 1966, 5,
625-635, and references cited therein.
(14) For reviews on post-metallocene polymerization catalysts see for
example: (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000,
100, 1169–1203. (b) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428–447. (c) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev.
2003, 103, 283–315.
ꢀ
ꢀ
ꢀ
(18) Perez-Puente, P.; de Jesus, E.; Flores, J. C.; Gomez-Sal, P.
J. Organomet. Chem. 2008, 693, 3902–3906.
(19) Hao, H.; Bhandari, S.; Ding, Y.; Roesky, H. W.; Magull, J.; Schmidt,
H. G.; Noltemeyer, M.; Cui, C. Eur. J. Inorg. Chem. 2002, 1060–1065.

11178 Inorganic Chemistry, Vol. 48, No. 23, 2009
Chart 2. Labeling of Ligand Atoms
Gomes et al.
Syntheses of 1-Formiminophenanthro[9,10-c]pyrrole Ligand
Precursors (IIa,b). A similar procedure used in previous pub-
lications of our group was followed.16a,17,20 Phenanthro[9,10-c]-
pyrrole-1-carboxaldehyde (10.0 mmol),25 the appropriate aniline
(10 mmol), a catalytic amount of p-toluenesulfonic acid and
MgSO4 (to remove any water from the reaction mixture) were
suspended in absolute ethanol (5 mL) in a 50 mL round-bottom
flask, fitted with a condenser and a CaCl2 guard tube. The
mixture was heated to reflux overnight turning yellow-orange.
After cooling to room temperature, CH2Cl2 was added and the
suspension was filtered through Celite and washed through with
more CH2Cl2. After removal of all volatiles, the product was
recrystallized in refluxing toluene. The solution, which was
stored at -20 ꢀC, yielded 2.24 g (70%) of a light brown solid
of IIa or 3.16 g (78%) of yellow crystals of IIb.
Data for IIa. Anal. Found (calcd) (C23H16N2): C, 86.26
(86.22); H, 5.13 (5.03); N, 8.63 (8.72). NMR [δH (400 MHz,
CDCl3)]: δ 9.18 (1H, s, NdCH), 8.56 (1H, d, J=2.4 Hz, H15 or
H12), 8.50 (1H, d, J=7.6 Hz, H18 or H9), 8.32 (1H, d, J=2.0 Hz,
H12 or H15), 8.07 (1H, d, J=6.8 Hz, H9 or H18), 7.79 (1H, s,
H5), 7.55-7.43 (6H, m, H10, H11, H16, H17 and aryl o-H),
7.33-7.25 (4H, m, aryl m-andp-H and NH);NMR[δC (100MHz,
CDCl3)]: δ 151.8 (aryl ipso-C), 149.2 (N=CH), 130.8 (C14 or
C13), 129.4 (aryl o-C), 128.4 (C13 or C14), 128.3 (C8 or C19),
128.1 (C19 or C8), 127.3 (C9 or C18), 127.2 (C18 or C9), 126.2
(C16 or C11), 125.62 (aryl p-C), 125.6 (C11 or C16), 125.4 (C17
or C10), 125.0 (C4 or C3), 124.1 (C10 or C17), 123.5 (C15 or
C12), 122.9 (C12 or C15), 122.5 (C3 or C4), 121.9 (C2), 121.0
(aryl m-C), 115.8 (C5). ESI-MS: m/z 321 [MþH]þ (calcd. for
C23H17N2 [MþH]þ, 321).
sieves and degassed by the freeze-pump-thaw method. Sol-
vents and solutions were transferred using a positive pressure of
nitrogen through stainless steel cannulae, and mixtures were
filtered in a similar way using modified cannulae that could be
fitted with glass fiber filter disks.
Nuclear magnetic resonance (NMR) spectra were recorded
on Varian Unity 300 (1H, 299.995 MHz; 13C, 75.4296 MHz),
Bruker Avance III 300 (1H, 300.130 MHz; 13C, 75.4753 MHz) or
Bruker Avance III 400 (1H, 400.132 MHz; 13C, 100.623 MHz)
spectrometers. Spectra were referenced internally using the
residual protio solvent resonance relative to tetramethylsilane
(δ=0). All chemical shifts are quoted in δ (ppm) and coupling
constants given in hertz. Multiplicities were abbreviated as
follows: broad (br), singlet (s), doublet (d), triplet (t), quartet
(q), heptet (h), and multiplet (m). For air- and/or moisture-stable
compounds, samples were dissolved in CDCl3 and prepared in
common NMR tubes. The NMR assignments of the pyrrole ring
or the phenanthro[9,10-c]pyrrole rings were made according to
the X-ray labeling (see Chart 2). For air- and/or moisture-
sensitive materials, samples were prepared in J. Young tubes in a
glovebox. Elemental analyses were obtained from the IST
elemental analysis services. Mass spectra were collected on a
500 MS LC Ion Trap (Varian, Inc., Palo Alto, CA, U.S.A.) mass
spectrometer equipped with an electrospray ionization (ESI)
source. The spray voltage was set at ( 5 kV, and the capillary
voltage was set at 10 V.
The 2-formiminopyrrole ligand precursors (see below in
Scheme 1) were prepared according to previous publications of
our group,16a,17,20 which were adapted from procedures reported
in the literature for Ia21,22 and Ib,23,24 by reaction of 2-formyl-
pyrrole with the appropriate aniline, in the presence of a
catalytic amount of p-toluenesulfonic acid (see Supporting
Information). 2-Formylphenanthro[9,10-c]pyrrole was synthe-
sized following a method described in the literature25 with some
modifications given in the Supporting Information.
Data for IIb. Anal. Found (calcd) (C29H28N2): C, 86.26
(86.10); H, 6.63 (6.98); N, 7.14 (6.92). NMR [δH (400 MHz,
CDCl3)]: δ 8.95 (1H, s, NdCH), 8.56 (1H, d, J=8.1 Hz, H15 or
H12), 8.49 (1H, d, J=7.8 Hz, H12 or H15), 8.20 (1H, d, J=
7.8 Hz, H18 or H9), 7.98 (1H, d, J = 6.9 Hz, H9 or H18),
7.52-7.43 (4H, m, H10, H11, H16 and H17), 7.40 (1H, s, H5),
7.29-7.23 (4H, m, aryl m- and p-H and NH), 3.20 (2H, hept, J=
6.9 Hz, CH(CH3)2), 1.21 (12H, d, J=6.9 Hz, CH(CH3)2); NMR
[δC (100 MHz, CDCl3)]: δ 152.4 (N=CH), 149.1 (aryl ipso-C),
139.2 (aryl o-C), 130.7 (C14 or C13), 129.6 (C13 or C14), 128.4
(C19 or C8), 128.3 (C8 or C19), 128.2 (C3 or C4), 128.1 (C4 or
C19), 127.4 (C17 or C10), 127.2 (C10 or C17), 126.2 (C16 or
C11), 125.5 (C11 or C16), 125.2 (C18 or C9), 124.7 (aryl p-C),
124.1 (C15 or C12), 123.9 (C12 or C15), 123.5 (aryl m-C), 122.8
(C9 or C18), 122.2 (C2), 116.5 (C5), 28.1 (CH(CH3)2), 23.7
(CH(CH3)2). ESI-MS: m/z 405 [MþH]þ (calcd. for C29H29N2
[MþH]þ, 405).
