Novel Re(i) dendrimers: Synthesis, characterization and theoretical studies

Article abstract of DOI:10.1039/b912939a

Four novel diimine rhenium(i) carbonyl complexes with the formula [Re(CO)₃(L)Br], where L = 2-(4-(9H-carbazol-9-yl)phenyl)-1H- imidazo[4,5-f][1,10]phenanthroline (P1), 2-(4-(3,6-di-tert-butyl-9H-carbazol-9- yl)phenyl)-1H-imidazo-[4,5-f][1,10]ph

Full text of DOI:10.1039/b912939a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Novel Re(I) dendrimers: synthesis, characterization and theoretical studies†
Zhenjun Si,a,b Xiaona Li,a Xiyan Li,a Guohai Xu,a Chenling Pan,a Zhiyong Guo,a Hongjie Zhang*a and
Liang Zhoua
Received 30th June 2009, Accepted 28th September 2009
First published as an Advance Article on the web 30th October 2009
DOI: 10.1039/b912939a
Four novel diimine rhenium(I) carbonyl complexes with the formula [Re(CO)3(L)Br],
where L = 2-(4-(9H-carbazol-9-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (P1),
2-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1H-imidazo-[4,5-f][1,10]phenanthroline (P2),
2-(4-(6-(9H-carbazol-9-yl)-9H-3,9¢-bicarbazol-9-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline
(D1), and 2-(4-(3¢,6¢-di-tert-butyl-6-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-3,9¢-bicarbazol-9-
yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (D2), have been successfully synthesized and fully
characterized by 1H NMR, IR, and UV–Vis, etc. The luminescence quantum yields (LQYs) of the
parent Re(I) complexes P1 and P2 are 0.13 and 0.16, respectively, which are much higher than the
previously reported Re(I) dendrimers. The HOMOs and the LUMOs of P1 and P2 are calculated to be
mainly composed of [d(Re) + p(CO + Br)] and p*(L) orbital, respectively. However, those of the Re(I)
dendrimers D1 (LQY = 0.066) and D2 (LQY = 0.0048) are mainly localized on ligand L, indicating
that the component of the metal-to-ligand charge-transfer dp (Re) → p*(N–N) (MLCT) transitions in
P1 and P2 should be more than those in D1 and D2. As a result, the higher LQYs of P1 and P2 are
tentatively assigned to the disturbance of the MLCT transitions during the photoluminescence
process.
report, in this paper, the synthesis of several novel phenanthroline
derivatives and the corresponding Re(I) complexes P1, P2, D1
and D2 as well as experimental and theoretical studies on their
Introduction
The study of the control of the photochemical, photophysical,
and electrochemical properties of transition metal complexes is a
photophysical properties. The LQYs of P1 and P2 in CH2Cl2 are
hot topic due to the advantages of their diverse applications,1–8
0.13 and 0.16, respectively, which are much higher than that (0.03)
which has led to intensive studies of d6 transition metal diimine
complexes including Ru(II), Ir(III), Pt(II), and Re(I). One of the
most efficient methods of tuning the properties of these complexes
is to change the properties and/or scale of the organic ligand.
For example, it is reported that purposefully designed dendrimers
can offer excellent luminescence properties,9–11 and it was recently
found that the luminescence quantum yield (LQY) of the Re(I)
dendrimer based on phenanthroline derivatives could be much
higher than that of the corresponding parent Re(I) complex due
to the suppression of the T–T annihilation by encapsulating the
emitting centers.12
of a previously reported Re(I) dendrimer.12 The higher LQYs of
P1 and P2 could be partly attributed to the larger contributions
of the metal-to-ligand charge-transfer dp(Re) → p*(L) (MLCT)
transitions of P1 and P2 according to the theoretical calculations.
Experimental section
Materials
N,N-Dimethylacetamide (DMA) was dried with P2O5, the
carbazole (R1) and other chemicals are commercially available
and used without further purification. 3,6-Diiodo-9-tosyl-
9H-carbazole (I2TsCz), 3,6-di-tert-butyl-9H-carbazole (R2),
6-(9H-carbazol-9-yl)-9H-3,9¢-bicarbazole (R3), 3,6-di-tert-butyl-
9-(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-carba-zol-6-yl)-
9H-carbazole (R4), 4-(9H-carbazol-9-yl)benzaldehyde (a),
4-(3,6-di-tert-butyl-9H-carbazol-9-yl)benzaldehyde (b), 4-(6-
According to the fact that the complexation of phenanthroline
derivatives with ions of transition metals and lanthanides provides
a virtually countless library of coordination compounds and their
rigidity gives them entropically better chelating abilities,13 we
aState Key Laboratory of Rare Earth Resource Utilization, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun,
130022, People’s Republic of China. E-mail: hongjie@ciac.jl.cn; Fax: +86
43185698041; Tel: +86 431 85262127
(9H-carbazol-9-yl)-9H-3,9¢-bicarbazol-9-yl)benzaldehyde
(c),
and 4-[3,6-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-carbazol-
9-yl]benzaldehyde (d) were prepared according to the literature.14
bSchool of Materials Science and Engineering, Changchun University of
Science and Technology, Changchun, 130022, People’s Republic of China.
