Intraligand charge-transfer excited states in Re(I) complexes with donor-substituted dipyridophenazine ligands

Article

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Intraligand Charge-Transfer Excited States in Re(I) Complexes with

Donor-Substituted Dipyridophenazine Ligands

Christopher B. Larsen,^†Holly van der Salm,^†Charlotte A. Clark,^‡Anastasia B. S. Elliott,^†

Michael G. Fraser,^†Raphael Horvath,^‡Nigel T. Lucas,^†Xue-Zhong Sun,^‡Michael W. George,*^,‡

and Keith C. Gordon*^,†

^†MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Otago, Union Place,

Dunedin 9001, New Zealand

^‡School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom

S

* Supporting Information

ABSTRACT: The donor−acceptor ligands 11-(4-

diphenylaminophenyl)dipyrido[3,2-a:2′,3′-c]phenazine (dppz-

PhNPh₂) and 11-(4-dimethylaminophenyl)dipyrido[3,2-a:2′,3′-c]-

phenazine (dppz-PhNMe₂), and their rhenium complexes, [Re-

(CO)₃X] (X = Cl⁻, py, 4-dimethylaminopyridine (dmap)), are

reported. Crystal structures of the two ligands were obtained. The

optical properties of the ligands and complexes are dominated by

intraligand charge transfer (ILCT) transitions from the amine to the

dppz moieties with λ_abs= 463 nm (ε = 13 100 M⁻¹cm⁻¹) for dppz-

PhNMe₂and with λ_abs= 457 nm (ε = 16 900 M⁻¹cm⁻¹) for dppz-

PhNPh₂. This assignment is supported by CAM-B3LYP TD-DFT

calculations. These ligands are strongly emissive in organic solvents and, consistent with the ILCT character, show strong

solvatochromic behavior. Lippert−Mataga plots of the data are linear and yield Δμ values of 22 D for dppz-PhNPh₂and 20 D for

dppz-PhNMe₂. The rhenium(I) complexes are less emissive, and it is possible to measure resonance Raman spectra. These data

show relative band intensities that are virtually unchanged from λ_exc= 351 to 532 nm, consistent with a single dominant

transition in the visible region. Resonance Raman excitation proﬁles are solvent sensitive; these data are modeled using

wavepacket theory yielding reorganization energies ranging from 1800 cm⁻¹in toluene to 6900 cm⁻¹in CH₃CN. The excited

state electronic absorption and infrared spectroscopy reveal the presence of dark excited states with nanosecond to microsecond

3

lifetimes that are sensitive to the ancillary ligand on the rhenium. These dark states were assigned as phenazine-based ILCT

states by time-resolved infrared spectroscopy. Time-resolved infrared spectroscopy shows transient features in which Δν(CO) is

approximately −7 cm⁻¹, consistent with a ligand-centered excited state. Evidence for two such states is seen in mid-infrared

transient spectra.

behavior was originally attributed to protonation of the phz

nitrogen atoms stabilizing the nonemissive b₁(phz) state in

aqueous media.¹⁸Brennaman et al.,¹⁹based on variable

temperature lifetime studies, further developed this model in

proposing that the dark b₁(phz) state is always the lowest

energy state, even in aprotic solvents, and that the “light-switch”

is driven by competition between enthalpic factors that favor

the dark b₁(phz) state and entropic factors that favor the

emissive b₁(phen) state.

Research in this area has focused on manipulating the relative

energies of the two MOs. This may be achieved by chelation to

various metals²⁰⁻²⁶which alters the b₁(phen) MO. Substitution

at the dppz ligand has mostly involved lowering the energy of

the b₁(phz) state through appending electron-withdrawing

groups to the E ring.^20,27−34In addition, in one study the

INTRODUCTION

■

Dipyridophenazine (dppz) based ligands and their complexes

have been extensively studied due to their interesting

photophysical properties.¹⁻⁶These are derived from the

presence of two close-lying acceptor molecular orbitals

(MOs) that lie on diﬀerent sections of the ligand.⁷⁻¹⁰In

dppz-based systems the b₁(phen) orbital has wave function

amplitude localized on the phenanthroline portion of the ligand

(rings A, B, and C in Figure 1), while the b₁(phz) orbital has

electron density localized on the phenazine portion of the

ligand (rings B, D, and E in Figure 1).⁴

The most well-known manifestation of these two acceptor

orbitals is the phenomenon known as the “light-switch” eﬀect,

in which [Ru(bpy)₂(dppz)]²⁺is nonemissive in protic media,

but emissive in aprotic media or in the presence of double-

stranded DNA.¹¹⁻¹⁷This eﬀect occurs because the b₁(phz)

metal to ligand charge transfer (MLCT) state is nonemissive

and the b₁(phen) MLCT state emissive.⁴The “light-switch”

Received: August 15, 2013

A

dx.doi.org/10.1021/ic402082m | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

The electronic donor−acceptor behavior of these com-

pounds has been characterized by B3LYP and CAM-B3LYP

TD-DFT calculations (validated by FT-Raman spectroscopy

and X-ray crystallography), cyclic voltammetry, and variable

temperature and variable solvent steady state absorption and

emission, transient absorption, and resonance Raman spectros-

copies. We observe an intense ILCT transition in all of the

studied compounds, as well as an interplay between dark and

emissive excited states that may be modulated by the ancillary

ligands of the rhenium.

EXPERIMENTAL SECTION

■

Materials. Commercially available reagents were used as received.

Literature procedures were used to prepare 1,10-phenanthroline-5,6-

dione⁵⁶and 11-bromodipyrido[3,2-a:2′,3′-c]phenazine.³²Rhenium

complexes were synthesized by previously reported methods.^23,34,57

General Procedure for the Preparation of dppz Ligands

dppz-PhNPh₂and dppz-PhNMe₂. To a deoxygenated mixture of

11-bromodipyrido[3,2-a:2′,3′-c]phenazine (∼1.5 mmol), boronic acid

(∼3.1 mmol), and K₂CO₃(∼15 mmol) in toluene (50 mL), water (20

mL), and EtOH (10 mL) was added dichloro[1,1′-bis-

(diphenylphosphino)ferrocene]palladium(II) (∼0.15 mmol). The

reaction mixture was heated at reﬂux under an Ar atmosphere

overnight and cooled and the volatiles removed under reduced

pressure. The reaction material was washed with copious amounts of

water and CH₃CN. The crude product was puriﬁed by preparative

column chromatography (CHCl₃, Al₂O₃) and recrystallized by hexane

diﬀusion into a CHCl₃solution.

11-(4-Diphenylaminophenyl)dipyrido[3,2-a:2′,3′-c]phenazine

(dppz-PhNPh₂). Following the general procedure with 11-

bromodipyrido[3,2-a:2′,3′-c]phenazine (0.504 g, 1.40 mmol), 4-

(diphenylaminophenyl)boronic acid (0.517 g, 1.79 mmol), K₂CO₃

(2.07 g, 14.9 mmol), and dichloro[1,1′-bis(diphenylphosphino)-

ferrocene]palladium(II) (0.177 g, 0.216 mmol), dppz-PhNPh₂was

isolated (0.696 g, 74%) as an orange crystalline solid. ¹H NMR

(CDCl₃, 500 MHz): δ 9.64 (m, 2H, H_1,8), 9.28 (td, J = 1.6, 4.4 Hz, 2H,

H_3,6), 8.50 (d, J = 1.9 Hz, 1H, H₁₀), 8.37 (d, J = 8.8 Hz, 1H, H₁₂), 8.20

(dd, J = 2.1, 8.9 Hz, 1H, H₁₃), 7.80 (dd, J = 7.9, 4.2 Hz, 2H, H_2,7), 7.75

(d, J = 8.7 Hz, 2H, H_Ph), 7.33 (t, J = 8.5 Hz, 4H, NPh₂), 7.24 (d, J =

8.7 Hz, 2H, Ph), 7.20 (dd, J = 8.5, 1.1 Hz, 4H, NPh₂), 7.10 (tt, J = 8.5,

1.2 Hz, 2H, NPh₂) ppm. ¹³C NMR (CDCl₃, 125 MHz): δ 152.6,

152.5, 148.7, 148.0, 147.5, 143.1, 142.9, 142.0, 141.6, 140.7, 134.0,

133.9, 132.6, 130.5, 129.9, 129.6, 129.4, 129.3, 128.4, 127.9, 127.8,

125.5, 125.1, 124.4, 124.3, 123.7, 123.4, 122.18 ppm. HRMS (ESI)

Calcd for C₃₆H₂₄N₅([M + H]⁺): m/z 526.2026. Found: 526.2035.

Anal. Calcd for C₃₆H₂₃N₅·CHCl₃: C, 68.90; H, 3.75; N, 10.86. Found:

C, 69.01; H, 3.71; N, 10.93.

Figure 1. Dppz ligand structure with ring labels and numbering, and

phenazine (phz) LUMO and phenanthroline (phen) LUMO + 1.

energy of the b₁(phen) state was modiﬁed through substitution

at the A and C rings.²⁰Recent studies have used dppz as an

acceptor motif, connected to sulfur-based donors.^{27,30,35−40}Jia

et al. demonstrated a tetrathiafulvalene (TTF)−dppz donor−

acceptor (D−A) couple with a change in dipole moment of 16

D.³⁸Goze et al. then demonstrated the same ligand as a series

of ruthenium complexes: [Ru(bpy)₂(dppz-TTF)]²⁺, [Ru(bpy)-

(dppz-TTF)₂]²⁺, and [Ru(dppz-TTF)₃]²⁺.³⁷They proposed

1

that initial excitation could occur as either ILCT or MLCT

1

transitions, and that the MLCT could undergo an electron-

transfer process with a neighboring ligand (either bpy or dppz-

TTF, depending on the complex) to do one of three things:

form a ligand−ligand charge-separated (LLCS) state, inter-

system cross to a ³MLCT state, or undergo an electron-transfer

event with the TTF of the same ligand to form the ¹ILCT state.

1

3

Both the ILCT and MLCT states are emissive, consistent

with the dual emission observed.

These complexes showed isolated ILCT and MLCT

transitions, whereas the {Re(CO)₃Cl} and {Cu(bpy)}⁺dppz

complexes with appended sulfur-based donors reported by

Fraser et al. show a charge-transfer event that has a

combination of MLCT and ILCT character.^40,41The donation

of electron density from the metal and sulfur-based donor into

the dppz manifests as an absorption band that is much more

intense than the sum of the MLCT and ILCT transitions.