In Situ Synthesis of the Sodium Salts of Compounds Ia,b and
IIa,b (IIIa,b and IVa,b). The same procedure followed in pre-
vious publications of the group was used:16a,17,20 In a typical
experiment, NaH (24 mg, 1.0 mmol; or 13 mg, 0.5 mmol) was
suspended in THF, and 1 mmol (or 0.5 mmol) of a neutral ligand
precursor Ia,b (or IIa,b) was slowly added as a solid under a
counterflow of nitrogen. An immediate evolution of hydrogen
occurred, and after some minutes, a solution of the sodium
ligand salt was obtained and stirred for 90 min.
(20) Gomes, C. S. B.; Suresh, D.; Gomes, P. T.; Veiros, L. F.; Duarte,
M. T.; Nunes, T. G.; Oliveira, M. C., Dalton Trans., in press, 2009; DOI:
10.1039/B905948B.
(21) Yeh, K.-N.; Barker, R. H. Inorg. Chem. 1967, 6, 830–833.
(22) Yoshida, Y.; Matsui, S.; Takagi, Y.; Mitani, M.; Nakano, T.;
Tanaka, H.; Kashiwa, N.; Fujita, T. Organometallics 2001, 20, 4793–4799.
(23) (a) Li, Y.-S.; Li, Y.-R.; Li, X.-F. J. Organomet. Chem. 2003, 667, 185–
191. (b) Antonelli, D. M.; Gomes, P. T.; Green, M. L. H.; Martins, A. M.;
Synthesis of Bis(pyrrole-1-ylmethylene-phenylamine)zinc (1a).
Zinc chloride (0.068 g, 0.5 mmol) was suspended in THF and
cooled to -78 ꢀC. A solution of sodium salt IIIa prepared in situ
(0.17 g, 1.0 mmol) was filtered and directly added dropwise to
the ZnCl2 suspension, which was then stirred for 1 h. The
mixture was allowed to warm to room temperature and further
stirred overnight. All volatiles were evaporated and the resulting
residue was washed with n-hexane and extracted with diethyl
ether until extracts were colorless. The resulting orange-yellow
solution was concentrated and cooled to -20 ꢀC, yielding 0.16 g
(80%) of a yellow-orange powder.
ꢀ
Mountford, P. J. Chem. Soc., Dalton Trans. 1997, 2435–2444. (c) Jimenez-
Tenorio, M.; Puerta, M. C.; Salcedo, I.; Valerga, P.; Costa, S. I.; Gomes, P. T.;
Mereiter, K. Chem. Commun. 2003, 1168–1169. (d) Liu, H.-R.; Costa, S. I.;
Gomes, P. T.; Duarte, M. T.; Branquinho, R.; Fernandes, A. C.; Chien, J. C. W.;
Marques, M. M. J. Organomet. Chem. 2005, 690, 1314–1323.
(24) Iverson, C. N.; Carter, C. A. S. G.; Baker, R. T.; Scollard, J. D.;
Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2003, 125, 12674–12675.
(25) (a) Lash, T. D.; Chandrasekar, P.; Osuma, A. T.; Chaney, S. T.;
Spence, J. D. J. Org. Chem. 1998, 63, 8455–8469. (b) Novak, B. H.; Lash, T. D.
J. Org. Chem. 1998, 63, 3998–4010. (c) Lash, T. D.; Bellettini, J. R.; Bastian,
J. A.; Couch, K. B. Synthesis 1994, 170–172.
Anal. found (calculated) (C22H18N4Zn): C 65.38 (65.44), H
4.79 (4.49), N 13.66 (13.88)%. NMR [δH (300 MHz, C6D6)]: δ

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11179
8.20 (2H, s, NdCH), 7.42-7.40 (2H, m, H5), 7.28-7.26 (2H, m, H3),
7.18-7.10 (4H, m, aryl o-H), 7.03-7.01 (2H, m, H4), 6.87-6.285
(6H, m, aryl m- and p-H); NMR [δH (300 MHz, C6D6)]: δ 154.1
(N=CH), 147.0 (aryl ipso-C), 138.6 (C5), 138.1 (C2), 129.6 (aryl m-
C), 125.6 (aryl p-C), 121.6 (C3), 120.5 (aryl o-C), 115.6 (C4). ESI-
MS: m/z 403 [MþH]þ (calcd. for C22H19N4Zn [MþH]þ, 403).
Synthesis of Bis(pyrrole-1-ylmethylene-(2,6-diisopropylphe-
nyl)amine)zinc (1b). This compound was already reported in
the literature, although synthesized by a different method.19 The
same procedure described for 1a was followed, using a solution
of sodium salt IIIb prepared in situ (0.25 g, 1.0 mmol), to give
0.13 g (44%) of a pale yellow powder.
Anal. found (calculated) (C34H42N4Zn): C 71.29 (71.38), H
7.49 (7.40), N 9.96 (9.79)%. NMR [δH (300 MHz, C6D6)]: δ 7.49
(2H, s, NdCH), 7.12 (2H, br s, H5), 6.98-6.92 (6H, m, aryl m-
and p-H), 6.84 (2H, dd, J1=3.6 Hz, J2=1.2 Hz, H3), 6.55 (2H,
dd, J1 = 3.6 Hz, J2 = 1.5 Hz, H4), 3.13 (4H, h, J = 6.6 Hz,
CH(CH3)2), 1.06 (12H, d, J=6.6 Hz, CH(CH3)2), 0.60 (12H, d,
J=6.6 Hz, CH(CH3)2). NMR [δC (75 MHz, C6D6)]: δ 161.3
(N=CH), 144.9 (aryl ipso-C), 141.9 (aryl o-C), 137.6 (C2), 136.6
(aryl p-C), 126.2 (C5), 124.0 (aryl m-C), 120.7 (C3), 115.1 (C4),
28.4 (CH(CH3)2), 24.6 (CH(CH3)2), 23.0 (CH(CH3)2). ESI-MS:
m/z 571 [MþH]þ (calcd. for C34H43N4Zn [MþH]þ, 571).
Synthesis of Bis(phenanthro[9,10-c]pyrrole-1-ylmethylene-phe-
nylamine)zinc (2a). Procedure A. The same procedure de-
scribed for 1a was followed, but a solution of sodium salt IVa
prepared in situ (0.16 g, 0.5 mmol) was employed. Extraction of
the reaction products with toluene, until extracts remained
colorless, and removal of the solvent by vacuum evaporation,
afforded a yellow-mustard powder, which was recrystallized in
CH2Cl2 and double-layered with n-hexane, at -20 ꢀC, giving
0.88 g (50%) of a bright green powder.