E-mail: sizj168@126.com; Fax: +86 43185583015; Tel: +86 43185583016
Synthesis of L1
1,10-Phenanthroline-5,6-dione (0.420 g, 2.000 mmol), compound
a (0.540 g 2.000 mmol), and NH4OAc (1.540 g, 20.000 mmol)
were added to 15 mL HOAc. The mixture was refluxed for 6 h.
The crude product was poured into 100 mL H2O and stirred for
15 min. The resulting yellow powder was collected by filtration and
† Electronic supplementary information (ESI) available: Tables S1–S4,
Fig. S1–S5: selected bond lengths and angles, frontier molecular orbital
compositions, triplet absorption parameters, absorption spectra of L1–L4
and electron density plots. CCDC reference numbers 722314 and 722315.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b912939a
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washed successively with 10 mL dilute aqueous NaOH solution,
50 mL H2O, and 10 mL methanol to get 0.740 g (80%) of yellow
powder. 1H NMR (CDCl3, 400 MHz): d 7.26 (5H, s), 7.29 (2H, d,
J = 8.8), 7.41 (2H, d, J = 8.8), 7.52 (1H, m), 7.94 (1H, m), 8.09
(2H, s), 8.81–8.96 (3H, m), 9.21–9.22 (1H, s), 14.40 (1H, s). Anal.
Calcd for C31H19N5: C, 80.68; H, 4.15; N, 15.17. Found: C, 80.35;
H, 4.39; N, 15.26%. IR (KBr): n 1452, 1607, 3465 cm-1.
Synthesis of D1
The procedure is similar to that of P1. Yield: 78% 1H NMR
(CDCl3, 400 MHz): d 7.29–7.33 (5H, m), 7.39–7.47 (8H, m), 7.63–
7.66 (1H, m), 7.768 (3H, m), 7.90 (3H, m), 8.09 (2H, d, J = 6.8),
8.15–8.21 (5H, m), 8.37 (2H, s), 8.46–8.61 (2H, m). Anal. Calcd
for C58H33BrN7O3Re: C, 61.00; H, 2.91; N, 8.59. Found: C, 61.32;
H, 2.58; N, 8.91%. IR (KBr): n 1451, 1600, 1902, 2023, 3411 cm-1.
Synthesis of L2
1
Synthesis of D2
The procedure is similar to that of L1. Yield: 75%. H NMR
(CDCl3, 400 MHz): d 1.48 (18H, s) 7.44 (5H, d, J = 8.8), 7.49
(1H, d, J = 3.2), 7.52 (2H, d, J = 3.2), 7.70 (1H, s), 7.77 (3H,
d, J = 8.4), 8.16 (2H, s), 8.50 (2H, d, J = 6.4), 9.12–9.18 (3H,
m), 10.64 (1H, s). Anal. Calcd for C39H35N5: C, 81.64; H, 6.15;
N, 12.21. Found: C, 81.45; H, 6.45; N, 12.10%. IR (KBr): n 1449,
1608, 3426 cm-1.
1
The procedure is similar to that of P1. Yield: 70%. H NMR
(CDCl3, 400 MHz): d 1.46 (36H, s), 7.33 (3H, d, J = 8.4), 7.40–
7.52 (7H, m), 7.60–7.76 (5H, m), 7.87–7.89 (1H, t, J = 4), 7.97 (2H,
d, J = 8), 8.09 (1H, d, J = 8), 8.16–8.19 (5H, m), 8.24–8.26 (3H,
m), 8.33 (1H, s), 10.19 (1H, s). Anal. Calcd for C74H65BrN7O3Re:
C, 65.04; H, 4.79; N, 7.18. Found: C, 65.37; H, 4.43; N, 7.42%. IR
(KBr): n 1476, 1602, 1908, 2024, 3423 cm-1.