In an attempt to further understand the properties of Re(I)

dppz complexes with appended donor groups, we have

synthesized two dppz ligands with amine donors, dppz-

PhNPh₂and dppz-PhNMe₂and their {Re(CO)₃X} (X = Cl,

pyridine and dimethylaminopyridine (dmap)) complexes

(Figure 2). This is the ﬁrst study to investigate amine-

substituted donor−acceptor Redppz systems although the use

of triarylamine-based donor groups has been heavily inves-

tigated in a range of donor−acceptor studies and for use in

photovoltaic dyes.⁴²⁻⁵⁵

11-(4-Dimethylaminophenyl)dipyrido[3,2-a:2′,3′-c]phenazine

(dppz-PhNMe₂). Following the general procedure with 11-

bromodipyrido[3,2-a:2′,3′-c]phenazine (0.524 g, 1.45 mmol), 4-

(dimethylaminophenyl)boronic acid (0.512 g, 3.10 mmol), K₂CO₃

(2.35 g, 17.0 mmol), and dichloro[1,1′-bis(diphenylphosphino)-

ferrocene]palladium(II) (0.133 g, 0.162 mmol), dppz-PhNMe₂was

isolated (0.420 g, 72%) as an orange crystalline solid. ¹H NMR

(CDCl₃, 500 MHz): δ 9.67−9.62 (m, 2H, H_1,8), 9.28−9.25 (m, 2H,

H_3,6), 8.47 (1H, d, J = 2.1 Hz, H₁₀), 8.34 (d, J = 8.9 Hz, 1H, H₁₃), 8.22

(dd, J = 8.9, 2.1 Hz, 1H, H₁₂), 7.81−7.78 (m, 4H, H_2,7, Ph), 6.89 (d, J

Figure 2. General structure of ligands and Re(I) complexes presented in this Article, where R = Me, Ph and X = Cl, py, dmap. n = 1 when X = py,

dmap; n = 0 when X = Cl.

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= 8.8 Hz, 2H, Ph), 3.08 (s, 6H, NMe₂) ppm. ¹³C NMR (CDCl₃, 125

MHz): 152.6, 152.4, 150.9, 148.5, 148.3, 143.5, 143.3, 141.8, 141.5,

140.3, 133.9, 133.7, 130.5, 129.7, 128.4, 128.0, 127.9, 124.2, 124.2,

112.9, 40.6 ppm. HRMS (ESI) calcd for C₂₆H₂₀N₅([M+H]⁺): m/z

402.1713; found: 402.1715. Elemental analysis calcd for C₂₆H₁₉N₅·

CHCl₃: C, 62.26; H, 3.87; N, 13.45. Found: C, 62.18; H, 3.67; N,

13.63.

General procedure for the preparation of [ReCl(CO)₃(dppz-

PhNPh₂)] and [ReCl(CO)₃(dppz-PhNMe₂)]. A mixture of dppz

ligand (∼0.6 mmol) and pentacarbonylrhenium(I) chloride (∼0.7

mmol) in EtOH (100 mL) was heated at reﬂux overnight. The

reaction mixture was allowed to cool, ﬁltered and the precipitate

washed with EtOH.

fac-Chlorotricarbonyl(11-(4-diphenylaminophenyl)dipyrido[3,2-

a:2′,3′-c]phenazine)rhenium(I), [ReCl(CO)₃(dppz-PhNPh₂)]. Follow-

ing the general procedure with 11-(4-diphenylaminophenyl)dipyrido-

[3,2-a:2′,3′-c]phenazine (0.298 g, 0.568 mmol) and

pentacarbonylrhenium(I) chloride (0.272 g, 0.751 mmol), [ReCl-

(CO)₃(dppz-PhNPh₂)] was isolated (0.429 g, 91%) as a black solid.

¹H NMR (CDCl₃, 400 MHz): δ 9.86 (t, J = 7.5 Hz, 2H), 9.45 (t, J =

5.0 Hz, 2H), 8.57 (s, 1H), 8.46 (d, J = 9.0 Hz, 1H), 8.33 (d, J = 9.3 Hz,

1H), 8.04−8.00 (m, 2H), 7.77 (d, J = 8.5 Hz, 2H), 7.34 (t, J = 7.6 Hz,

4H), 7.23−7.20 (m, 6H), 7.12 (t, J = 7.5 Hz, 2H) ppm. HRMS (ESI)

calcd for C₃₉H₂₃ClN₅NaO₃Re ([M+Na]⁺): m/z 854.0939; found:

854.0810. Elemental analysis calcd. for C₃₉H₂₃ClN₅O₃Re·H₂O: C,

55.15; H, 2.97; N, 8.25. Found: C, 54.89; H, 3.01; N, 8.23.

fac-Chlorotricarbonyl(11-(4-dimethylaminophenyl)dipyrido[3,2-

a:2′,3′-c]phenazine)rhenium(I), [ReCl(CO)₃(dppz-PhNMe₂)]. Follow-

ing the general procedure with 11-(4-dimethylaminophenyl)dipyrido-

[3,2-a:2′,3′-c]phenazine (0.251 g, 0.624 mmol) and

pentacarbonylrhenium(I) chloride (0.258 g, 0.712 mmol), [ReCl-

(CO)₃(dppz-PhNMe₂)] was isolated (0.370 g, 84%) as a dark purple

solid. ¹H NMR (CDCl₃, 400 MHz): δ 9.87−9.82 (m, 2H), 9.46−9.43

(m, 2H), 8.53 (s, 1H), 8.42 (d, J = 8.9 Hz, 1H), 8.35 (d, J = 9.0 Hz,

1H), 8.03−7.99 (m, 2H), 7.85 (d, J = 8.4 Hz, 2H), 7.05−6.91 (m,

2H), 3.14 (s, 6H) ppm. HRMS (ESI) Calcd for C₂₉H₂₁N₅O₄Re ([M −

Cl + H₂O]⁺): m/z 690.1146. Found: 690.1107. Anal. Calcd for

C₂₉H₁₉ClN₅O₃Re: C, 49.26; H, 2.71; N, 9.90. Found: C, 49.27; H,

2.69; N, 9.96.

= 8.9 Hz, 1H), 8.45 (dd, J = 13.0, 7.1 Hz, 2H), 7.96 (d, J = 8.5 Hz,

2H), 6.98 (d, J = 8.6 Hz, 2H), 3.10 (s, 6H), 2.21 (s, 3H) ppm.

General Procedure for the Preparation of [Re(CO)₃(dppz-

PhNPh₂)(py)]PF₆and [Re(CO)₃(dppz-PhNMe₂)(py)]PF₆. A mix-

ture of acetonitrile complex (∼0.15 mmol) and pyridine (∼3 mmol) in

THF (50 mL) was heated at reﬂux under an Ar atmosphere overnight.

The reaction was allowed to cool, and then ﬁltered through Celite.

Aqueous NH₄PF₆(∼1.8 mmol) was added and the reaction mixture

concentrated under reduced pressure. The crude product was washed

with copious amounts of water.

fac-Pyridinetricarbonyl(11-(4-diphenylaminophenyl)dipyrido-

[3,2-a:2′,3′-c]phenazine)rhenium(I) Hexaﬂuorophosphate, [Re-

(CO)₃(dppz-PhNPh₂)(py)]PF₆. Following the general procedure with

fac-acetonitriletricarbonyl(11-(4-diphenylaminophenyl)dipyrido[3,2-

a:2′,3′-c]phenazine)rhenium(I) hexaﬂuorophosphate (0.126 g, 0.128

mmol), pyridine (0.3 mL, 3.71 mmol), and NH₄PF₆(0.300 g, 1.84

mmol), [Re(CO)₃(dppz-PhNPh₂)(py)]PF₆was isolated (0.122 g,

93%) as a purple solid. ¹H NMR (CDCl₃, 400 MHz): δ 9.98 (t, J = 8.8

Hz, 2H), 9.59 (t, J = 5.9 Hz, 2H), 8.54 (d, J = 1.8 Hz, 1H), 8.42 (d, J =

9.0 Hz, 1H), 8.38 (d, J = 4.9 Hz, 2H), 8.33−8.30 (m, 3H), 7.74 (d, J =

8.6 Hz, 2H), 7.70 (t, J = 8.0 Hz, 1H), 7.38−7.33 (m, 2H), 7.33 (t, J =

7.9, 4H), 7.21 (t, J = 8.5 Hz, 4H), 7.21−7.17 (m, 2H), 7.12 (t, J = 7.4

Hz, 2H) ppm. HRMS (ESI) Calcd for C₄₄H₂₈N₆O₃Re ([M]⁺): m/z

875.1780. Found: 875.1772. Anal. Calcd for C₄₄H₂₈F₆N₆O₃PRe·H₂O:

C, 50.92; H, 2.91; N, 8.10. Found: C, 51.06; H, 2.86; N, 8.06.

fac-Pyridinetricarbonyl(11-(4-dimethylaminophenyl)dipyrido-

[3,2-a:2′,3′-c]phenazine)rhenium(I) Hexaﬂuorophosphate, [Re-

(CO)₃(dppz-PhNMe₂)(py)]PF₆. Following the general procedure with

fac-acetonitriletricarbonyl(11-(4-dimethylaminophenyl)dipyrido[3,2-

a:2′,3′-c]phenazine)rhenium(I) hexaﬂuorophosphate (0.130 g, 0.151

mmol), pyridine (0.3 mL, 3.71 mmol), and NH₄PF₆(0.300 g, 1.84

mmol), [Re(CO)₃(dppz-PhNMe₂)(py)]PF₆was isolated (0.126 g,

93%) as a purple solid. ¹H NMR ((CD₃)₂CO, 400 MHz): δ 10.03 (m,

2H), 9.96 (m, 2H), 8.70 (d, J = 5.7 Hz, 2H), 8.55 (s, 1H), 8.53−8.43

(m, 4H), 7.93 (d, J = 8.3 Hz, 3H), 7.41 (t, J = 6.8 Hz, 2H), 6.96 (d, J =

8.4 Hz, 2H), 3.09 (s, 6H) ppm. HRMS (ESI) Calcd for

C₃₄H₂₄N₆O₃Re ([M]⁺): m/z 751.1467. Found: 751.1495. Anal.

Calcd for C₃₄H₂₄F₆N₆O₃PRe·H₂O: C, 44.69; H, 2.87; N, 9.20.

Found: C, 44.71; H, 2.83; N, 9.10.