Table 1. Crystal Data and Structure Refinement for Compounds IIb and 2b
IIb
2b
formula
M
λ/ A
T/ K
C
29H28N2
C58H54N4Zn
872.42
0.71073
150
404.53
0.71073
150
˚
crystal system
space group
monoclinic
P21/c
12.6922 (12)
20.6575 (21)
8.8788 (9)
90
108.119 (6)
90
2212.5 (4)
4
1.214
monoclinic
Cc
16.105(8)
12.486(7)
24.603(13)
90
108.52(3)
90
4691(4)
4
1.235
˚
a/ A
˚
b/ A
˚
c/ A
R/ deg
β/ deg
γ/ deg
3
˚
V/ A
Z
F
calc/ g cm-3
μ/ mm-1
max/ deg
0.071
25.74
0.566
25.03
θ
total data
unique data
Rint
R [I > 3σ(I)]
wR
goodness of fit
F min, F max
30099
4179
0.1099
0.0628
0.1484
1.013
37322
7901
0.3488
0.0898
0.1710
0.835
-0.407
0.519
-0.360
0.441
Procedure A. A solution of sodium salt IVb prepared in situ
(0.20 g, 0.5 mmol) was employed. After extraction of the
reaction products with toluene, the resulting solution was con-
centrated and stored at -20 ꢀC, giving 0.15 g (70%) of a bright
yellow powder.
Procedure B. A solution of 1-formiminophenanthro[9,10-c]-
pyrrole IIa (0.32 g, 1.0 mmol) in a mixture of solvents, toluene
(25 mL), and THF (10 mL), was added to a solution of ZnMe2
(0.5 mL of a 2.0 M solution in toluene, 1.0 mmol) diluted with
toluene (20 mL), at -80 ꢀC. After the addition, the mixture was
warmed slowly to room temperature and stirred overnight. All
volatiles were evaporated, and the resulting residue was washed
with n-hexane and extracted with toluene until the extracts were
colorless. The remaining residue was extracted with THF and
filtered. The resulting yellow-greenish solutions were evapo-
rated, and the residue redissolved in toluene, double-layered
with n-hexane, and cooled to -20 ꢀC, to yield 0.34 g (95% based
on IIa) of a yellow-green powder.
Procedure B. A solution of 1-formiminophenanthro[9,10-c]-
pyrrole IIb (0.20 g, 0.5 mmol) in a mixture of solvents, toluene
(10 mL), and THF (5 mL) was added to a solution of ZnMe2
(0.25 mL of a 2.0 M solution in toluene, 0.5 mmol) diluted with
toluene (10 mL), at -80 ꢀC. After the addition, the mixture was
warmed slowly to room temperature and stirred overnight. All
volatiles were evaporated, and the resulting residue was washed
with n-hexane and extracted with toluene. The resulting bright
yellow solution was evaporated, redissolved in toluene, double-
layered with n-hexane and cooled to -20 ꢀC, to yield 0.19 g (89%
based on IIb) of a yellow powder.
Anal. Found (calcd) (C58H54N4Zn): C 79.63 (79.85), H 6.55
(6.24), N 6.13 (6.42). NMR [δH (300 MHz, CD2Cl2)]: δ 8.80 (2H,
s, NdCH), 8.66 (2H, d, J=7.8 Hz, H15 or H12), 8.58 (2H, d, J=
7.8 Hz, H12 or H15), 8.27 (2H, d, J = 8.4 Hz, H18 or H9),
8.21-8.19 (4H, m, H9 or H18 and H16 or H11), 7.58-7.48 (8H,
m, H10, H11 or H16, H17 and aryl p-H), 7.16-7.06 (6H, m, H5
and aryl m-H), 3.23 (4H, br s, CH(CH3)2), 1.20 (12H, d, J=6.6 Hz,
CH(CH3)2), 0.52 (12H, d, J=6.3 Hz, CH(CH3)2); NMR [δH
(300 MHz, C6D6)]: δ 8.85 (2H, s, NdCH), 8.46 (2H, d, J =
8.0 Hz, H15 or H12), 8.43 (2H, d, J=8.5 Hz, H12 or H15), 8.24
(2H, d, J=7.8 Hz, H18 or H9), 8.14 (2H, d, J=7.6 Hz, H9 or
H18), 8.04 (2H, s, H5), 7.44 (2H, t, J=7.1 Hz, H16 or H11),
7.38-7.27 (6H, m, H10, H11 or H16 and H17), 7.05-6.95 (6H,
m, aryl m- and p-H), 3.28 (4H, br s, CH(CH3)2), 1.15 (24H, d, J=
6.8 Hz, CH(CH3)2). NMR [δC (75 MHz, CD2Cl2)]: δ 161.0
(N=CH), 145.0 (aryl ipso-C), 142.4 (aryl o-C), 132.1 (C9 and
C18), 131.2 (C13 or C14), 130.8 (C14 or C13), 129.2 (C8 or C19),
128.6 (C19 or C8), 128.1 (C4 or C3), 127.6 (C17 or C10), 127.4
(C10 or C17), 127.1 (C3 or C4), 127.0 (C2), 126.5 (C16 or C11),
126.3 (C11 or C16), 125.1 (aryl p-C), 124.5 (C15 and C12), 124.4
(aryl m-C), 123.8 (C12 or C15), 123.3 (C5), 28.7 (CH(CH3)2),
24.8 (CH(CH3)2), 23.0 (CH(CH3)2). ESI-MS: m/z 871 [MþH]þ
(calcd. for C58H55N4Zn [MþH]þ, 871).
Anal. Found (calcd) (C46H30N4Zn): C 78.92 (78.46), H 4.02
(4.29), N 8.12 (7.96). NMR [δH (300 MHz, CD2Cl2)]: δ 9.47 (2H, s,
NdCH), 8.68 (2H, d, J=8.1 Hz, H15 or H12), 8.58 (2H, d, J=
8.1 Hz, H18 or H9), 8.53 (2H, d, J=8.4 Hz, H12 or H15), 8.23
(2H, s, H5), 8.15 (2H, d, J=6.6 Hz, H9 or H18), 7.67-7.49 (8H,
m, H10, H11, H16 and H17), 7.33-7.24 (6H, m, aryl m- and
p-H), 7.10-7.08 (4H, m, aryl o-H); NMR [δH (300 MHz, C6D6)]:
δ 9.14 (2H, s, NdCH), 8.54 (2H, dd, J=6.9 Hz, J=2.4 Hz, H15
or H12), 8.47 (2H, dd, J=8.4 Hz, J=1.2 Hz, H18 or H9), 8.36
(2H, dd, J=6.3 Hz, J=2.4 Hz, H12 or H15), 8.08 (2H, dd, J=
7.5 Hz, J=1.5 Hz, H9 or H18), 7.84 (2H, s, H5), 7.48-7.35 (8H,
m, H10, H11, H16 and H17), 6.97-6.88 (6H, m, aryl m- and
p-H), 6.80-6.75 (4H, m, aryl o-H). NMR [δC (75 MHz,
CD2Cl2)]: δ 160.7 (aryl ipso-C), 152.4 (N=CH), 139.2 (aryl o-
C), 133.6 (C14 or C13), 130.8 (C13 or C14), 128.6 (C8 or C19),
128.4 (C19 or C8), 127.8 (C9 or C18), 127.6 (C18 or C9), 126.6
(C16 or C11), 126.2 (C4 or C3), 126.0 (C11 or C16), 125.7 (C17
or C10), 125.0 (C4 or C3), 124.7 (aryl p-C), 124.4 (C10 or C17),
123.9 (C15 or C12), 123.6 (aryl m-C), 123.3 (C12 or C15), 122.7
(C2), 116.2 (C5). ESI-MS: m/z 703 [MþH]þ (calcd. for
C46H31N4Zn [MþH]þ, 703).