Synthesis of L3
1
The procedure is similar to that of L1. Yield: 84%. H NMR
Spectroscopy
(CDCl3, 400 MHz): d 7.29–7.31 (7H, m), 7.38–7.43 (10H, m),
7.60–7.69 (4H, m), 7.75 (1H, d, J = 8.4), 7.83 (1H, d, J = 8), 7.98
(1H, d, J = 8), 8.15–8.17 (5H, m), 8.25–8.30 (3H, m), 8.57 (1H, d,
J = 8), 10.20 (1H, s). Anal. Calcd for C55H33N7: C, 83.42; H, 4.20;
N, 12.38. Found: C, 83.75; H, 4.02; N, 12.23%. IR (KBr): n 1451,
1599, 3406 cm-1.
The IR spectra were acquired using a Magna560 FT-IR spec-
trophotometer. Elemental analyses were performed using a Vario
Element Analyzer. 1H NMR spectra were obtained using a Bruker
AVANVE 400 MHz spectrometer with tetramethylsilane as the
internal standard. The absorption and photoluminescence (PL)
spectra of the degassed solutions with ca. 10-4 mol L-1 samples
were recorded on a Lambda 750 spectrometer and a Hitachi model
F-4500 fluorescence spectrophotometer, respectively. The LQYs
were measured by comparing fluorescence intensities (integrated
areas) of a standard sample (quinine sulfate) and the unknown
sample according to eqn (1).
Synthesis of L4
1
The procedure is similar to that of L1. Yield: 70%. H NMR
(CDCl3, 400 MHz): d 1.46 (36H, s), 7.32–7.35 (5H, m), 7.45–7.47
(5H, m), 7.63–7.65 (3H, m), 7.79 (3H, m), 7.90 (1H, m), 7.96–7.98
(1H, d, J = 8.4), 8.03–8.08 (1H, m), 8.16 (4H, s), 8.24–8.26 (3H,
m), 8.54 (1H, d, J = 8), 9.18 (1H, d, J = 10), 10.19 (1H, s). Anal.
Calcd for C71H65N7: C, 83.91; H, 6.45; N, 9.65. Found: C, 84.17;
H, 6.57; N, 9.26%. IR (KBr): n 1490, 1602, 3406 cm-1.
Uunk = Ustd(Iunk/Aunk)(Astd/Istd)(hunk/hstd)2
(1)
where Uunk is the LQY of the unknown sample; Ustd is the LQY of
quinine sulfate and taken as 0.546;15
unk and Istd are the integrated
I
Synthesis of P1
fluorescence intensities of the unknown sample and quinine
sulfate, respectively; Aunk and Astd are the absorbances of the
unknown sample and quinine sulfate, respectively, with excitation
wavelength of 365 nm. The hunk and hstd are the refractive indices
of the corresponding solvents (pure solvents were assumed). The
excited-state lifetimes were detected by a system equipped with
a TDS 3052 digital phosphor oscilloscope pulsed Nd:YAG laser
with a THG 355 nm output and a computer-controlled digitizing
oscilloscope according to the reported method.16 The Origin 7.0
program by OriginLab Corporation was used for the curve-fitting
analysis. The absorption and emission spectral data of P1, P2, D1,
and D2 in CH2Cl2 and solid state are listed in Table 1.
L1 (0.090 g, 0.210 mmol) and Re(CO)5Br (0.080 g, 0.200 mmol)
were refluxed in 15 mL of toluene for 6 h. After the mixture was
cooled to room temperature (RT), the solvent was removed in a
water bath under reduced pressure. The resulting yellow solid was
purified by silica gel column chromatography with acetic acid ethyl
1
ester and CH2Cl2 (v/v = 10 : 1). Yield: 0.124 g (80%). H NMR
(CDCl3, 400 MHz): d 7.41 (3H, t, J = 7.2), 7.57 (3H, t, J = 7.2),
7.68 (3H, d, J = 8), 7.90 (3H, d, J = 8), 8.24 (3H, d, J = 8), 8.45
(3H, s). Anal. Calcd for C34H19BrN5O3Re: C, 50.31; H, 2.36; N,
8.63. Found: C, 50.65; H, 2.15; N, 8.97%. IR (KBr): n 1450, 1607,
1898, 2023, 3415 cm-1.
Synthesis of P2
Crystallography
1
The procedure is similar to that of P1. Yield: 75%. H NMR
(CDCl3, 400 MHz): d 1.53 (18H, s), 7.52–7.55 (1H, m), 7.63 (4H,
s), 7.70 (1H, m), 7.88 (2H, d, J = 8.4), 8.23 (2H, s), 8.39–8.48 (3H,
m), 8.87 (1H, s), 9.04 (1H, s), 9.30 (1H, s), 11.41 (1H, s). Anal.