General Procedure for the Preparation of [Re(CO)₃(dppz-

PhNPh₂)(dmap)]PF₆and [Re(CO)₃(dppz-PhNMe₂)(dmap)]PF₆. A

mixture of acetonitrile complex (∼0.15 mmol) and 4-dimethylamino-

pyridine (∼3 mmol) in THF (50 mL) was heated at 70 °C under an

Ar atmosphere overnight. The reaction was allowed to cool, and then

ﬁltered through Celite. Aqueous NH₄PF₆(∼1.8 mmol) was added and

the reaction mixture concentrated under reduced pressure. The crude

product was washed with copious amounts of water.

General Procedure for the Preparation of [Re(CO)₃(dppz-

PhNPh₂)(CH₃CN)]PF₆and [Re(CO)₃(dppz-PhNMe₂)(CH₃CN)]PF₆.

A mixture of rhenium complex (∼0.3 mmol) and AgOTf (∼1 mmol)

in CH₃CN (100 mL) was heated at reﬂux under an Ar atmosphere

overnight. The reaction was allowed to cool, and then ﬁltered through

Celite. To the reaction was added aqueous NH₄PF₆(∼1.8 mmol), and

the reaction was concentrated under reduced pressure. The crude

1

product was washed with water, and then characterized by H NMR

fac-(4-Dimethylaminopyridine)tricarbonyl(11-(4-

diphenylaminophenyl)dipyrido[3,2-a:2′,3′-c]phenazine)rhenium(I)

Hexaﬂuorophosphate, [Re(CO)₃(dppz-PhNPh₂)(dmap)]PF₆. Follow-

ing the general procedure with fac-acetonitriletricarbonyl(11-(4-

diphenylaminophenyl)dipyrido[3,2-a:2′,3′-c]phenazine)rhenium(I)

hexaﬂuorophosphate (0.120 g, 0.122 mmol), 4-dimethylaminopyridine

(0.372 g, 3.04 mmol), and NH₄PF₆(0.300 g, 1.84 mmol),

[Re(CO)₃(dppz-PhNPh₂)(dmap)]PF₆(0.121 g, 93%) was isolated

as a purple solid. ¹H NMR (CDCl₃, 400 MHz): δ 9.98 (t, J = 7.0 Hz,

2H), 9.51 (t, J = 4.7 Hz, 2H), 8.55 (s, 1H), 8.43 (d, J = 8.9 Hz, 1H),

8.31 (d, J = 8.3 Hz, 1H), 8.25 (m, 2H), 7.75 (d, J = 8.6 Hz, 2H), 7.67

(m, 2H), 7.34 (t, J = 8.1 Hz, 4H) 7.24−7.19 (m, 6H), 7.12 (t, J = 7.4

Hz, 2H), 6.34 (m, 2H), 2.84 (s, 6H) ppm. HRMS (ESI) Calcd for

C₄₆H₃₃F₆N₇O₃PRe ([M]⁺): m/z 918.2202. Found: 918.2184. Anal.

Calcd for C₄₆H₃₃F₆N₇O₃PRe·2H₂O: C, 50.27; H, 3.39; N, 8.92.

Found: C, 50.61; H, 3.40; N, 8.93.

fac-(4-Dimethylaminopyridine)tricarbonyl(11-(4-

dimethylaminophenyl)dipyrido[3,2-a:2′,3′-c]phenazine)rhenium(I)

Hexaﬂuorophosphate, [Re(CO)₃(dppz-PhNMe₂)(dmap)]PF₆. Follow-

ing the general procedure with fac-acetonitriletricarbonyl(11-(4-

dimethylaminophenyl)dipyrido[3,2-a:2′,3′-c]phenazine)rhenium(I)

hexaﬂuorophosphate (0.120 g, 0.122 mmol), 4-dimethylaminopyridine

(0.372 g, 3.04 mmol), and NH₄PF₆(0.300 g, 1.84 mmol),

[Re(CO)₃(dppz-PhNMe₂)(dmap)]PF₆was isolated (0.132 g, 95%)

spectroscopy and reacted without further puriﬁcation or further

characterization.

fac-Acetonitriletricarbonyl(11-(4-diphenylaminophenyl)dipyrido-

[3,2-a:2′,3′-c]phenazine)rhenium(I) Hexaﬂuorophosphate, [Re-

(CO)₃(dppz-PhNPh₂)(CH₃CN)]PF₆. Following the general procedure

with fac-chlorotricarbonyl(11-(4-diphenylaminophenyl)dipyrido[3,2-

a:2′,3′-c]phenazine)rhenium(I) (0.232 g, 0.278 mmol), AgOTf

(0.239 g, 0.932 mmol), and NH₄PF₆(0.300 g, 1.84 mmol),

[Re(CO)₃(dppz-PhNPh₂)(CH₃CN)]PF₆was isolated (0.249 g,

1

91%) as a purple solid. H NMR (CDCl₃, 400 MHz): δ 10.02 (t, J

= 7.5 Hz, 2H), 9.36 (t, J = 5.5 Hz, 2H), 8.57 (s, 1H), 8.46 (d, J = 9.0

Hz, 1H), 8.32 (d, J = 9.0 Hz, 1H), 8.15 − 8.10 (m, 2H), 7.76 (d, J =

8.3 Hz, 2H), 7.34 (t, J = 7.7 Hz, 4H), 7.26 − 7.20 (m, 6H), 7.12 (t, J =

7.7 Hz, 2H), 2.20 (s, 3H) ppm.

fac-Acetonitriletricarbonyl(11-(4-dimethylaminophenyl)dipyrido-

[3,2-a:2′,3′-c]phenazine)rhenium(I) Hexaﬂuorophosphate, [Re-

(CO)₃(dppz-PhNMe₂)(CH₃CN)]PF₆. Following the general procedure

with fac-chlorotricarbonyl(11-(4-dimethylaminophenyl)dipyrido[3,2-

a:2′,3′-c]phenazine)rhenium(I) (0.246 g, 0.348 mmol), AgOTf

(0.239 g, 0.932 mmol), and NH₄PF₆(0.300 g, 1.84 mmol),

[Re(CO)₃(dppz-PhNMe₂)(CH₃CN)]PF₆was isolated (0.257 g,

1

86%) as a purple solid. H NMR ((CD₃)₂CO, 400 MHz): δ 10.06

(m, 2H), 9.71 (m, J = 4.6, 8.6 Hz, 2H), 8.60−8.57 (m, 2H), 8.51 (d, J

C

dx.doi.org/10.1021/ic402082m | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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1

as a purple solid. H NMR (CDCl₃, 400 MHz): δ 10.04−9.99 (m,

(Fc*) couple (−0.012 V in CH₂Cl₂, 0.001 V in CH₃CN) and are

reported relative to the saturated calomel electrode (SCE) for

comparison with other data by subtracting 0.045 V. All IR

spectroelectrochemical measurements were performed in an in-house

built OTTLE cell which has been described previously.⁶⁵Brieﬂy, the

cell comprises two CaF₂windows sandwiched to give a path length of

approximately 200 μm. The working and counter electrodes are pieces

of Pt gauze (area approximately 0.25 mm²). Speciﬁc alignment of the

cell allows only the measurement of the working electrode gauze. The

pseudoreference electrode of the cell is Ag wire. For each

measurement the potential was decreased in gradual steps and held

at the particular potential until stabilization had been observed, and IR

spectra were recorded at each point. All spectra acquired with this

setup were recorded with a Nicolet Nexus 380 FTIR spectrometer,

ﬁtted with an MCT detector, and recorded at a resolution of 2 cm⁻¹.

Spectroscopy. Steady state absorption spectra were recorded as

solutions on a Perkin-Elmer Lambda 950 UV−vis−NIR spectrometer.

FT-Raman spectra were recorded on solid samples at room

temperature, using 196−256 scans and 30 mW power, with a spectral

resolution of 4 cm⁻¹. A Bruker Equinox IFS-55 interferometer was

used with a FRA/106 S attachment and an excitation wavelength of

1064 nm from a Nd:YAG laser source, and was controlled using the

Bruker Opus v5.5 software package. Raman photons were detected by

a liquid nitrogen cooled Ge diode (D418T).

Resonance Raman spectra were recorded using a previously

described setup.⁶⁶⁻⁶⁹Solutions were ∼1 mM in spectroscopic grade

toluene, CH₂Cl₂, CHCl₃, CH₃CN, and dimethylformamide. Excitation

wavelengths of 350.7, 356.4, 406.7, and 413.1 nm were obtained from

a krypton-ion laser (Innova I-302, Coherent Inc.), 448.0 and 532.0 nm

from solid-state diode lasers (CrystaLaser), and 488.0 and 514.5 nm

from an argon-ion laser (Innova Sabre, Coherent Inc.). Laser power at

the sample was typically 30 mW. The incident beam and collection

lens were arranged in a 135° backscattering geometry to reduce loss of

Raman intensity by self-absorption. Scattered light was focused by an

aperture-matched lens through a narrow-bandpass ﬁlter (Ruggate) to

remove Rayleigh scattering and a quartz wedge (Spex) to remove

polarization bias, and onto the entrance slit (50 μm) of a spectrograph

(Acton Research SpectraPro 500i). The collected light was dispersed

in the horizontal plane by a 1200 grooves mm⁻¹ruled diﬀraction

grating (blaze wavelength 500 nm). The light was detected by a liquid-

nitrogen cooled back-illuminated Spec-10:100B CCD controlled by a

ST-133 controller and WinSpec32 (v2.5.8.1) software (Roper

Scientiﬁc, Princeton Instruments). Calibration was performed using

a 1:1 v/v toluene/acetonitrile solution, with Raman peak positions

reproducible to ∼1 cm⁻¹. Samples were held in a spinning NMR tube

and spectra obtained with a 5 cm⁻¹resolution.

Steady state emission spectra of the ligands were recorded on a

Perkin-Elmer LS50B luminescence spectrometer. Steady state

emission spectra of the Re complexes were recorded on the same

setup as resonance Raman spectra, as previously described,⁶⁶⁻⁶⁹with

two alterations: ﬁrst, the removal of the notch ﬁlter before dispersion

of the photons at the spectrograph, and, second, the use of a 300

grooves mm⁻¹grating. Variable temperature emission spectra were

recorded on this same setup, using a TLC50/Edinburgh/E (Quantum

Northwest) temperature control system.