Synthesis of Bis(phenanthro[9,10-c]pyrrole-1-ylmethylene-(2,6-
diisopropylphenyl)amine)zinc (2b). The same procedures de-
scribed for 2a were followed.
X-ray Experimental Data. Crystallographic and experimental
details of crystal structure determinations are listed in Table 1.
The crystal of complex 2b was selected under an inert atmosphere,

11180 Inorganic Chemistry, Vol. 48, No. 23, 2009
Gomes et al.
covered with polyfluoroether oil, and mounted on a nylon loop.
Crystallographic data for the ligand precursor IIb and complex 2b
were collected using graphite monochromated Mo KR radiation
Computational Details. DFT calculations34 were performed
using the Amsterdam Density Functional (ADF) pro-
gram package.35 Gradient corrected geometry optimizations,36
without symmetry constraints, were performed using the
local density approximation of the correlation energy
(Vosko-Wilk-Nusair),37 and the generalized gradient approxi-
mation (Becke’s exchange38 and Perdew’s correlation
functionals39). The core orbitals were frozen for Zn ([1-2]s,
2p); C and N (1s). Triple ζ Slater-type orbitals (STO) were used
to describe the valence shells of C, N (2s and 2p), and Zn (3d, 4s).
A set of two polarization functions was added to C, N (single ζ,
3d, 4f), and Zn (single ζ, 4p, 4f). Triple ζ Slater-type orbitals
(STO) were used to describe the valence shells of H (1s) with two
polarization function (single ζ, 2s, 2p). Relativistic effects were
treated with the ZORA approximation.40 TD-DFT calcula-
tions41 with the ADF implementation were used to determine
the excitation energies. The 10 lowest singlet-singlet excitation
energies for L1 and L2 and the 40 lowest singlet-singlet excita-
tion energies for C1 and C2 were calculated, using the optimized
geometries. Unrestricted calculations were performed for the
first excited singlet state structure, obtained from a HOMO (H)
to LUMO (L) excitation, which was fully optimized without
symmetry constraints. The solvent effects were included using
the COnductor like Screening Model (COSMO)42,43 implemen-
ted in ADF on the gas phase optimized structures. Three-
dimensional representations of the orbitals were obtained with
Molekel,44 and structures and electronic spectra with Chem-
craft.45
˚
(λ=0.71073 A) on a Bruker AXS-KAPPA APEX II diffracto-
meter equipped with an Oxford Cryosystems open-flow nitrogen
cryostat, at 150 K. Cell parameters were retrieved using Bruker
SMART software and refined using Bruker SAINT on all observed
reflections. Absorption corrections were applied using SADABS.26
Structure solution and refinement were performed using direct
methods with the programs SIR9727 and SHELXL28 both in-
cluded in the package of programs WINGX-Version 1.70.01.29
For compound 2b, the poor diffracting power, size, and crystal
quality (Rint=0.3488) prevented the anisotropic refinement of all
the non-hydrogen atoms. Only Zn and the N atoms were refined
with anisotropic displacement parameters. Because of the number
of atoms present in the asymmetric unit, the anisotropic refinement
would lead to a very poor ratio of number of refined parameters/
number of reflections that could prevent a good, stable, and
reliable refinement. When analyzing the diffraction data, a very
low value of |E2 - 1|=0.541 indicated the presence of twinning.
Using the program TwinRotMat routine included in PLATON,30
we determined a possible twin matrix, indicating general and
racemic twinning. The TWIN law used was -1 0 0 0 -1 0 1 0 1
and the BASF components refined equally to 0.17. All hydrogen
atoms, except the NH proton in IIb, were inserted in idealized
positions and allowed to refine riding on the parent carbon atom.
Figures were generated using ORTEP3.31 Data was deposited
in CCDC under the deposit numbers 742158 for IIb and 742157
for 2b.
Photophysical Characterization. Solution samples were pre-
pared in a glovebox by dissolving the complexes (or the ligand
precursors) in freshly distilled THF (dried over sodium under
nitrogen). Their concentrations (c) were: [Ia]=5.2 ꢀ 10-6 M,
[Ib]=5.4ꢀ10-6 M, [1a]=9.9ꢀ10-6 M, and [1b]=7.1ꢀ10-6 M;
[IIa]=9.5ꢀ10-6 M, [IIb]=5.6ꢀ10-6 M, [2a]=3.4ꢀ10-6 M, and
[2b]=9.4ꢀ10-7 M. The solutions were kept in quartz cells, under
inert atmosphere. Absorption and fluorescence spectra were
recorded with a Beckman DU-70 spectrophotometer and a
SPEX Fluorolog 212I fluorimeter, respectively. The fluores-
cence spectra were corrected for the wavelength response of the
instrumental system. Fluorescence quantum yields (φf) were
measured using R-oligothiophenes as standards.32 Fluorescence
decays were measured using the time-correlated single-photon
counting (TCSPC) technique with picoseconds resolution
(excitation pulse fwhm=19 ps). The experimental setup employed
is described elsewhere.33
Results and Discussion
Synthesis of Complexes. The formiminopyrrole ligand
precursors Ia,b and IIa,b herein reported (Scheme 1) were
prepared and characterized according to a method described
in previous publications.16a,17,20 It consists in the conden-
sation of 2-formylpyrrole or 2-formylphenanthro[9,10-c]-
pyrrole with two types of arylamines (aniline or 2,6-
diisopropylaniline), employing standard conditions.15-17,21-24
Despite that 2-formylpyrrole is available commercially,
its preparation from pyrrole, dimethylformamide (DMF),
and phosphorus oxychloride is a straightforward reac-
tion, known as the Vilsmeier-Haak acylation.46 This
formylation reaction is also employed in the last step of
the preparation of 2-formylphenanthro[9,10-c]pyrrole
from phenanthro[9,10-c]pyrrole. However, the prepara-
(26) Sheldrick, G. M. SADABS, Program for Empirical Absorption
(36) (a) Versluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322–328. (b) Fan,
L. Y.; Ziegler, T. J. Chem. Phys. 1991, 95, 7401–7408.
(37) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(38) Becke, A. D. J. Chem. Phys. 1987, 88, 1053–1062.
(39) (a) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (b) Perdew, J. P.
Phys. Rev. B 1986, 34, 7406.
€
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Correction; University of Gottingen: Gottingen, Germany, 1996.