Calcd for C42H35BrN5O3Re: C, 54.60; H, 3.82; N, 7.58. Found:
C, 54.93; H, 3.45; N, 7.92%. IR (KBr): n 1454, 1608, 1905, 2026,
3417 cm-1.
The crystals of the parent Re(I) complexes P1 and P2 were mea-
sured on a Bruker Smart Apex CCD single-crystal diffractometer
˚
using Mo Ka radiation, l = 0.7107 A at 215 K. An empirical
absorption based on the symmetry-equivalent reflections was
applied to the data using the SADABS program. The structure
was solved using the SHELXL-97 program.17–19
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Table 1 Photophysical parameters of P1, P2, D1, and D2 in CH2Cl2 and solid state at 293 K
Absorption
Excitation
Emission
max/nm (W1/2/cm-1)
Complex
P1
Medium
l/nm
l/nm
l
t(A)/ms
f (degassed)
0.13
CH2Cl2
Solid
CH2Cl2
Solid
CH2Cl2
Solid
CH2Cl2
Solid
226, 262, 344, 430
226, 262, 284, 350,
238, 264, 344, 438
240, 268, 350, 446
256, 323, 390, 443
275, 323, 478
328, 406, 452
267, 333, 436, 497
328, 397, 446
272, 362, 458, 475
322, 387, 429
554 (73)
561 (83)
554 (74)
553 (70)
554 (74)
562 (89)
554 (102)
559 (77)
0.05 (0.05), 0.29 (0.35)
0.05 (0.05), 0.31 (0.33)
0.14 (0.89), 0.02 (0.03)
0.06 (0.07), 0.49 (0.31)
P2
0.16
D1
0.066
D2
0.0048
274, 360, 456, 473
Cyclic voltammetry
and Br atoms, and LANL2DZ on Re atom were employed for all
Re(I) dendrimers to ensure that the calculations were performed on
the same level. The calculated electronic density plots for frontier
molecular orbitals were prepared by using the GaussView 3.07
software. All the calculations were performed with the Gaussian
03 software package.25
Cyclic voltammetry measurements were conducted on a voltam-
metric analyzer (CH Instruments, Model 620B) with a polished Pt
plate as the working electrode, Pt mesh as the counter electrode,
and a saturated calomel electrode (SCE) as the reference electrode,
at a scan rate of 0.1 V s-1. The voltammograms were recorded in
CH3CN solutions with ~10-3 M sample P1, P2, D1, and D2 and
0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as
the supporting electrolyte. Prior to each electrochemical measure-
ment, the solution was purged with nitrogen for ~10–15 min to
remove the dissolved O2 gas.
Results and discussion
Synthesis and characterization
Both R3 and R4 were prepared from 3,6-disubstituted carbazole
building blocks by copper-catalyzed Ullmann reactions. The
precursors aldehydes a, b, c, and d were prepared by reactions
between the carbazole derivatives of Rn (n = 1, 2, 3, and 4) and
4-iodobenzaldehyde, and the corresponding products were used to
synthesize the dendritic ligands L1–L4 by a condensed method.8
Finally, the dendronized L1–L4 were complexed to Re(I) cations
to obtain the final Re(I) complexes P1, P2, D1, and D2 according
to the previous report,26 and the synthetic routes to L1–L4 and
P1, P2, D1, and D2 are illustrated in Scheme 1 and Scheme 2,
Computational details
The geometrical structures of the ground states were optimized in
gas phase by the density functional theory (DFT)20 with Becke’s
LYP (B3LYP) exchange–correlation functional calculus.21 On the
basis of the optimized ground state geometry structures, the
absorption spectral properties in CH2Cl2 media were calculated by
time-dependent DFT (TDDFT),22 associating with the polarized
continuum model (PCM). The 6-31G23, 24 basis set on C, H, N, O,
Scheme 1 Synthetic routes to L1–L4. (i) AlCl3, tBuCl, RT, 24 h; (ii) Cu2O, 4-iodobenzaldehyde, DMA, 170 ◦C, 24 h; (iii) HOAc, NH4OAc,
[1,10]phenanthroline-5,6-dione, reflux, 6 h; (iv) I2TsCz, Cu2O, DMAc, 170 ◦C, 24 h; (v) KOH, THF, DMSO, H2O, reflux, 2 h.