Excited-state emission and absorption transients were acquired

using a LP920K transient absorption (TA) system (Edinburgh

Instruments). Excitation was carried out using pulsed second-

harmonic radiation (532 nm) from a Brilliant (Quantel) Nd:YAG

laser operating at 1 Hz and, in TA mode, a Xe900 450W xenon arc

lamp controlled by an xP900 lamp pulser was used as the probe

source. The photons were dispersed using a TMS300-A Czerny−

Turner monochromator with a 1800 grooves mm⁻¹grating, recorded

on a R928 (Hamamatsu) photomultiplier, and transcribed on a

TDS3012C (Tektronix) digital oscilloscope. A TLC50/Edinburgh/E

(Quantum Northwest) temperature control system was used.

Adjustment of slit width allowed a single point on the wavelength

axis to be recorded with a spectral bandwidth of 0.1−18 nm.

Fluorescence correction was applied for the transient absorption

2H), 9.51−9.48 (m, 2H), 8.60 (s, 1H), 8.53−8.51 (m, 2H), 8.26−8.22

(m, 2H), 7.97−7.95 (m, 2H), 7.82−7.80 (m, 1H), 7.66−7.63 (m, 2H),

6.74−6.72 (m, 1H), 6.37−6.34 (m, 2H), 3.20 (s, 6H), 2.86 (s, 6H)

ppm. HRMS (ESI) Calcd for C₃₆H₂₉N₇O₃Re ([M]⁺): m/z 794.1889.

Found: 794.1918. Anal. Calcd for C₃₆H₂₉F₆N₇O₃PRe·H₂O: C, 45.19;

H, 3.27; N, 10.25. Found: C, 44.96; H, 3.55; N, 10.24.

X-ray Crystallography. Crystals were attached with Paratone N

to a ﬁber loop supported in a copper mounting pin, and then

quenched in a cold nitrogen stream. Data were collected at 93(1) K

using Mo Kα radiation (sealed tube, graphite monochromated) using a

Bruker Kappa X8 diﬀractometer with APEX II detector. The data

processing were undertaken with SAINT and XPREP.⁵⁸The crystal of

dppz-PhNMe₂was nonmerohedrally twinned with orientation

matrices for the two components identiﬁed using CELL_NOW.⁵⁹

A

multiscan absorption correction was applied to the data.^60,61The

structures were solved by direct methods with SHELXS-97, and

extended and reﬁned with SHELXL-97.^62,63The non-hydrogen atoms

in the asymmetric unit were modeled with anisotropic displacement

parameters and a riding atom model with group displacement

parameters used for the hydrogen atoms. X-ray crystallographic data

of dppz-PhNPh₂and dppz-PhNMe₂ligands is available in CIF format

(Supporting Information). CCDC 953106 and CCDC 953107 contain

the supplementary crystallographic data for this Article. These data can

be obtained free of charge from The Cambridge Crystallographic Data

Centre via www.ccdc.cam.ac.uk/data_request/cif.

Crystal data for dppz-PhNPh₂·CHCl₃follow: C₃₇H₂₄Cl₃N₅, M =

644.96, orange plate, 0.57 × 0.30 × 0.03 mm³, triclinic, a = 6.047(2) Å,

b = 10.171(2) Å, c = 25.288(6) Å, α = 85.675(9)°, β = 83.516(9)°, γ =

76.627(9)°, V = 1501.5(5) Å³, space group P1 (No. 2), Z = 2, μ(Mo

̅

Kα) = 0.343 mm⁻¹, 2θ_max= 50.12°, 24 095 reﬂections measured, 5268

independent reﬂections (R_int= 0.0772). The ﬁnal R1(F) = 0.0501 (I >

2σ(I)); 0.1048 (all data). The ﬁnal wR2(F²) = 0.1006 (I > 2σ(I));

0.1199 (all data). GOF = 1.006. CCDC 953106.

Crystal data for dppz-PhNMe₂·CHCl₃: C₂₇H₂₀Cl₃N₅, M = 520.83,

orange needle, 0.53 × 0.08 × 0.07 mm³, triclinic, a = 6.892(1) Å, b =

10.453(2) Å, c = 16.852(2) Å, α = 89.638(7)°, β = 85.067(6)°, γ =

73.619(6)°, V = 1160.3(3) Å³, space group P1 (No. 2), Z = 2, μ(Mo

̅

Kα) = 0.423 mm⁻¹, 2θ_max= 50.16°, 23 700 reﬂections measured, 4047

independent reﬂections (R_int= 0.0674). The ﬁnal R1(F) = 0.0614 (I >

2σ(I)); 0.0955 (all data). The ﬁnal wR2(F²) = 0.1296 (I > 2σ(I));

0.1484 (all data). GOF = 1.060. CCDC 953107.

Physical Measurements. Aldrich spectroscopic grade or HPLC

grade solvents were used for all spectroscopic measurements.

¹H NMR spectra were recorded at either 400 MHz on a Varian

400MR spectrometer or at 500 MHz on a Varian 500AR spectrometer.

¹³C NMR spectra were recorded at 125 MHz on a Varian 500AR

spectrometer. All samples were recorded at 25 °C in 5 mm diameter

tubes. Chemical shifts were referenced internally to residual non-

perdeuterated solvent using δ values as reported by Gottlieb et al.⁶⁴

Electrospray ionization high resolution mass spectra were recorded

on a Bruker MicroTOF-Q mass spectrometer operating in positive

mode.

Analysis of elemental composition was made by the Campbell

Microanalytical Laboratory at the University of Otago, Dunedin, New

Zealand, using a Carlo Erba 1108 CHNS combustion analyzer. The

estimated error in the measurements is 0.3%.

Electrochemistry and Spectroelectrochemistry. The electro-

chemical cell for cyclic voltammetry was made up of a 1 mm diameter

platinum rod working electrode embedded in a KeL-F cylinder with a

platinum auxiliary electrode and a Ag/AgCl reference electrode. The

potential of the cell was controlled by an EG&G PAR 273 A

potentiostat with model 270 software. Solutions were typically about

10⁻³M in CH₂Cl₂and CH₃CN with 0.1 M tetrabutylammonium

hexaﬂuorophosphate (Bu₄NPF₆) as supporting electrolyte, and were

purged with nitrogen for approximately 5 min prior to measurement.

The Bu₄NPF₆was recrystallized from ethyl acetate and dried under

reduced pressure at 70 °C for 10 h, and then kept at 90 °C until use.

The scanning speed was 1 V s⁻¹, and the cyclic voltammograms were

calibrated against the decamethylferrocenium/decamethylferrocene

D

dx.doi.org/10.1021/ic402082m | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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a

Scheme 1. Synthesis of Compounds Presented in This Article (R = Ph, Me; X = H, NMe₂)

a

Reaction conditions: (i) 4-bromo-1,2-phenylenediamine, EtOH, room temperature; (ii) 4-(diphenylamino)phenylboronic acid or 4-

(dimethylamino)phenylboronic acid, K₂CO₃, PdCl₂(dppf), toluene, water, EtOH, reﬂux; (iii) pentacarbonylrhenium chloride, EtOH, reﬂux; (iv)

AgOTf, CH₃CN, reﬂux; (v) pyridine or 4-(dimethylamino)pyridine, THF, reﬂux.

spectra as part of the optical density (ΔOD) calculation. An

exponential decay curve was ﬁtted to the output to obtain the lifetime.

TRIR spectroscopy was carried out at the University of

Nottingham, on an apparatus that has been described previously.^70,71

Brieﬂy, the output from a commercial Ti:sapphire oscillator (MaiTai)/

regenerative ampliﬁer system (Spitﬁre Pro, Spectra Physics) is split

and used to generate 400 nm pump pulses and a tunable mid-IR pulse

with a spectral bandwidth of 180 cm⁻¹and a pulse energy of ca. 2 μJ at

2000 cm⁻¹. Half of the IR pulse is reﬂected onto a single-element

mercury cadmium telluride (MCT) detector (Kolmar Technology) to

serve as a reference, and the other half serves as the probe beam, which

is focused and overlaps with the pump beam at the sample position.

The 400 nm pump pulse is optically delayed (up to 3 ns) by a

translation stage (LMA Actuator, Aerotech) and focused onto the

sample with a quartz lens. The focus spot of the probe beam was

adjusted to be slightly smaller than that of the pump beam in order to

ensure that the area probed corresponds to excited molecules. The

broad-band-transmitted probe pulse is detected with an MCT array

detector (Infrared Associates), which consists of 128 elements (1 mm

high and 0.25 mm wide). The array detector is mounted in the focal

plane of a 250 mm IR spectrograph (DK240, Spectra Product) with a

150 g mm⁻¹grating, resulting in a spectral resolution of ca. 4 cm⁻¹at

2000 cm⁻¹. The signals from the array detector elements and the

single-element detector are ampliﬁed with a 144-channel ampliﬁer and

digitized by a 16-bit analogue-to-digital converter (IR-0144, Infrared

Systems Development Corp.). A Harrick ﬂowing solution cell with 2

mm thick CaF₂windows and a path length of 350 μm is mounted on a

motorized cell mount that moves the cell rapidly in x and y dimensions

throughout the experiment. Consequently, each laser pulse illuminates

a diﬀerent volume of the sample, reducing heating and degradation of

the sample solution.

vacuo and in a solvent ﬁeld. Time-dependent DFT calculations (TD-

DFT) using the B3LYP and CAM-B3LYP functionals were performed

in a solvent ﬁeld using the polarizable continuum model, implemented

using the keyword “scrf” in dichloromethane, chloroform, toluene, and

acetonitrile, due to the environmental sensitivity these compounds

exhibit. The energy of charge-transfer transitions was underestimated

by B3LYP calculations, while this was overestimated by CAM-B3LYP

calculations, but to a lesser extent, and therefore spectra were

interpreted using CAM-B3LYP calculations. In all calculations, the Re

atom was described using the LANL2DZ basis set, while the remaining

atoms were described using 6-31G(d). Calculated frequencies were

scaled to give the lowest mean absolute deviation (MAD) value. MO

models were generated in Gaussview 4.1, and vibrational modes were

illustrated using Molden.⁷⁴

The electronic absorption and resonance Raman intensities of the

studied compounds were simulated using ﬁtting programs developed

by the Myers-Kelley group⁷⁵⁻⁷⁷which are based on a time-dependent

formulation originally developed by Lee and Heller.⁷⁸The theory

involves a consideration of overlap between an excited wavepacket

(|χ_i(t)⟩) propagating along the Franck−Condon surface with the

ground state wave function (|χ_i⟩ when rationalizing the electronic

absorption process) and the ﬁnal wave function (|χ_f⟩ when

rationalizing the resonance Raman intensities). Experimental Raman

intensities were corrected for self-absorption, diﬀerential bandpass,

spectrograph throughput, and detector sensitivity. Band areas were

obtained using the peak-ﬁtting module in OriginPro v7.5 (Origin Lab

Corporation). The measured solute Raman cross-sections were

converted to absolute diﬀerential Raman cross sections (dσ_R/dΩ)_u

(units Å²sr⁻¹) by calibration relative to solvent absolute cross-section

standards using eq 1

⎛

⎜

⎝

⎞

⎟

⎠

⎛

⎞

⎟

⎠

dσ_R

dΩ

I_uc_vdσ_R

I_vc_udΩ

Spectral data was analyzed using GRAMS A/I (Thermo Scientiﬁc)

and OriginPro v8.0 (Origin Lab Corporation).