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Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R.
J. Appl. Crystallogr. 1999, 32, 115-119.
(28) Sheldrick, G. M. SHELX97 - Programs for Crystal Structure
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Analysis (Release 97-2); Instit€ut f€ur Anorganische Chemie der Universitat:
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Tammanstrasse 4, D-3400 Gottingen, Germany, 1998.
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J. G.; Baerends, E. J. J. Phys. Chem. A 1999, 103, 6835–6844. (b) Rosa, A.;
Baerends, E. J.; van Gisbergen, S. J. A.; van Lenthe, E.; Groeneveld, J. A.;
Snijders, J. G. J. Am. Chem. Soc. 1999, 121, 10356–10365. (c) van Gisbergen,
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(29) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.
(30) (a) Spek, A. L. Acta Crystallogr., Sect. A 1990, C34, 46. (b) PLATON, A
Multipurpose Crystallographic Tool; Utrecht University: Utrecht, The Netherlands,
Spek, A. L., 1998.
(31) ORTEP3 for Windows; Farrugia, L. J. J. Appl. Crystallogr. 1997, 30,
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Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11181
Scheme 1. Syntheses of the Ligand Precursors
Scheme 2. Synthetic Routes Used in the Present Work for the Synth-
eses of Zinc Complexesa
a Route B was only employed for phenanthro[9,10-c]iminopyrrolyl
derivatives. Sodium derivatives were generated in situ.
tion of the latter compound consisted of a sequence of
several steps starting from phenanthrene, in a synthetic
sequence slightly adapted from the literature25 (see Sup-
porting Information). The corresponding formimines
were obtained as powder or crystalline materials, with
colors ranging from pale yellow to light brown and yields
varying from 70 to 78%. These ligand precursors were
Compound 1b has already been described in the
literature by Roesky et al.,19 being prepared by reaction
of an excess of ZnMe2 with the protonated ligand
precursor Ib, in a toluene/n-hexane solvent mixture.
However, a low yield was reported (20%) in compar-
ison with that of the present reaction of ZnCl2 with IIIb
(44%). In this work, complexes 2a,b were also synthe-
sized via the ZnMe2 method (Scheme 2, route B). The
use of a toluene/THF solvent mixture (instead of to-
luene/n-hexane) led to the improved syntheses of
formiminophenanthro[9,10-c]pyrrolyl zinc complexes
2a and 2b, which were obtained in very high yields
(95 and 89%, respectively).
X-ray Diffraction. For compounds IIb and 2b, crystals
suitable for X-ray diffraction were obtained, allowing the
determination of their crystal structures. Compound IIb
crystallizes in the space group P21/c. The molecular
structure of the ligand precursor IIb is represented in
Figure 1, and selected bond distances and angles are listed
in Table 2. The asymmetric unit of compound IIb con-
tains a single molecule.
1
characterized by H and 13C{1H} NMR spectroscopy.
The crystal structure of ligand precursor IIb was deter-
mined (see X-ray Diffraction Section).
Treatment of the ligand precursors Ia,b and IIa,b with
sodium hydride, in THF, resulted in the deprotonation of
the NH proton of the pyrrole ring and consequent
formation of either the pyrrolyl or the phenanthro[9,10-c]-
pyrrolyl sodium salts, respectively, IIIa,b and IVa,b.
These sodium salts were prepared in situ and subsequently
reacted with the corresponding metal halides.
Slow addition of a THF solution of salts IIIa,b or IVa,b
prepared in situ to a suspension of ZnCl2 inthesamesolvent,
in a molar ratio of 2:1 (ligand precursor:ZnCl2), gave rise to
complexes 1a,b and 2a,b (Scheme 2, route A). The zinc
complexes formed in the reaction were extracted with
diethyl ether, in the case of the simple iminopyrrolyl ligand
derivatives, or with toluene, for the iminophenanthro-
[9,10-c]pyrrolyl ones. Further workup yielded powders with
colors orange-yellow (1a), pale yellow (1b), bright green (2a)
and bright yellow (2b), in moderate to high yields (80, 44, 50
and 70%, respectively). Deprotonation of the ligand pre-
cursors with n-LiBu followed by reaction with ZnCl2 was
also attempted, but lower yields than those produced with
NaH were obtained. All compounds showed to be sensitive
to air and moisture, either in solid or in solution. This
instability was also observed in the case of the related zinc
complexes prepared by Wang et al.47
For compound IIb, the phenanthro[9,10-c]pyrrole sys-
tem shows a planar backbone, where the phenanthrene
moiety displays the usual delocalized π system and
the longest and shortest bonds in the pyrrole ring are
˚
˚
C(3)-C(4) (1.417(4) A) and N(1)-C(5) (1.350(3) A),
respectively, as can be seen in Table 2. The angle centered
at N(1) is 111.3(2)ꢀ, while that at N(2) is 118.3(2)ꢀ. The
˚
imine N(2)-C(6) exhibits a distance of 1.276(3) A, which
is typical for this type of groups.16,17,20 The coplanarity of
the phenanthro[9,10-c]pyrrole backbone with the for-
mimino group (torsion angle N(2)-C(6)-C(2)-N(1)
˚
of 3.4(4)ꢀ) and a distance C(2)-C(6) (1.429(4) A)
shorter than normal values for typical C-C single bond,
points to an extension of the pyrrole ring π-electronic
ꢀ
(47) Liu, S.-F.; Wu, Q.; Schmider, H. L.; Aziz, H.; Hu, N.-X.; Popovic, Z.;
Wang, S. J. Am. Chem. Soc. 2000, 122, 3671–3678.

11182 Inorganic Chemistry, Vol. 48, No. 23, 2009
Gomes et al.
Figure 1. ORTEP III diagram of the ligand precursor IIb using 50%
probability level ellipsoids. Calculated hydrogen atoms have beenomitted
for clarity.
Figure 2. Crystal packing structure of ligand precursor IIb.
(see Supporting Information, Figure S1 and Table S1).
However, only the Zn and the N atoms were refined with
anisotropic displacement parameters because of the poor
quality of the crystal (see Experimental Section).
˚
Table 2. Selected Bond Distances (A) and Angles (deg) for the Ligand
Precursor IIb
parameter
IIb
Photophysical Characterization. The 2-aryliminopyr-
role ligand precursors Ia and Ib (emission bands at λmax
=
˚
Distances (A)
357 and 322 nm, respectively) show negligible fluorescence
quantum yields (e0.07%) in THF. The coordination of the
resulting 2-aryliminopyrrolyl ligands in complexes 1a and
1b exhibit slightly higher fluorescence quantum yields (φf) in
relation to Ia and Ib, the corresponding values (φf=0.26 and
0.16%) being close to those obtained by Ma et al.11b for a
related iminopyrrolyl Zn system studied for applications as
Zn2þ sensor. Absorption and fluorescence spectra, as well as
photophysical data for Ia,b and 1a,b are shown in the
Supporting Information, Figure S2 and Table S2.