10594 | Dalton Trans., 2009, 10592–10600
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Table 2 Crystal data for 2P1·3CHCl3 and P2·CH3OH·CH2Cl2
2P1·3CHCl3
P2·CH3OH·CH2Cl2
Formula
C71H41Br2Cl9N10O6Re2
1981.41
215 K
C44H41BrCl2N5O4Re
1040.83
215 K
Formula weight
T/K
˚
l/A
0.71073
0.71073
¯
¯
Crystal system
Space group
P1
P1
Triclinic
11.9408(8)
12.7779(9)
13.1483(9)
70.5850(10)
88.5970(10)
79.0500(10)
1856.0(2)
1
Triclinic
11.9617(7)
13.7719(8)
13.8699(8)
80.0310(10)
66.9410(10)
84.7430(10)
2069.9(2)
2
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
Scheme 2 Synthetic route to P1, P2, D1, and D2. (i) Toluene, N2, reflux,
6 h.
V/A
Z
rc/Mg m-3
1.773
4.716
958
2.44–25.95
1.670
4.079
1032
1.61–28.31
m/mm-1
respectively. The structures of the four synthesized Re(I) complexes
were fully verified with IR, UV–Vis, elemental analysis, and
1H NMR spectroscopy.
F(000) (e)
Range of transm
factors (deg)
Reflns collected/
unique
The needle-like yellow crystals for P1 and P2 were obtained at
RT over a period of a week from the evaporation of the solutions
of P1–methanol–chloroform and P2–methanol–dichloromethane,
respectively. The solid-state structures of P1 and P2 were de-
termined by single-crystal X-ray diffraction and the ORTEP
diagrams of P1 and P2 are presented in Fig. 1. The crystal
data of P1 and P2 are listed in Table 2, while the selected bond
distances and the angles are listed in Table S1†. The coordination
geometry at the Re cations is a distorted octahedron with the three
carbonyl ligands arranged in a facial fashion, and the coordination
atmospheres of the Re atoms in P1 and P2 are similar to each other.
For example, the bond distance of Br–Re and the average bond
9935/6956
13152/9549
R(int)
0.0257
0.0333
Completeness
R1, wR2 [I > 2s(I)]
R1, wR2 (all data)
97.1% (q = 25.00)
0.0789/0.2220
0.0940/0.2366
99.4% (q = 25.00)
0.0482/0.1022
0.0769/0.1141
carbon atoms in P1 and P2 are within the ranges 87.5(3)–89.8(3)◦
and 88.4(3)–90.4(3)◦, respectively, which are close to 90◦. But the
N–Re–N angles in P1 (75.64(17) ◦) and P2 (75.75 (19) ◦) are much
smaller than 90◦, which is most likely due to the result of the steric
requirement of the bidentate coordination of L1 and L2. At the
same time, the planes of the pyrrole ring and the imidazole ring of
P1 and P2 which are connected by a phenylene intersect at angles
of 7.38◦ and 18.42◦, respectively. The serious thermal vibration of
the solvent molecules in the single crystal structure of P1 leads to
˚
distances of Re–C and Re–N are 2.6246(7), 1.915, and 2.1785 A for
˚
P1, and 2.617(4), 1.883, and 2.105 A for P2, respectively. The bond
angles in Table S1† clearly indicate that the CO ligands are almost
linearly coordinated. The bond angles between the adjacent CO
Fig. 1 ORTEP drawing of the crystal of P1 (upper) and P2 (lower) with displacement ellipsoids at the 30% probability level. Hydrogen atoms are omitted
for clarity.
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the disorder of C(35) and Cl(1)–Cl(3) and the bigger temperature
factors of C(36) and Cl(4)–Cl(6).
Electrochemistry
Cyclic voltammograms of P1, P2, D1, and D2 presented in
Fig. 2 exhibit irreversible redox behaviors in CH3CN solution.
The anodic waves of these Re(I) dendrimers are associated
with a ReI-based oxidation process (ReI/ReII), and the cathodic
waves are associated with a ligand-based reduction process
([ReIBr(CO)3(L)]/[ReIBr(CO)3(L∑)]-).27 As presented in Fig. 2, P1
shows an anodic wave peak at +1.41 V with the onset oxidation
potential (Vonset) of +1.08 V and cathodic wave peak at -1.44 V
with the Vonset of -1.34 V. For P2, the anodic wave and cathodic
wave peaks shift to + 1.22 V with Vonset of +1.09 V and -1.40 V
with the Vonset of -1.29 V, respectively. For D1 and D2, the peaks of
the anodic waves are located at + 1.27 V with the Vonset of +1.11 V
and + 1.21 V with the Vonset of +1.13 V, and the peaks of the
cathodic waves are located at -1.46 V with the Vonset of -1.33 V
and -1.39 V with the Vonset of -1.29 V, respectively. The energy
levels of the highest-occupied molecular orbital (HOMO) and the
lowest-unoccupied molecular orbital (LUMO) are calculated from
the onset potentials of the oxidation (Eonset(Ox)) and reduction
(Eonset(Red)) peaks with the formula EHOMO = -4.74 - Eonset(Ox)
and ELUMO = -4.74 - Eonset(Red) (-4.74 V for SCE with respect
to the zero vacuum level,28) respectively. These calculations give
the EHOMOs of -5.82, -5.83, -5.85 and -5.87 eV, and the ELUMO s
of -3.40, -3.45, -3.41, and -3.45 eV for P1, P2, D1, and D2,
respectively. As presented above, both the carbazole (Cz) group
and the tert-butyl group in the Re(I) complexes lower the molecular
orbital energy levels in CH3CN solution. As a result, the order of
the EHOMO is: P1 > P2 > D1 > D2, and the order of the ELUMO is
P1 > D1 > P2 = D2.