⎜

=

⎝

(1)

u

v

Computational Methods. Calculations were performed using the

Gaussian09 package,⁷²and calculated spectra generated using

GaussSum 2.2.5.⁷³DFT calculations utilizing the B3LYP functional

were used to optimize the geometry and calculate vibrational

frequencies for the lowest energy singlet and triplet states, both in

where u refers to solute and v to solvent, I is the intensity of the peak,

and c is the concentration. Various solvents were used in this study

(CH₂Cl₂, CHCl₃, CH₃CN, toluene, DMF), of which the Raman

spectra have been previously reported.^68,79−83

E

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Inorganic Chemistry

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relatively planar, with N5 positioned only 0.053 Å above the

plane deﬁned by the ipso-carbons of the three aryl substituents

(C22, C25, C31). The two triarylamine phenyl groups are

rotated from the phenylene linker, with torsion angles of

−38.2° and +130.9°. The phenylene linker is rotated −22.8°

from the dppz body in dppz-PhNPh₂, while only −5.7° in dppz-

PhNMe₂. As this deviation from coplanarity is only slight in

both cases, some retention of conjugation from the dppz to the

donor nitrogen is likely. This conjugation pathway manifests

itself in the bifurcated hydrogen-bonding of CHCl₃to N1 and

N2 of dppz, which is biased toward N2 (from the CHCl₃

proton, 2.243 Å to N2 versus 2.515 Å to N1 for dppz-PhNPh₂

and 2.252 Å to N2 versus 2.372 Å to N1 for dppz-PhNMe₂).

The experimental crystallographic data were compared to

calculated data. Calculations modeled the rotation of the

phenylene linker from the dppz body to be 36.9° for dppz-

PhNPh₂and 38.5° for dppz-PhNMe₂, overestimated compared

to the experimental values. The twist of the two triarylamine

phenyl groups from the phenylene linker for dppz-PhNPh₂was

calculated with torsion angles of −44.0° and +137.0°,

consistent with the experimental results. The greatest diﬀerence

in bond length between experimental and calculated values is

0.026 Å for dppz-PhNPh₂. There is greater deviation in bond

length between experimental and calculated values for dppz-

PhNMe₂, with the largest diﬀerence being 0.053 Å. Both of

these deviations indicate that there is a good ﬁt between

calculated and experimental results.

RESULTS AND DISCUSSION

■

Synthesis. The synthesis of the ligands and complexes is

summarized in Scheme 1. The two ligands, dppz-PhNPh₂and

dppz-PhNMe₂, were prepared by Suzuki−Miyaura palladium-

catalyzed cross-coupling between 11-bromodipyridophenazine

and an amino-functionalized phenylboronic acid, R₂NC₆H₄B-

(OH)₂(R = Ph, Me), in yields of 74% and 72%, respectively.

Both ligands were isolated as bright orange solids, in contrast to

the yellow color characteristic of previously reported dppz

ligands.^31,34,38These two ligands are the ﬁrst reported examples

of diaryl and dialkyl amine-appended dppz systems.

Following previously reported methods for the preparation

of [ReCl(CO)₃(dppz)] complexes,^34,57[ReCl(CO)₃(dppz-

PhNPh₂)] and [ReCl(CO)₃(dppz-PhNMe₂)] were synthesized

in yields of 91% and 84%, respectively, as dark solids. Exchange

of the chloride ligand for an CH₃CN ligand was accomplished

by chloride abstraction with AgOTf in CH₃CN,²³to give

[Re(CO)₃(dppz-PhNPh₂)(CH₃CN)]PF₆and [ReCl-

(CO)₃(dppz-PhNMe₂)(CH₃CN)]PF₆in yields of 91% and

86%, respectively. Reaction of these CH₃CN complexes with

pyridine and 4-(dimethylamino)pyridine yielded [Re-

(CO)₃(dppz-PhNPh₂)(py)]PF₆(93%), [Re(CO)₃(dppz-

PhNPh₂)(dmap)]PF₆(93%), [Re(CO)₃(dppz-PhNMe₂)(py)]-

PF₆(93%), and [Re(CO)₃(dppz-PhNMe₂)(dmap)]PF₆(95%).

X-ray crystal structures were obtained for both of the ligands,

dppz-PhNPh₂and dppz-PhNMe₂(Figure 3), using a crystal

In order to eﬀectively use the resonance Raman data one can

calculate the vibrational frequencies and resulting FT-Raman

spectra of the complexes and compare these to the

experimental data;^29,40,41,71in addition to assisting in the

band assignments, this is also a test of the reliability of the

calculation.^{6,31,40,41,84−87}The goodness-of-ﬁt between the

experimental and calculated Raman data may be parametrized

by the mean absolute deviation (MAD) between the

frequencies of the most intense peaks.^{40,41,66,71,85,88,89}Figure

4 shows a comparison between the experimental and calculated

Raman spectrum for dppz-PhNPh₂, which has a MAD of 7

cm⁻¹. The MADs for the other studied compounds range

between 5 and 11 cm⁻¹. These are suﬃciently small enough to

suggest that the calculations adequately model the studied

compounds.^40,41,90,91

Electrochemistry. Electrochemical data for the two ligands

and their Re(I) complexes in CH₂Cl₂are presented in Table 1.

A number of computational and electrochemical studies have

indicated that the LUMO in a wide range of dppz ligands and

Re(I) complexes is phenazine-based,^{1,4,8,33,40,92}on the basis of

the observation that the reduction potentials of dppz (−1.28 V

vs SCE) occur much closer to that of phenazine (−1.4 V vs

SCE) than that of 2,2′-bipyridine (−2.35 V vs SCE).¹This is

further supported by the results obtained from IR spectroelec-

trochemical measurements. Following reduction of [ReCl-

(CO)₃(dppz-PhNPh₂)] (Supporting Information Figure S1) in

addition to a shift to lower energy of the ν(CO) bands, several

bands were also observed in the 1350−1300 cm⁻¹region of the

IR spectrum. The positions of these bands are consistent with

the formation of a [phz]⁻species.⁹³Similar spectral features

were observed in the spectra of the [Re(CO)₃(dppz-PhNPh₂)-

(dmap)]PF₆and [Re(CO)₃(dppz-PhNPh₂)(py)]PF₆com-

plexes (Supporting Information Figures S2 and S3). The

change in reduction potential is therefore a good indicator of

the alteration to the phz MO by the substitution and

complexation. Although it is expected that appending

Figure 3. X-ray crystal structures of dppz-PhNPh₂(top) and dppz-

PhNMe₂(bottom), both H-bonded with a molecule of CHCl₃. Atomic

displacement ellipsoids are shown at 50% probability.

grown from hexane diﬀusion into a CHCl₃solution in both

cases. Both ligands crystallized in the triclinic P1 space group.

̅

The dppz skeleton is virtually planar in both cases, with a

maximum deviation from planarity at C18 of 0.171 Å for dppz-

PhNPh₂and a maximum deviation at C16 of 0.175 Å for dppz-

PhNMe₂. For dppz-PhNPh₂, the triarylamine adduct is

F

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Inorganic Chemistry

Article

have very little wave function amplitude on the chelating

nitrogens. This implies that the segregated orbital model does

not necessarily apply to these compounds. As expected, the

oxidation potentials of the amine-based donors were similar

across the range of compounds, and are consistent with

literature values.^43,47,94,95

In order to probe the solvent interaction of these species, the

electrochemical properties of dppz-PhNPh₂, [ReCl-

(CO)₃(dppz-PhNPh₂)], dppz-PhNMe₂, and [ReCl-

(CO)₃(dppz-PhNMe₂)] were further investigated in CH₃CN

and compared to those in CH₂Cl₂(Table 2). The reduction

Table 2. Electrochemical Data for the Ligands and

[ReCl(CO)₃(dppz-Ph-NR₂)]ⁿ⁺(R = Ph, Me) Complexes in

CH₂Cl₂and CH₃CN

CH₂Cl₂

CH₃CN

E

_red/V

E_ox/V E_ox

−

E_red/V

E_ox/V E_ox−

compd

vs SCE vs SCE E_red

dppz-PhNPh₂

−1.23

−0.97

1.03

1.05

2.26

2.02

−0.87

−0.81

1.04

1.07

1.91

1.88

[ReCl(dppz-

PhNPh₂)

(CO)₃]

dppz-PhNMe₂

−1.24

−1.01

0.91

0.95

2.15

1.96

−1.15

−0.77

0.91

1.08

2.06

1.85

[ReCl(dppz-

PhNMe₂)

(CO)₃]

potential (E_red) shifts to more positive potentials in CH₃CN;

this suggests that the LUMO is decreasing in energy upon

increasing solvent polarity. This is further examined using

resonance Raman excitation proﬁles, vide infra. The oxidation

potential was relatively unaﬀected upon changing solvent.

Electronic Absorption Spectroscopy. The electronic

absorption spectra of dppz-PhNPh₂and its Re(I) complexes are

presented in Figure 5. The electronic absorption spectra of the

Figure 4. Calculated (black) versus experimental (red) Raman spectra

of dppz-PhNPh₂.

Table 1. Electrochemical Data for the Ligands and

Complexes in CH₂Cl₂

a

b

a

b

E_red(ΔR) /V vs

E_ox(ΔR) /V vs

compd

SCE

c

dppz

dppz-PhNPh₂

−1.28

−1.23

1.03

[ReCl(dppz-PhNPh₂)(CO)₃]

−0.97 (0.26)

−0.88 (0.35)

1.05 (0.02)

[Re(dppz-PhNPh₂)

(CO)₃(dmap)]PF₆

[Re(dppz-PhNPh₂)(CO)₃py]PF₆

dppz-PhNMe₂

−0.85 (0.38)

−1.24

1.05 (0.02)

0.91

[ReCl(dppz-PhNMe₂)(CO)₃]

−1.01 (0.23)

−0.92 (0.32)

0.95 (0.04)

0.96 (0.05)

[Re(dppz-PhNMe₂)

(CO)₃(dmap)]PF₆

[Re(dppz-PhNMe₂)(CO)₃py]PF₆

−0.81 (0.43)

1.04 (0.13)

a

E_redand E_oxare recorded against (Fc*)⁺/Fc* and converted to SCE.