Absorption and emission spectra of the 2-arylimino-
phenanthro[9,10-c]pyrrole ligand precursors (IIa and IIb)
and their corresponding Zn complexes (2a and 2b)
are shown in Figure 3, and their photophysical pro-
perties such as the absorption and emission maxima
wavelengths (λabs and λem, respectively), molar extinc-
tion coefficient (εmax), fluorescence quantum yields and
average fluorescence lifetimes (Æτfæ) are listed in Table 3.
Radiative kf and radiationless knr rate constants derived
from fluorescence quantum yields and lifetimes are also
shown.
The absorption spectra of compounds of the type a
(aryl=phenyl) are substantially red-shifted (ca. 15 and
40 nm, for ligand precursors and complexes, respectively)
with respect to those of the type b (aryl=2,6-diisopropyl-
phenyl), indicating extension of the delocalized π-orbitals
to the phenyl group in compounds a. This delocalization
occurs to a lesser extent in compounds b because of the
expectable larger dihedral angle of the 2,6-diisopropyl-
phenyl ring in relation to the iminopyrrole/yl backbone,
resulting from the high steric hindrance of the isopropyl
groups. This effect has already been observed in the
molecular structures of the simple iminopyrrolyl ligand
precursors Ia and Ib.20
N(1)-C(2)
1.373(3)
1.350(3)
1.412(4)
1.417(4)
1.381(4)
1.429(4)
1.276(3)
1.427(3)
N(1)-C(5)
C(3)-C(2)
C(3)-C(4)
C(5)-C(4)
C(6)-C(2)
N(2)-C(6)
N(2)-C(7)
Angles (deg)
C(6)-N(2)-C(7)
N(1)-C(2)-C(6)
N(2)-C(6)-C(2)
C(3)-C(2)-C(6)
N(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(5)-C(4)-C(3)
N(1)-C(5)-C(4)
C(5)-N(1)-C(2)
118.3(2)
118.5(2)
121.8(3)
134.9(2)
106.5(2)
106.5(2)
108.1(2)
107.6(2)
111.3(2)
delocalization toward its formimino substituent. In this
compound, the steric hindrance produced by the two
bulky isopropyl substituents, at the 2 and 6 positions of
the phenyl ring, makes it nearly perpendicular (80.50ꢀ) to
the formiminophenanthro[9,10-c]pyrrole plane defined
by atoms N(2)-C(6)-C(2)-N(1). The supramolecular
arrangement shows dimerization of the iminopyrrole
molecules, which are coplanar and connected by comple-
mentary hydrogen bonds between the imino group and
the pyrrole NH of the adjacent molecule.48 One can also
observe π-stacking of the phenanthrene rings of adjacent
dimers (Figure 2).
Complex 2b crystallized in the monoclinic Cc space
group. Its X-ray molecular structure reveals a distorted
tetrahedral geometry about the Zn atom with the four
nitrogen atoms of two chelating 2-formiminophenan-
thro[9,10-c]pyrrolyl ligands arranged at the vertices
Another interesting observation is the red-shift of the
absorption spectra of complexes 2a and 2b (65 and 36 nm,
respectively) relative to those of the corresponding ligand
precursors (Table 3). These large shifts suggest an
(48) Munro, O. Q.; Joubert, S. D.; Grimmer, C. D. Chem.;Eur. J. 2006,
12, 7987–7999.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11183
Figure 3. Normalized (a) absorption and (b) fluorescence spectra of ligand precursors IIa,b and complexes 2a,b (for each compound, obtained by exciting
at its absorption maximum wavelength), in THF, at 293 ꢀC. The ligand precursors fluorescence spectra are magnified 50-fold for better visibility.
Table 3. Photophysical Parameters of Zn Complexes 2a and 2b and of Their Corresponding Neutral Ligand Precursors IIa and IIb, in THF, at 293 K
εmax ꢀ 10-4
kf ꢀ 10-7 c
knr ꢀ 10-9 d
compound
λabs (nm)
(L mol-1 cm-1
)
λem (nm)
φf (%)
Æτfæa (ps)
τradb (ns)
(s-1
)
(s-1
)
IIa
IIb
2a
383
369
448
405
2.9
1.6
2.0
4.2
432
419
494
459
0.16
0.23
8.8
425
532
435
180
263
233
4.8
4.8
0.38
0.43
21
2.4
1.9
2.2
5.2
2b
3.9
21
P
a Æτfæ= ai τi. b τrad=1/kf. c kf=φf/Æτfæ. d knr=(1 - φf)/Æτfæ.
enhancement of the π-conjugation due to the bidentate
complexation of the ligand to the d10 spectroscopically
inactive Zn2þ ion. The bidentate coordination imposes a
planar conformation in the iminopyrrolyl backbone by
restricting the rotation about the C2-C6 bond.
The values of the radiative kf and radiationless rate
constants knr (Table 3) show that the fluorescence quan-
tum yields enhancement in the complexes results from a
50-fold increase in the radiative rate constant values
(knr values are similar for all four compounds). This
indicates a much more allowed transition (larger transi-
tion dipole moment) from the first singlet excited state
(S1) to the ground state (S0) in the case of the complexes.
In fact, the kf values of the complexes are typical
of allowed transitions, and are consistent with the order
of magnitude of their εmax values (also proportional to the
transition dipole moment), while those of the ligand
precursors are typical of forbidden transitions. The εmax
values of both complexes and ligands are of the same
order of magnitude, which strongly indicates that the
S1 f S0 transition, in the case of the ligand precursors,
does not correspond to the main absorption band (where
εmax was measured). In fact, the absorption spectra of
the ligand precursors show long wavelength tails (e.g.,
The fluorescence spectra of complexes 2a and 2b show
red-shifts (62 and 40 nm, respectively) relative to the
ligand precursors, similar to those observed in the absorp-
tion spectra. As in the case of absorption, these red shifts
may be attributed to the highly resonant π-conjugated
framework of the deprotonated iminophenanthropyrrole
ligand precursor, giving rise to a decrease in the π*-π
energy gap of the ligand molecular orbitals responsible
for the transitions.49,50 More importantly, a substantial
increase (ca. 60 and 20-fold, respectively) in the fluores-
cence quantum yields, φf, is observed. This is in accor-
dance with the results obtained by other authors for the
spectroscopic properties observed with other zinc com-
plexes and corresponding ligand precursors.47,49,51
Fluorescence decays of complexes 2a and 2b were
measured at two emission wavelengths of the onset and
tail of the fluorescence bands (see Supporting Informa-
tion, Figure S4). The decay times were well fitted with
double exponential functions, being the average fluores-
cence lifetimes listed in Table 3. The decays of the ligand
precursors IIa and IIb (see Supporting Information,
Figure S3), measured at a single emission wavelength,
were also obtained as multiexponentials with average
fluorescence lifetimes indicated in Table 3.
ε
459nm = 580 L mol-1 cm-1 and ε403nm = 832 L mol-1
cm-1, for IIa and IIb, respectively) that are absent in
the complexes. These tails are due to the presence of
transitions with strong n-π* character resulting from
the excitation of a nitrogen lone pair electron to a
delocalized π* orbital of the aromatic system (see below
in Molecular Orbital Calculations). Such transitions are
shifted to very high energies since the nitrogen non-
bonding orbitals become bonding upon coordination to
the metal, thus increasing the difference between
the energies of the π* and the energies of the former
n orbitals.