Fig. 3 Absorption and excitation spectra of P1, P2, D1, and D2 in
CH2Cl2.
the visible region from ca. 400 to ca. 500 nm, which are tentatively
assigned to the MLCT transitions. These assignments are based
on the absorption spectra of the free ligands5 shown in Fig. S1
(ESI†) and the theoretical studies (vide infra).
Fig. 4 shows the PL spectra of P1, P2, D1, and D2 in CH2Cl2.
The PL spectra of these samples, except that the width of the half
maximum (W1/2) of D2 is ca. 102 nm (Table 1), present similar
emission bands peaking at ca. 554 nm with W1/2 of ca. 74 nm. The
LQYs of P1, P2, and D1 are measured to be 0.13, 0.16, and 0.066,
respectively, which are much higher than that of the reported Re3
(0.03).12 But the LQY of D2 (0.0048) is almost six times lower than
that of Re3, which should be partly attributed to the more pliable
molecular structures of D2 in CH2Cl2 with the addition of the tert-
butyl groups.12 The broad and low-energy band in the absorption
spectra partially resolve into two bands in scanning the excitation
spectra, namely,1MLCT transition states with peaks at 390, 410,
401, and 388 nm and 3MLCT transition states with peaks at 444,
453, 445, and 434 nm for P1, P2, D1, and D2, respectively.26,29
Excitation at either the p → p* absorption region (~ 324 nm) or
the MLCT absorption region (~ 386 nm for P1 and D2, ~ 450 nm
for P2 and D1) leads to the same MLCT emission band at RT for
P1, P2, D1, and D2, which indicates there should be a potential
surface crossing (PSC) from the higher p → p* state to the lower
MLCT state, and the major contribution of the observed emission
is from the latter.30 This character of the PL spectra is further
revealed by the measurement of the excited-state lifetimes.
Fig. 2 Cyclic voltammograms of P1, P2, D1, and D2 measured in CH3CN
(vs SCE) at a scan rate of 0.1 V s-1. A polished Pt plate and a Pt mesh
were used as the working electrode and the counter electrode, respectively.
TBAPF6 was taken as supporting electrolyte.
Photophysical properties
Fig. 3 shows UV–Vis absorption and the excitation spectra of
P1, P2, D1, and D2 in CH2Cl2. The dominant absorption bands
of P1, P2, D1, and D2 in the ca. 220–400 nm region should be
assigned to the ligand-centred p → p* absorptions. These bands
are also accompanied by the lower-energy features extending into
Fig. 4 PL spectra of P1, P2, D1, and D2 in CH2Cl2.
10596 | Dalton Trans., 2009, 10592–10600
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Fig. 5 PL lifetime decay measurements of the powder samples of P1 (a), P2 (b), D1 (c), and D2 (d).