Solvent CH₂Cl₂, supporting electrolyte Bu₄NPF₆. Irreversible

b

oxidations were observed toward the solvent limit. The change in

potential from that of the corresponding ligand. Previously studied,³¹

c

included for comparison.

electron-donating groups to the 11-position of dppz can result

in an increase in the reduction potential of the phz MO, a

decrease is observed. This was also observed for sulfur-based

donor dppz systems by Fraser et al.,⁴¹who attributed the

lowering of the reduction potential to the extension of

conjugation of the phz MO. It can also be observed that the

ancillary ligand of the Re complexes aﬀects the reduction

potential, which lowers in the order Cl > dmap > py. This order

is consistent with the π-accepting capabilities of the ancillary

ligands, with py being the most capable and Cl the worst π-

acceptor. The fact that the Re ancillary ligand aﬀects the

reduction potential is unexpected, as phenazine-based orbitals

Figure 5. Absorption spectra of dppz-PhNPh₂and [Re(CO)₃(dppz-

PhNPh₂)X]ⁿ⁺(X = Cl, n = 0; X = py, dmap, n = 1) complexes in

CH₂Cl₂.

two ligands show a low energy band at 457 nm (dppz-PhNPh₂)

and 463 nm (dppz-PhNMe₂). This band is signiﬁcantly red-

shifted from that of dppz (378 nm). A similar band was

observed by Fraser et al. for dppz ligands appended with sulfur-

based donors and assigned as an ILCT band.⁴⁰In {Re(CO)₃X}

systems it is possible to vary the dπ MO energies by the

ancillary substituents at X such as py, Cl⁻, and DMAP.¹⁰The

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Table 3. Experimental and Calculated Electronic Absorption Data and Change in Mulliken Charge Density Calculations

expt

calcd

Mulliken charge density change (%)

λ/

ε/M⁻¹

λ/

compd

dppz-PhNPh₂

nm

cm⁻¹

nm

f

major MO conﬁguration

metal

Phen

Phz

adduct

457

492

522

519

463

501

530

518

16 900

374

399

414

421

373

395

438

432

0.84

0.82

0.76

0.83

0.69

0.72

H-1 → LUMO (18%), HOMO →

LUMO (69%)

8 → 23

(15)

16 → 71

(55)

75 → 6

(−69)

85 → 5

(−80)

87 → 3

(−84)

86 → 4

(−82)

71 → 3

(−68)

77 → 3

(−74)

49 → 2

(−47)

81 → 3

(−78)

[ReCl(CO)₃(dppz-PhNPh₂)]

17 710

14 400

13 050

13 100

12 400

10 600

9600

H-3 → LUMO (13%), HOMO →

LUMO (75%)

0 → 0(0) 2 → 32

13 → 63

(50)

(30)

[Re(CO)₃(dppz-PhNPh₂)

(py)]PF₆

H-1 → LUMO (12%), HOMO →

LUMO (74%)

1 → 2(1) 2 → 43

11 → 52

(41)

[Re(CO)₃(dppz-PhNPh₂)

(dmap)]PF₆

H-2 → LUMO (11%), HOMO →

LUMO (76%)

0 → 1(1) 2 → 40

12 → 54

(42)

(38)

dppz-PhNMe₂

H-2 → LUMO (12%), HOMO →

LUMO (79%)

8 → 24

(16)

21 → 73

(52)

[ReCl(CO)₃(dppz-

PhNMe₂)]

HOMO → LUMO (80%)

0 → 1(1) 4 → 34

19 → 62

(43)

(30)

[Re(CO)₃(dppz-PhNMe₂)

(py)]PF₆

0.7494 HOMO → LUMO (80%)

0.75 HOMO → LUMO (81%)

4 → 1

(−3)

1 → 1(0) 2 → 43

7 → 44

(37)

41 → 52

(11)

[Re(CO)₃(dppz-PhNMe₂)

(dmap)]PF₆

16 → 52

(36)

(41)

eﬀect of these substituents is to stabilize the dπ MOs (in the

order py, Cl⁻, and DMAP). The shifting of the intense

transition at 450 nm for [ReCl(CO)₃(dppz-PhNPh₂)] on going

to X = py or DMAP is not consistent with an MLCT band

(Figure 5). The experimentally observed shifts in wavelength

and relative absorption intensity are correctly predicted by

TDDFT (Table 3).

For all of the studied compounds, CAM-B3LYP TD-DFT

calculations predicted the charge-transfer band to be predom-

inantly HOMO → LUMO, where the HOMO is mostly

localized on the donor moiety and the LUMO is largely

phenazine-based (Figure 6). Unlike the charge-transfer

phz, and 40% on phen, while for [ReCl(CO)₃(dppz-PhNPh₂)],

50% of the electron density change is predicted to terminate on

phz, and 30% on phen.

Resonance Raman Spectroscopy. The nature of the

Franck−Condon state can be probed by resonance Raman

spectroscopy.^29,96,97Resonance Raman spectroscopy involves

excitation of the probed molecule at the wavelength of an

electronic transition, which results in an enhancement of

vibrational modes that mimic the nature of the electronic

excitation.^71,96,98

The resonance Raman spectra of dppz-PhNPh₂show the

strongest enhancement of a band at 1532 cm⁻¹, with no

obvious frequency shift across multiple excitation wavelengths.

This mode was assigned using TD-DFT calculations as a

phenazine-based mode. Interestingly, the dominant enhance-

ment in the resonance Raman spectra of dppz-PhNMe₂ranged

from 1539 cm⁻¹at the shortest excitation wavelengths to 1530

cm⁻¹at the longest wavelengths. This mode was assigned as

being delocalized over the entire molecule. Both of the ligands

show enhancement of phenylene linker and donor group

modes (1595 cm⁻¹and 1607 cm⁻¹for dppz-PhNPh₂, 1600

cm⁻¹for dppz-PhNMe₂) at excitation wavelengths coinciding

with the charge-transfer band. This is consistent with the donor

group and the dppz body being involved in the electronic

transition in the ligands.

All of the complexes have similar band enhancements to that

of [ReCl(CO)₃(dppz-PhNPh₂)] (Figure 7), ranging between

1530 and 1540 cm⁻¹, all of which are assigned as phenazine

based, delocalized dppz, or delocalized ligand based modes.

The enhancement of phenylene linker and donor-group based

modes at ∼1600 cm⁻¹is also observed, while there appears to

be only very small enhancement of carbonyl based modes at

∼2040 cm⁻¹, indicating very little inﬂuence of the Re in the

electronic transitions. The Franck−Condon state appears to be

dominated by a single charge-transfer transition from the donor

group to dppz. There are only small changes in resonance

Raman spectra across a wide excitation wavelength range. TD-

DFT calculations predict the lowest energy transition in all

compounds as this charge-transfer type from donor to dppz.

Emission Spectroscopy. Solutions of the two ligands are

emissive at room temperature. The emission is unaﬀected by

degassing, suggesting it is singlet in origin. The emission data

are presented in Table 4. The emission from each ligand was

Figure 6. (a) dppz-PhNPh₂HOMO, (b) dppz-PhNPh₂LUMO, (c)

[ReCl(CO)₃(dppz-PhNPh₂)] HOMO, and (d) [ReCl(CO)₃(dppz-

PhNPh₂)] LUMO.

observed by Fraser et al.,⁴⁰our calculations predict no

MLCT character in the charge-transfer bands of the Re

complexes. The ILCT nature of this low-energy band may also

be investigated using Mulliken charge density calculations

(Table 3), which show the loss of electron density on the donor

and gain of electron density on the dppz body as a result of the

transition. It is also interesting to note that, for [Re-

(CO)₃(dppz-PhNPh₂)(py)]PF₆and [Re(CO)₃(dppz-

PhNPh₂)(dmap)]PF₆, which have very similar absorption

spectra, the lowest energy transition is predicted to involve

approximately 40% of the electron density change to end up on

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Figure 7. Resonance Raman spectra of [ReCl(CO)₃(dppz-PhNPh₂)]

recorded at various excitation wavelengths for a 1 mM dichloro-

methane solution. FT-Raman spectrum is shown in red, and was

recorded on a solid sample. Solvent bands are denoted with *.

found to be highly solvatochromic. Ligands dppz-PhNPh₂and

dppz-PhNMe₂show a variability in the emission wavelengths of

3050 and 2150 cm⁻¹, respectively, upon changing solvent from

toluene to CH₂Cl₂. There was no detectable emission in more

polar solvents, such as CH₃CN and DMSO, or protic solvents.

The complexes are weakly emissive in a deoxygenated

solution, suggesting triplet state emission. Again, no emission

was observed in high polarity solvents. The complexes are also

solvatochromic, while the coordination of the ligand to

rhenium results in a bathochromic shift. The relationship

between the Stokes shift and solvent polarity parameter can be

related to a change in dipole moment between the ground and

emissive excited states through the Lippert−Mataga equation

(Figure 8).^99,100With a ﬁt of the emission data to this model, it

was found that the complexes had very similar Δμ to the

ligands (∼22 D and ∼20 D for dppz-PhNPh₂and dppz-

PhNMe₂, respectively). Despite the ligand emission being

attributed to a singlet state and complex emission to a triplet

state, they both appear to have similar character, this being

ILCT.

The variable-temperature emission spectra of dppz-PhNPh₂

in CH₂Cl₂are presented in Supporting Information Figure S4

and show a blue-shift and increased emission intensity with

increasing temperature.