Molecular Orbitals Calculations. DFT calculations34
using the ADF program35 were performed for the ligands
IIa and IIb, and for the two complexes 2a and 2b. The
geometries were fully optimized, and the agreement be-
tween the calculated distances (see below) and angles for
IIb and 2b with the corresponding values obtained from
(49) Su, Q.; Gao, W.; Wu, Q.-L.; Ye, L.; Li, G.-H.; Mu, Y. Eur. J. Inorg.
Chem. 2007, 4268–4175.
(50) (a) Yang, W.; Schmider, H.; Wu, Q.; Zhang, Y.-S.; Wang, S. Inorg.
Chem. 2000, 39, 2397–2404. (b) Wu, Q.; Lavigne, J. A.; Tao, Y.; D'Iorio, M.;
Wang, S. Inorg. Chem. 2000, 39, 5248–5254.
(51) Ghedini, M.; La Deda, M.; Aiello, I.; Grisolia, A. J. Chem. Soc.,
Dalton Trans. 2002, 3406–3409.

11184 Inorganic Chemistry, Vol. 48, No. 23, 2009
Gomes et al.
Table 4. Composition, Energy (eV), Calculated and Experimental Absorption Wavelengths (nm), and Oscillator Strength (OS) of the More Relevant Electronic Transitions
of Ligand Precursors IIa and IIb and of Zn Complexes 2a and 2b
no.
composition
energy (eV)
wavelength (nm)
λexpa (nm)
OS
Ligand Precursor IIa
1
2
3
4
H-1fL (58%); HfL (28%)
3.017
3.118
3.229
3.565
411
398
384
348
383
0.097
0.051
0.071
0.207
H-1fL (35%); HfL (28%); HfLþ1 (14%); H-2fL (10%); H-3fL (8%)
H-2fL (44%); H-3fL (25%); HfLþ1 (14%); HfL (11%)
H-3fL (43%); HfL (19%); H-2fL (12%); H-1fLþ1 (12%)
Ligand Precursor IIb
1
2
3
4
5
6
H-1fL (54%); HfL (45%)
2.751
3.110
3.170
3.240
3.544
3.554
450
399
391
383
350
349
369
448
405
0.031
0.070
0.078
0.068
0.048
0.185
HfLþ1 (38%); HfL (19%); H-1fL (16%); H-2fL (15%)
HfLþ1 (43%); H-1fLþ1 (21%); H-1fL (13%); HfL (12%)
H-2fL (52%); H-4fL (22%); H-1fLþ1 (12%)
HfLþ2 (39%); H-1fLþ2 (33%); H-4fL (15%)
H-4fL (37%); H-1fLþ2 (17%); H-2fL (12%); HfLþ2 (10%)
Complex 2a
1
2
3
4
5
6
H-1fLþ1 (22%); H-3fLþ1 (18%); H-2fL (18%); HfL (17%);
H-3fL (20%); H-2fLþ1 (14%); HfLþ1 (14%); H-5fL (14%); HfL (14%)
H-3fLþ1 (23%); H-1fLþ1 (16%); H-5fLþ1 (14%); H-2fL (14%)
H-5fL (35%); H-4fLþ1 (28%); H-1fLþ2 (8%)
2.857
2.907
2.934
3.153
3.164
3.176
434
426
423
393
392
390
0.166
0.409
0.214
0.190
0.183
0.101
H-5fLþ1 (30%); HfLþ2 (21%); H-4fL (16%)
HfLþ2 (39%); H-1fLþ3 (30%)
Complex 2b
1
2
3
4
5
6
HfLþ1 (54%); H-2fLþ1 (17%)
2.795
2.805
2.900
3.151
3.156
3.172
444
442
428
393
393
391
0.220
0.172
0.259
0.032
0.084
0.032
H-1fL (45%); H-2fL (17%); H-1fLþ1 (11%)
H-3fL (30%); H-2fLþ1 (26%); H-3fLþ1 (11%); HfLþ1 (10%)
H-1fLþ2 (47%); H-5fL (31%)
H-4fLþ1 (39%); H-1fLþ2 (14%); H-5fL (14%)
H-5fL (23%); H-1fLþ2 (22%); H-5fLþ1 (20%)
a From Table 3.
single crystal X-ray diffraction studies (see above) is very
good. Therefore it is expected that the structures calcu-
lated for IIa and 2a will be reliable.
From a qualitative point of view the orbitals involved in
the transitions of ligand IIb are very similar to those
described above (see Supporting Information, Figure S5).
The transitions start from H-4, H-2, H-1, and H, and end
in L, Lþ1, and Lþ2. In terms of composition, the nitro-
gen lone pair only contributes to H-1. H-4 of IIb is the
analogue of H-3 in IIa and localized in the phenyl ring,
but with some contribution from the isopropyl groups in
IIb. On the other hand, two unoccupied orbitals (L and
Lþ2) are strongly localized in the imine CdN π bond,
while the major contribution to Lþ1 are the benzene rings
of the phenanthrene fragment (see Supporting Informa-
tion, Figure S5). The energies of the orbitals of the ligand
precursor IIb are higher than those of IIa.
The nature of the orbitals changes when the ligand
precursors lose a proton and bind the metal, since the
nitrogen σ lone pairs (now also a second one on the
pyrrolyl group) are involved in Zn-N σ bonds and
therefore they no longer belong to the group of frontier
orbitals. In the complexes, the frontier orbitals are mostly
localized in the ligands, since the d10 Zn based levels are all
filled and have lower energies, as can be seen in the
Supporting Information, Table S4. The orbitals come in
pairs, corresponding to the symmetric and antisymmetric
combinations of a given orbital of each ligand, or loca-
lized in each of them. Lþ2 and Lþ3 of 2a are almost
totally localized in the phenanthrene of one ligand in Lþ2
and in the phenanthrene of other ligand in Lþ3, and have
almost the same energy. L and Lþ1 are more delocalized
over the two ligands. The energy of the frontier orbitals of
the complexes are close to those of the ligand precursors,
TD-DFT calculations41 were performed on the four
molecules to calculate the nature and energy of the lowest
energy transitions. The results, namely, the energy, wave-
length, and oscillator strength of the more intense bands,
are collected in Table 4.
There are two groups of bands, the first closer to
400 nm, defining the absorption maximum, and the
second which appears as a shoulder at higher energy.
The shift to lower energies of the absorption maxima
upon moving from the ligand precursors IIa and IIb to the
complexes of the deprotonated ligands 2a and 2b is well
reproduced. It is not so easy to see the effect of the
isopropyl substituents, as a global maximum can be adjusted
when simulating the spectra, but the trends are kept. The
calculated maxima for complexes 2a and 2b can be found
at 433 and 422 nm, not too far from the experimental 448
and 405 nm.