As indicated in Fig. 5, the PL spectra are composed of
biexponential decays with t1 = 0.05 (A1=0.053), 0.05 (A1=0.05),
0.14 (A1=0.89), 0.06 ms (A1=0.07), and t2 = 0.29 (A2=0.35), 0.31
(A2=0.33), 0.02 (A2=0.03), 0.49 ms (A2=0.31) for P1, P2, D1, and
D2, respectively. In general, if there is PSC from the higher p →
p* state to the lower MLCT state, then strong excitation bands in
the p → p* absorption region and a shorter decay lifetime from
the p → p* state than those from the lower MLCT state should be
observed based on the energy law.31 As can be seen from Fig. 3, the
strength of the excitation peaks of the samples show the following
order: P1 > P2 > D2 >> D1 in the p → p* absorption region
with the normalization according to the maximum peak values,
suggesting that the order of the efficiency of the PSC from the p →
p* state to the MLCT state should be in the same order. Therefore,
the assignment of t1 of P1, P2, D1, and D2 to the emission from
those from the experimental measurements: the bond distance
of Br–Re and the average bond distances of Re–C and Re–N
of P1, which are the same as those of P2, are calculated to be
2.677, 1.921 and 2.186 A, respectively. The difference between
these parameters and those from the experimental measurements
˚
˚
is less than 0.06 A. The calculated bond angles of C–Re–C and
O–C–Re are within the ranges 90.7–92.2◦ and 177.2–179.8◦ for P1
and P2, respectively, indicating that the CO ligands almost linearly
coordinate to Re ions, and have an angle of close to 90◦ between
them. The difference between the theoretical calculations and the
experimental measurements should be attributed to the fact that
the former are optimized in the gas phase and the latter are in a
tight crystal lattice.32
As can be seen from Fig. S2–S5,† the LUMOs of P1, P2, D1,
and D2, which have similar distribution, are composed of the p*
orbitals localized on the 1H-imidazo[4,5-f][1,10]phenanthroline
(L¢) moiety with ca. 90% composition (Table S2 and S3†), while
their LUMO+1 and LUMO+2 extend to the p* orbitals of the
bridging phenylene. The LUMO+3 of P1 (P2) is mainly composed
of the p* orbitals localized on almost the whole L ligand, while
that of D1 (D2) is mainly composed of the p* orbitals localized
on R3 (R4).
The HOMO and HOMO-1 of P1 (P2) are mainly composed
of the p orbitals of the carbonyl group, the bromine, and the d
orbitals of Re cations which are in antibonding coordination with
the axial bromine group and bonding with the three carbonyl
groups, while those of D1 (D2) are localized predominantly on the
p-conjugated [9,3¢;6¢,9¢¢]tercarbazole (TCz) moiety. The
HOMO-2 and the HOMO-3 of P1 (P2) are mainly composed
of the p orbitals of the carbazole moiety, and the composition
of the HOMO-2 and the HOMO-3 of D1 (D2) are similar to
the p → p* state should be reasonable.29 The average lifetimes (
of P1, P2, D1, and D2 are calculated according to eqn (2).
)
t
(2)
t = (A1t1 + A2t2 )/(A1 + A2 )
The average lifetimes of P2 (0.28 ms) and D2 (0.41 ms) are longer
than those of P1 (0.26 ms) and D1 (0.14 ms), respectively, suggesting
that the attachment of the tert-butyl substituents to P2 and D2 is
helpful in extending their radiative lifetimes due to the fact that the
bigger molecular radii will reduce the T–T annihilation between
the light-emitting centers.
Theoretical calculations
Frontier molecular orbital properties. As can be seen from
Table S1,† the selected bond distances and the angles of the simu-
lated molecular structures of P1 and P2 are nicely coincident with
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Table 3 Selected singlet-absorption parameters of P1, P2, D1, and D2 in CH2Cl2 according to TDDFT (LANL2DZ/6-31G) calculations
Complex
State
Transition
l/nm/E/eV
Oscillator
Assignment
lexp/nm
P1
S1
S4
S10
S14
S22
S26
S1
H-1 → L (97%)
H → L+1 (100%)
H → L+2 (98%)
H-4 → L+1 (73%)
H → L+5 (93%)
H-4 → L+2 (78%)
H → L (100%)
482/2.57
421/2.95
370/3.35
334/3.72
304/4.07
296/4.19
501/2.47
439/2.82
336/3.69
296/4.19
287/4.31
274/4.53
510/2.43
406/3.06
391/3.17
336/3.69
322/3.86
286/4.34
534/2.32
421/2.94
402/3.08
335/3.70
322/3.85
292/4.24
0.0036
0.2561
0.3959
0.0907
0.1041
0.3087
0.0535
0.2310
0.1374
0.4086
0.1494
0.4259
0.0189
0.0214
0.2590
0.1291
0.3513
0.1333
0.0133
0.0003
0.2333
0.4231
0.1946
0.2512
d→p*/p→p*
d→p*/p→p*
d→p*/p→p*
d→p*/p→p*
d→p*/p→p*
d→p*/p→p*
d→p*/p→p*
d→p*/p→p*
d→p*/p→p*
d→p*/p→p*
d→p*/p→p*
d→p*/p→p*
p→p*
430
344
282
450
350
284
438
P2
D1
D2
S4
H → L+1 (100%)
H-4 → L+1 (84%)
H-4 → L+2 (79%)
H-9 → L (48%)
H-9 → L+1 (55%)
H → L (100%)
S13
S29
S32
S37
S1
S9
H-3 → L (46%)
H → L+2 (100%)
H-5 → L+2 (76%)
H-8 → L+1 (55%)
H-15 → L (29%)
H → L (100%)
d→p*/p→p*
p→p*
S12
S24
S31
S54
S1
p→p*
342
p→p*
p→p*
282
446
p→p*
S8
H-3 → L (100%)
H → L+2 (100%)
H-4 → L+2 (79%)
H-11 → L+1 (31%)
H-8→ L+2 (74%)
d→p*/p→p*
p→p*
S10
S22
S31
S52
p→p*
336
298
p→p*
p→p*
the HOMO and the HOMO-1 of P1 (P2), respectively. These
results present that the difference among the diimine ligands
L1–L4 dramatically influences the distribution of the occupied
molecular orbitals in the corresponding Re(I) dendrimers, which
should also result in the different electronic transition character
upon excitation.