The increase in intensity can be attributed to the emissive

state being in thermal equilibrium with a nonemissive (dark)

state of lower energy, analogous to the behavior of the

molecular light switch, although in this case the nature of these

states need not be MLCT.¹⁹The presence of the dark state is

observed in transient absorption spectroscopy, vide infra. The

dark state has a greater population at low temperature, and the

emissive state becomes more thermally populated with

increasing temperature. It is important to note that the d−d

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based. This implies that the lowest energy triplet state is the

dark state, consistent with the variable-temperature emission

spectroscopy. In the optimized geometry of the lowest energy

triplet state the phenyl linker group is closer to coplanar relative

to the ground state geometry, indicative of extended

conjugation in the excited state. This behavior has been

previously reported.^102,103

The temperature-dependent behavior of [ReCl(CO)₃(dppz-

PhNPh₂)] in CH₂Cl₂, and dppz-PhNPh₂, dppz-PhNMe₂, and

[ReCl(CO)₃(dppz-PhNMe₂)] in toluene, is presented in

Supporting Information Figures S5−S8. The increase in

emission intensity with increasing temperature is even more

pronounced for [ReCl(CO)₃(dppz-PhNPh₂)] in CH₂Cl₂,

indicating a greater separation between the emissive and dark

states than dppz-PhNPh₂. [ReCl(CO)₃(dppz-PhNMe₂)] in

toluene has a decrease of emission intensity with increasing

temperature, indicative of the dark state being higher in energy

than the emissive state.

Typically, an increase in temperature results in a bath-

ochromic shift of the emission band because the solvent−

ﬂuorophore interaction increases, resulting in a stabilization of

the emissive state (and consequent lowering of emission

energy).^104,105However, a slight hypsochromic shift (∼150

cm⁻¹over 70 °C) has been observed for the donor−acceptor

compound laurdan (6-dodecanoyl-2-dimethylaminonaptha-

lene) in EtOH.¹⁰⁶MacGregor and Weber predicted this

phenomenon for the donor−acceptor compound prodan (6-

propionyl-2-dimethylaminonaphthalene).¹⁰⁷They identify

Figure 8. Lippert−Mataga plot for dppz-PhNPh₂and its [Re(CO)₃X]

complexes.

states that can be occupied in Ru-dppz complexes are too high

in energy to be thermally occupied in Re-dppz complexes.¹⁰¹

Triplet state calculations indicate, for all of the studied

compounds, that in the lowest energy triplet state the

electronically excited electron occupies a phenazine-based

orbital and the positive hole is based on the donor moiety

(Supporting Information Table S1). In the segregated orbital

model for dppz excited states,⁴the dark state is phenazine-

Figure 9. Electronic absorption spectra (ﬁtted red line and experimental black line) and resonance Raman excitation proﬁles for several vibrational

bands (experimental black bars and ﬁts red line) of [ReCl(CO)₃(dppz-PhNPh₂)] in CH₂Cl₂and dimethylformamide.

J

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three temperature regimes: (1) a low temperature blue-shift in

which the ﬂuorescence lifetime (τ) is signiﬁcantly less than the

correlation time for solvent relaxation (ρ_s); (2) an intermediate

temperature red-shift in which τ and ρ_sare of comparable

magnitude; (3) a high-temperature blue-shift in which τ is

signiﬁcantly greater than ρ_s. Viard et al. described this same

behavior as a maximum stabilization of the emissive state by the

solvent at a point where the thermal motion is not able to

completely disrupt the orientation of the solvent dipoles, but

the solvent is still ﬂuid enough to completely relax before

emission occurs.¹⁰⁶At this stabilization maximum, an increase

in temperature increases thermal motion enough to disrupt the

orientation of the solvent dipoles, while a decrease in

temperature lessens the interaction between solvent and

ﬂuorophore due to a decrease in solvent ﬂuidity, in both

cases decreasing the stabilization and resulting in a

hypsochromic shift.

Table 5. Electronic Parameters Used To Model the Lowest

Energy Band of [ReCl(CO)₃(dppz-PhNPh₂)]

CH₂Cl₂CHCl₃Toluene DMF CH₃CN

electronic origin (E₀₀)/

17 050 15 800 17 900 16 700 14 800

cm⁻¹

homogeneous broadening

1800

1.46

2150

1.36

2000

1.4

2300

1.79

3900

1.03

(Γ_o)/cm⁻¹

electronic dipole length

(μ)/Å

classical reorganization

1467

2092

1811

2394

6885

energy (λ_s)/cm⁻¹

The ∼230 cm⁻¹hypsochromic shift over 40 °C observed in

the variable-temperature emission studies of dppz-PhNPh₂in

CH₂Cl₂is more signiﬁcant than that observed for laurdan (in

EtOH). It has been shown that increasing temperature

decreases the dielectric constant of a solvent, which in turn

decreases the solvent polarity parameter.¹⁰⁸As the variable-

solvent emission studies have shown the studied compounds to

be very sensitive to a change in solvent parameter, we propose

an analogous model to that of Viard et al.,¹⁰⁶in which there is

competition between the inﬂuence of the decreasing dielectric

constant of the solvent with increasing temperature and the

increased thermal motion of the solvent on the stability of the

emissive state. The decreasing solvent dielectric is a

destabilizing eﬀect, vide supra, and results in a hypsochromic

shift, whereas the increasing thermal motion is a stabilizing

eﬀect, due to increased interaction between the solvent and the

emissive state resulting in a bathochromic shift. According to

this model, all of the compounds studied herein appear to be

more heavily inﬂuenced by the decreasing dielectric constant

with increasing temperature.

Resonance Raman Excitation Proﬁles. The strong

solvent-dependence of the emission, coupled with the modest

solvatochromism in absorption spectra and the evidence of a

number of excited states, encouraged us to examine the

absorbing chromophore to investigate the structural distortion

of the Franck−Condon (FC) state. The solvent-dependence of

the FC states of dppz-PhNPh₂, [ReCl(CO)₃(dppz-PhNPh₂)],

dppz-PhNMe₂, and [ReCl(CO)₃(dppz-PhNMe₂)] was inves-

tigated using resonance Raman excitation proﬁles. Absorption

spectra and resonance Raman cross sections in toluene,

CH₂Cl₂, CHCl₃, dimethylformamide, and CH₃CN were

modeled using Heller and co-worker’s time-dependent wave

packet model.^78,109Resonance Raman excitation proﬁles for

[ReCl(CO)₃(dppz-PhNPh₂)] are presented in Figure 9,

indicating how well the model ﬁts the experimental data.

These analyses provide the structural distortions on going

from ground to absorbing state (ΔQ), and this may be related

to the reorganization energy (λ). The data are summarized in

Table 5 which shows that ΔQ (λ) is larger in DMF and

CH₃CN than in toluene, CH₂Cl₂, or CHCl₃. This suggests that

the energy minimum for the absorbing state is lower in energy

Figure 10. Schematic describing the nature of the Franck−Condon

state leading to the observed optical properties.

emissive in CH₂Cl₂but not in CH₃CN is in accord with the

energy gap law.¹¹⁰The mode speciﬁc data for this compound

are presented in Supporting Information Table S5.

Transient Absorption Spectroscopy. The transient

absorption data for the two ligands and their complexes are

presented in Table 6. Spectral data for [Re(CO)₃Cl(dppz-

PhNR₂)] are shown in Figure 11. All measurements were made

with an excitation wavelength of 532 nm. The emissive lifetimes

of all compounds were less than 10 ns. For the ligands, no

transient absorption measurements were observed at 532 or

Table 6. Transient Absorption Lifetimes (μs) of the Dark

States

dppz-PhNMe₂

dppz-PhNPh₂

CH₂Cl₂

toluene

CH₂Cl₂

toluene

1

in DMF and CH₃CN (Figure 10). If one assumes a ILCT

L

<0.01

2.2

<0.01

6.1

<0.01

3.8

<0.01

1.4

absorbing state, then the lowering of its energy would also

result in a lower energy ³ILCT state. The transient lifetime and

[ReCl(CO)₃(L)]

[Re(CO)₃(L)(dmap)]PF₆

[Re(CO)₃(L)(py)]PF₆

3

2.5

1.1

10.1

1.6

1.8

infrared data support the assignment of a long-lived ILCT

6.2

0.9

1.8

state (vide infra). The observation that the compounds are

K

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TRIR spectra were measured for [ReCl(CO)₃(dppz-

PhNPh₂)] in CH₂Cl₂, toluene, and CH₃CN, and for [Re-

(CO)₃(dppz-PhNPh₂)(py)]PF₆and [Re(CO)₃(dppz-

PhNPh₂)(dmap)]PF₆in CH₂Cl₂. Figure 12 shows the ps-

Figure 11. Change in absorption spectra measured for [Re(CO)₃Cl-

(dppz-PhNPh₂] (black line) and [Re(CO)₃Cl(dppz-PhNMe₂] (red

line) in CH₂Cl₂with 532 nm excitation. Spectra shown are measured

300 ns after excitation. Spectral data for the other complexes are

shown in Supporting Information Figure S9.

Figure 12. (a) ps-TRIR spectra of [Re(CO)₃(dppz-PhNPh₂)(dmap)]-

PF₆in the carbonyl region, at a number of time-delays after

photoexcitation with a 400 nm pulse. (b) The FTIR spectrum of

[Re(CO)₃(dppz-PhNPh₂)(dmap)]PF₆. Marked peak positions were

determined using band-ﬁtting methods.

355 nm. Long-lived excited state TA signals are observed for

the complexes.

The transient absorption spectra (358−798 nm) for the

complexes show two main features: a strong absorption at

∼400 nm and bleaching of the charge-transfer band (∼450−

500 nm). The transient electronic absorption spectra of dppz

complexes show similar features. The signal from [Cu-

(PPh₃)₂dppz]⁺* shows transient absorption at 460 nm.³⁴The

absorption proﬁle of [Re(CO)₃(py)(dppz)]⁺* shows a 460 nm

feature with decreasing absorption to the red;⁹very similar

features are observed for [Re(CO)₃(4Mepy) (dppz)]⁺*.⁵

These features are attributed to an IL ππ* of dppz. The

radical anion UV−vis spectrum of dppz has been reported. This

shows a distinct band at 584 nm for [Re(CO)₃Cl(dppz)]^•−and

587 nm for [Cu(PPh₃)₂(dppz)]^•−.⁹²In our data the appearance

of a band at approximately 400 nm provides little detailed

information on the excited state nature. However, the

observation of absorption bands at λ > 600 nm is atypical of

dppz systems. Such an absorption, at longer wavelengths, is not

TRIR spectra of [Re(CO)₃(dppz-PhNPh₂)(dmap)]PF₆at a

number of time-delays, acquired in CH₂Cl₂. Immediately after

photoexcitation with a 400 nm pulse, depletion of the parent

bands is observed, coincident with the appearance of a new set

of red-shifted peaks. Peak-ﬁtting reveals three sets of bands, as

summarized in Table 7. A decrease in vibrational frequency

Table 7. IR Stretching Frequencies Observed in CH₂Cl₂of

the Excited States of the Complex [Re(CO)₃(dppz-

PhNPh₂)(dmap)]PF₆, after Photoexcitation at 400 nm, and

the Ground State FTIR Spectrum

lifetime/ns

carbonyl stretching

frequencies/cm⁻¹

ligand stretching

frequencies/cm⁻¹

(relative

amplitude)

state

GS

inconsistent with oxidation of the donor moiety. For example,

a

2034, 1930

1634, 1606, 1591, 1558,

•+

a

the radical cation of tris(4-chlorophenyl)amine (TPACl₃

)

1361, 1335, 1317

shows bands at 590 and 680 nm¹¹¹and, for tris(4-

bromophenyl)amine (TPBA^•+), a strong band at 700 nm in

MeCN.¹¹²These data are consistent with the presence of an

excited state with ILCT character.