In the smaller ligand IIa, the transitions are mixed,
starting from H-3, H-2, H-1, and H, and ending in L and
Lþ1. The nitrogen atom attached to the phenyl ring
contributes to the four occupied orbitals, but while
H and H-2 are completely π systems, the nitrogen lone
pair has a strong contribution to H-1 and a much smaller
in H-2. Therefore, the transitions can be assigned as a
combination of nfπ*, π fπ*, and intraligand (IL)
π fπ*, as can be seen in Figure 4. The compositions of
the orbitals of the ligands IIa and IIb are given in
Supporting Information, Table S3.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11185
Table 5. Emission Energy (eV) and Wavelength (nm) from the First Excited
Singlet State of Ligand Precursors IIa and IIb, and the Complexes 2a and 2b, in
the Gas Phase and in THF
gas phase
wavelength
THF
wavelength
compound
Egp
ETHF
IIa
IIb
2a
1.792
1.879
1.822
1.965
693
660
681
631
1.814
1.922
1.853
1.978
684
645
669
626
2b
The comparison between the first and third row pic-
tures of Supporting Information, Figure S7 shows the
effect of coordinating ligand precursor IIa after deproto-
nation to Zn to afford 2a. The asymmetry between the
two halves of the molecule results from the promotion of
one electron to the LUMO, differently localized in the
two sides (recall the nature of the frontier orbitals dis-
cussed above). The effect is clearly seen on the right side of
the molecule (shortening of the NC-CN bond, lengthen-
ing of the C-N bonds, similar torsion angle).
Figure 4. Orbitals involved in the stronger absorptions of ligand pre-
cursor IIa.
Because configuration interaction is computationally
very demanding and not easily carried out, especially for
large molecules as complexes 2a and 2b, the emission
energy (Egp) was estimated as the difference between the
energy of this singlet excited state and the energy of a
ground state with the same geometry (gas phase), and is
listed in Table 5. The calculations were repeated in the
presence of the solvent (THF; details in Experimental
Section) and another energy (ETHF) was calculated.
Not surprisingly, the results do not agree with the
experimental values (Table 3), and the solvent effect is
negligible. However, the trend within each group (ligand
precursors or complexes) is kept, with lower emission
wavelengths in the compounds bearing the isopropyl
substituents.
Figure 5. Orbitals involved in the stronger absorptions of complex 2a.
Conclusions
as might be expected, since they are essentially π orbitals,
only very weakly involved in binding the metal.
The synthesis and molecular characterization of new
homoleptic Zn(II) complexes containing aryliminophenant-
zhro[9,10-c]pyrrolyl ligands was achieved inmoderate to very
high yields. We have measured the photophysical properties
of both ligand precursors and complexes and compared them
with those of analogues containing simple iminopyrrolyl
ligands. The fluorescence spectra of the metal complexes
show much more intense π-π* fluorescence emissions than
those of the free ligand precursors, which are partially
forbidden because of their n-π* character. The great en-
hancement in the fluorescence quantum yields observed upon
deprotonation and coordination of the iminophenanthro-
pyrrolyl ligands to the d10 spectroscopically inactive Zn2þ ion
is due to their highly resonant π-extended conjugation and to
the use of the nitrogens non-bonding electrons in the
σ-coordination to the metal. The DFT calculations con-
firmed the role of n-π* transitions in the electronic spectra
of the ligand precursors and their absence in the complexes.
Complex 2a is a green emitter and presents the highest
fluorescence quantum yield. The bulkiness of the 2,6-diiso-
propylphenyl substituent of the iminic group highly restricts
the aryl group rotation, inducing a shift in the emission to the
blue region. In summary, the new aryliminophenanthro[9,10-
c]pyrrolyl framework exhibits very interesting luminescent
properties and tuning capabilities when coordinated to Zn2þ
In terms of transitions, the oscillator strengths are larger
in the complexes than in the ligand precursors, while the
transitions involve only π and π* orbitals of the ligands.
The orbitals involved in the electronic transitions of com-
plex 2a are shown in Figure 5 and show a complete
localization on the π systems, either belonging to the
phenyl, to the phenanthrene, or the N-CH-C-N regions.
These calculations were also performed taking into
account the solvent (THF), but there were no significant
differences from the behavior in gas-phase.
To study the emission of the molecules, the geometry of
the first singlet excited state was optimized, promoting
one electron to the LUMO. Some relevant structural
parameters of the ground and excited state are given in
Supporting Information, Figure S7.
The excitation of one electron leads to changes in
several bond lengths, namely, the shortening of the
NC-CN bond, and lengthening of the C-N bonds, in
both ligands. The angle between the two rings, measured
by the C-N-C-C torsion, changes from 76.1ꢀ, in the
ground state of IIa, to 21.2ꢀ, in the singlet excited state.
The change is less pronounced in ligand precursor IIb
(from 79.9ꢀ to 51.8ꢀ), reflecting the steric hindrance of the
isopropyl substituents.

11186 Inorganic Chemistry, Vol. 48, No. 23, 2009
Gomes et al.
ions. The results herein obtained anticipate the possibility of
improvement of the luminescence properties of this type of
complexes by further chemical modifications of the ligand
framework and/or by varying the metal. Studies aiming at the
use of these compounds in light emitting devices or as sensors
are in progress.
and thermal parameters, and bond distances and angles, for
compounds IIb and 2b; syntheses of 2-formiminopyrrole ligand
precursors Ia,b and corresponding analytical data; schemes
containing the multistep preparation of 2-formylphenanthro-
[9,10-c]pyrrole (Scheme S1 and S2); X-ray molecular structure
of complex 2b (Figure S1) and its discussion, and corresponding
table with selected bond distances and angles (Table S1);
absorption and emission spectra of ligand precursors Ia,b and
complexes 1a,b (Figure S2); photophysical parameters of com-
plexes 1a and 1b and their corresponding neutral ligand pre-
cursors Ia and Ib (Table S2); fluorescence decays of ligand
precursors IIa,b (Figure S3) and of complexes 2a,b (Figure
S4); orbitals involved in the stronger absorptions of ligand
precursor IIb (Figure S5) and complex 2b (Figure S6); energy
and composition of the frontier orbitals of ligand precur-
sors IIa,b (Table S3) and complexes 2a,b (Table S4); optimized
geometries of the ground and first excited singlet states of ligand
precursors IIa,b and complexes 2a,b (Figure S7). This material is
available free of charge via the Internet at http://pubs.acs.org.
~
Acknowledgment. The authors thank the Fundac-ao
para a Ciencia e Tecnologia, Portugal, for financial sup-
^
port (Projects PTDC/QUI/65474, PPCDT/QUI/59025/
2004, and PPCDT/QUI/58925/2004, co-financed by
FEDER) and for fellowships to C.S.B.G. (SFRH/BD/
16807/2004) and R.E.D.P. (SFRH/BPD/34558/2007),
and the Portuguese NMR Network (IST-UTL Center)
for providing access to the NMR facility.
Supporting Information Available: CIF files containing X-ray
structural data, including data collection parameters, positional