As presented in Table S2 and Table S3, the theoretical calcu-
lations on the EHOMO of P1, P2, D1, and D2 exhibit the order
P1 (-5.51 eV) < P2 (-5.49 eV) < D1 (-5.42 eV) < D2 (-5.23
eV), while their ELUMO s exhibit the order P2 (-2.76 eV) > P1
(-2.78 eV) > D2 (-2.85 eV) > D1 (-2.87). The different orders
of the EHOMOs and ELUMO s of P1, P2, D1, and D2 between
the experimental measurements and the theoretical calculations
should be attributed to the fact that the solution effect is ignored
during the optimization of the geometrical structures of their
ground states.
are nicely fitted to those from the experimental measurements. As
presented in Table 3, the lowest lying singlet → singlet absorptions
of P1, P2, D1, and D2 are calculated at 482, 501, 510, and
534 nm, respectively. The absorption of D1 (D2) is red-shifted
according to that of P1 (P2), which should be attributed to the
intense participation of the TCz part which narrows the energy
gaps between the HOMOs and LUMOs of D1 and D232 (Fig. 7).
The configuration of HOMO-1 → LUMO is responsible for the
lowest lying transition of P1, while that of HOMO → LUMO
is responsible for those of P1, P2, D1, and D2. The HOMO-1
of P1 is mainly composed of ca. 17.5% Re(I) d orbital (13.9%
dxz +3.6% dxy), 11.1% carbonyl group, and 59.6% bromine group.
The LUMO of P1, which is similar to those of P2, D1, and D2,
is a p* (90.4%, L¢) type orbital in nature. Thus, the transition
at 482 nm for P1 should be described as the metal/ligand-to-
Absorption spectra. The curves of the theoretically simulated
absorption spectra of P1, P2, D1, and D2 presented in Fig. 6
Fig. 7 Pictorial representation of the frontier molecular orbital energy
levels of P1, P2, D1, and D2 calculated in the gas phase at the
DFT/B3LYP/6-31G level.
Fig. 6 Simulated absorption spectra of P1, P2, D1, and D2 at the TDDFT
(B3LYP)/6-31G level in CH2Cl2.
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ligand charge transfer [d(Re) + p(CO+Br)] → [p*(L)] (MLLCT)
transition. The composition of the HOMO of P2 is similar to
that of the HOMO-1 of P1 in nature, as a result, the lowest lying
transition at 501 nm for P2 is similar to that of P1. The HOMOs
of D1 and D2 are mainly composed of ca. 97.2% R3 and 97.7%
R4, respectively, and the transitions of D1 at 510 nm and D2 at
534 nm should be attributed to the p (R3 for D1 and R4 for
D2) → p*(L¢)transitions. The absorptions with the largest oscil-
lator strength at 370, 296, 322, and 335 nm are in agreement with
the experimental values of 344, 284, 342, and 350 nm for P1, P2,
D1, and D2, respectively. The dominant character of these higher
energy absorptions is the MLLCT and p → p* transitions for
P1 (P2) and D1 (D2), respectively. The excitation of HOMO →
LUMO+2 (98%) contributes to the absorption at 370 nm of P1.
The HOMO of P1 is mainly composed of ca. 20% metal Re(I)
extending their radiative lifetimes. The LQYs of P1 and P2 are
measured to be 0.13 and 0.16 which are much higher than those of
D1 (0.066), D2 (0.0048), and the previously reported Re3 (0.03).
Acknowledgements
The authors are grateful for financial aid from the National
Natural Science Foundation of China (Grant No. 20631040 and
20771099) and the MOST of China (Grant No. 2006CB601103,
2006DFA42610).
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Re(I) complexes P1, P2, D1, andD2 with similar PL spectrapeaked
at 554 nm are synthesized, and their photophysical behavior
is experimentally and theoretically studied. P1, P2, D1, and
D2 display biexponential light-emitting properties with average
lifetimes of 0.26, 0.28, 0.14, and 0.41 ms, respectively, which
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