2015, 1875

2026, 1898

2029, 1906

0.020 (0.22)

0.300 (0.78)

2800

state I

1594, 1571, 1331, 1319

1600, 1547, 1337, 1307

state II

state

III

a

From FTIR spectrum.

Time-Resolved Infrared Spectroscopy. The carbonyl

bands in the infrared may be used to determine the nature of

excited states in {Re(CO)₃X} systems using time-resolved

infrared spectroscopy (TRIR). The backbonding between the

dπ and CO π* orbitals means that oxidation of the metal, that

occurs with MLCT excitation, results in an upshift in frequency

Δν. The magnitude of this shift is modulated by backbonding

across the dπ to CO π* from the ligand π* MO. Thus, in the

case of dppz complexes it is possible to spectroscopically

diﬀerentiate between MLCT in which the dppz phen MO is

populated and that in which the phz MO is populated.

Excitation of [Re(CO)₃(dppzCl₂)(dmap)]⁺forms a mixture of

excited states, one of which is MLCT(phen) in nature. For this

state the Δν = +61 cm⁻¹. For [Re(CO)₃(dppzCl₂)Cl] the

excited state is MLCT(phz) in nature and has Δν = +81 cm⁻¹.

suggests an increased amount of electron density in the

carbonyl antibonding orbitals.^{10,24,26,113,114}The IR spectrum

obtained 1 ps following photolysis shows excited bands at

2026/1898 cm⁻¹and 2015/1875 cm⁻¹. The TRIR reveals the

presence of three excited states (I−III), the spectral features of

which are presented in Table 7. The shortest lived of these

(state I) has bands at 2015 and 1875 cm⁻¹. The decay of this

state occurs with the grow in of a long-lived state (III) with

features at 2029, 1906, and 1547 cm⁻¹. An additional state (II)

with a 300 ps lifetime is observed with bands at 2026, 1898, and

1571 cm⁻¹.

Examination of the CO bands suggests that none of these

states have any MLCT character as Δν (CO) < 0 cm⁻¹. The

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CO bands for states II and III are very similar. For the highest

frequency CO band Δν (CO) = −8 and −5 cm⁻¹, respectively.

These features may be associated with an intraligand state

which may involve the donor moiety. Such shifts have been

characterized for other dppz systems with IL(π,π*)

states.^{24,26,115,116}

Spectroelectrochemical FTIR on the [Re(CO)₃Cl(dppz-

PhNPh₂)]⁰(Supporting Information Figure S1) shows a CO

band at 2025 cm⁻¹. As the reduction is phz-based which is the

likely acceptor MO in the ILCT transition, this supports the

assignment of state II as ILCT. The shifts observed for state III

are also consistent with ILCT state, and these data are

supported by the transient electronic absorption spectra. The

Δν(CO) = −19 cm⁻¹for the highest frequency mode of state I

is greater than normally observed for IL states; these features

may be due to vibrationally hot states, as have been observed in

MLCT states of the TRIR of [Re(CO)₃(dmap)(dppzNO₂)]⁺.²¹

The involvement of the ancillary ligand in the excited state or a

breakdown product of the complex may also play a role.¹¹⁷

State I cannot be unambiguously assigned.

a broad absorption centered around 1325 cm⁻¹. The band at

1571 cm⁻¹together with another band at 1325 cm⁻¹decays

(300 ps) at the same rate as the decay of the ν(CO) bands

assigned to state II. A band at 1547 cm⁻¹grows within ca. 20

ps. The growth of this band is concomitant with the decay of

the 2015 cm⁻¹feature (Figure 14). This implies that state I

decays as state III appears.

The dynamic behavior of the TRIR suggests the presence of

more than one state in the ultrafast time regime. The excited

state bands centered at 1898 and 2026 cm⁻¹decay with a

lifetime of ca. 300 ps concomitant with the partial recovery for

the ground state bands. At early times weak bands at 1875 and

2015 cm⁻¹are observed that decay (in 20 ps) to form a new set

of peaks at 1906 and 2029 cm⁻¹. The positions of these bands

appear unchanged in the ns-TRIR spectra and decay with a

lifetime of 2.8 μs which is consistent with the transient

absorption data described above given the diﬀerent concen-

trations used in these experiments.

Figure 14. Evolution of prominent features in the ps-TRIR spectra of

[Re(CO)₃(dppz-PhNPh₂)(dmap)]PF₆after photoexcitation with a

400 nm pulse.

The small spectral shift in carbonyl vibrations, between 2026

and 2029 cm⁻¹, coupled with the paucity of the bands at 2015

and 1875 cm⁻¹makes interpretation of these data diﬃcult.

Thus, we investigated this further by examining ps-TRIR

spectra in the region of ligand-vibrations. Clearer spectral

diﬀerences exist between spectra in this region, acquired at 50

and 1000 ps, indicating two states of signiﬁcantly diﬀerent

electronic structures are being probed (Figure 13). The

vibrational frequencies are summarized in Table 7. After 1 ps

there are two clear peaks at 1571 and 1543 cm⁻¹together with

These data suggest that states II and III are IL in nature and

have charge-transfer character. State I may be due to a

vibrationally hot state.

The photophysics of [ReCl(CO)₃(dppz-PhNPh₂)] has been

examined in a range of solvents, and in these experiments, we

see the same IR features in the TRIR spectra. In CH₂Cl₂, the

parent bands are depleted, and excited-state features are

observed with a red-shift of ca. 7 cm⁻¹(Supporting Information

Figure S10 and Table S3). We observe no evidence for excited-

state interconversion. The kinetics of the excited-state

absorptions can be ﬁtted to a double exponential with both

components being associated with the ligand. No signiﬁcant

solvent-dependent shifts were observed, Supporting Informa-

tion Table S3. The excited state lifetimes range from 1.6 μs for

CH₃CN to 1.2 μs for toluene.

The TRIR spectra of [Re(CO)₃(dppz-PhNPh₂)(py)]PF₆

showed very similar behavior to [ReCl(CO)₃(dppz-

PhNPh₂)], Supporting Information Table S3. TRIR spectros-

copy revealed two ligand-centered excited states, which can be

assigned as IL states, in accordance with transient absorption,

emission, and spectroelectrochemistry measurements. No

evidence for MLCT states was observed for any of the

complexes, indicating a dominance of the intraligand

transitions, consistent with ﬁndings from other techniques.

CONCLUSIONS

■

The synthesis and characterization of two dppz ligands with

donor substitution (dppz-Ph-NPh₂and dppz-Ph-NMe₂) has

been reported. The spectral properties of these materials are

dominated by very strong ILCT transitions which occur at 450

nm. These transitions shift with complexation to [Re(CO)₃R]ⁿ

Figure 13. ps-TRIR spectra of [Re(CO)₃(dppz-PhNPh₂)(dmap)]PF₆

in the region of ligand vibrations, at a number of time-delays after

photoexcitation with a 400 nm pulse.

M

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Inorganic Chemistry

Article

(8) Atsumi, M.; Gonzal

2007, 190, 310.

́

ez, L.; Daniel, C. J. Photochem. Photobiol., A

(R = Cl, n = 0; R = py, DMAP, n = +1), but they remain

electronically ligand-centered in nature. The MLCT state is not

in evidence. The Franck−Condon state has been probed using

resonance Raman spectroscopy and TDDFT calculations, and

these are consistent with an ILCT transition. The observation

of minimal enhancement of the carbonyl bands suggests the

MLCT transition contributes very little to the observed optical

spectrum.

The ligands are emissive showing strong solvent dependence

consistent with an emissive state that has a large dipole

moment. Resonance Raman excitation proﬁles of the

complexes, which are not emissive, have been quantiﬁed

using wavepacket theory. These reveal that the structural

distortions between ground and excited states are solvent

dependent in a way that is similar to the emissive ligands. Time-

resolved infrared studies show spectral shifts in the CO region

that support the assignment of ligand-centered excited states.

At short time scales there is evidence for three ligand-centered

excited states in the observed Δν(CO) band shifts. By using

mid-infrared ultrafast spectroscopy the presence of two distinct

states is conﬁrmed with bands observed at 1571 and 1547 cm⁻¹,

respectively, for each.

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ASSOCIATED CONTENT

* Supporting Information

■

S

Figures showing FTIR spectroelectrochemical spectra, variable-

temperature emission spectra, transient absorption data, and

TRIR data. Tables detailing MOs of the triplet states, resonance

Raman excitation proﬁle electronic parameters, TRIR data, xyz

coordinates for DFT calculations, and resonance Raman cross

sections. Crystallographic data in CIF format. This material is

available free of charge via the Internet at http://pubs.acs.org.

(23) Dyer, J.; Creely, C. M.; Penedo, J. C.; Grills, D. C.; Hudson, S.;

Matousek, P.; Parker, A. W.; Towrie, M.; Kelly, J. M.; George, M. W.

Photochem. Photobiol. Sci. 2007, 6, 741.

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1158.

AUTHOR INFORMATION

Corresponding Author

*E-mail: kgordon@chemistry.otago.ac.nz.

Notes

■

The authors declare no competing ﬁnancial interest.

(25) Kuimova, M. K.; Grills, D. C.; Matousek, P.; Parker, A. W.; Sun,

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ACKNOWLEDGMENTS

■

Support from the University of Otago, The MacDiarmid

Institute for Advanced Materials and Nanotechnology, MBIE,

and Royal Society of New Zealand is gratefully acknowledged.

M.W.G. gratefully acknowledges receipt of a Royal Society

Wolfson Merit Award.

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383, 292.

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Intraligand charge-transfer excited states in Re(I) complexes with donor-substituted dipyridophenazine ligands

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