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Cite this: DOI: 10.1039/c2dt30296a

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Inorganic photoisomerization: the case study of rhenium(I) complexes†

Megumi Kayanuma,a,b Etienne Gindenspergera and Chantal Daniel*a

Received 9th February 2012, Accepted 14th March 2012DOI: 10.1039/c2dt30296a

The mechanism of photoisomerization of stilbene-like ligands coordinated to rhenium polypyridine orα-diimine carbonyls is deciphered in the light of recent theoretical results obtained at various levels oftheory, density functional theory (DFT, time-dependent DFT) and state-of-the-art ab initio methods,complete active space self-consistent field (CASSCF) and multi-state CAS perturbation theory 2nd Order(MS-CASPT2). On the basis of the electronic absorption spectra and potential energy profiles (PEF)associated to the low-lying singlet and triplet intra-ligand (IL) and metal-to-ligand-charge-transfer(MLCT) excited states, coupled by spin–orbit interactions and calculated for the series [Re(CO)3(N,N)-(L)]+ with N,N = bpy, phen and ph2phen (bpy = 2,2′-bipyridine; phen = 1,10-phenanthroline; ph2phen =4,7-diphenyl-1,10-phenanthroline) and L = stpy or bpe (stpy = 4-styrylpyridine; bpe = 1,2-bis(4-pyridylethylene) it is shown that the dynamics of the isomerization process are controlled by severalfeatures: (i) the occurrence of a 1ILL/

1MLCTNN conical intersection in the vicinity of the 1MLCTNN

absorption domain (ii) the kinetics of 1MLCTNN/3MLCTNN intersystem crossings; (iii) the kinetics of

3MLCTNN/3ILL internal conversion. The branching ratio between the two main isomerization pathways,

namely along the 1ILL PEF via 1MLCTNN/1ILL internal conversion at the conical intersection or along the

3ILL PEF after 3MLCTNN/3ILL charge transfer is largely influenced by the nature of the N,N antenna and

L isomerizable ligands.

Introduction

Whereas organic photoisomerization is well documented, bothexperimentally and theoretically,1 the use of light to induce theisomerization of a ligand coordinated to a metal centre is a morerecent field of research. Tentative experiments have been reportedfor Ru, Os and Fe complexes,2,3 with more or less success,but most of the time the quantum yield of isomerization is verylow and the mechanism is far from being understood as com-pared to processes occurring in stilbene-like organic molecules.Especially the role of the metal atom and of the associated low-lying triplet states is not clear.

The class of rhenium(I) polypyridyl complexes is particularlyappealing and a huge amount of experimental data, from syn-thesis to sophisticated fs–ps time-resolved experiments havebeen reported over the past few decades.4–9 Their tunable chemi-cal, electrochemical, photochemical and photophysical propertiesmake them the best candidates for various applications in thefield of catalysis, light emitting devices, polymerization, non-linear optical materials, bio-diagnostic and -therapeutic tools,optical switches, for instance. This wide range of technological

resources is based on their ability to link covalently chromo-phoric ligands, on their long lived metal-to-ligand-charge-transfer(MLCT) excited states, on the occurrence of various types ofelectronic excited states, so-called intra-ligand (IL), sigma-bond-to-ligand-charge-transfer (SBLCT), ligand-to-ligand-charge-transfer (LLCT). The presence of this assortment of excitedstates in the near-UV/visible energy domain is at the origin of arange of functions such as intra- or inter-molecular electron/energy transfers, luminescence phenomena, ligand dissociations,radicals formation and isomerizations. These processes are con-current but the functionality may be controlled by the introduc-tion of specific ligands and/or experimental conditions.

Irradiation with UV or visible light is one of the externalstimuli that activate the molecular switchable componentoperating as probe and conformation tuning of bio-molecules,supramolecular machines, memory units and metal ionsensors.10–14 Most of the photoswitches rely on ring-opening/closing and isomerization of a pivotal CvC (stilbene) orNvN (azobenzene) bond. Whereas organic photoisomerizationis activated by means of UV light, inorganic photoisomeri-zation can also be controlled by the wavelength of irradiationranging from 450 nm to 300 nm. This opens the door to alarger number of applications, especially in biosystems. Theprice to pay is the occurrence of competitive elementary pro-cesses such as dissociation, luminescence, energy/electrontransfer that can influence the efficiency of the chromophore asfar as reversibility, stability and isomerization quantum yieldsare concerned.

†Based on the presentation at Dalton Discussion No. 13, 10–12 Septem-ber 2012, University of Sheffield, UK.

aLaboratoire de Chimie Quantique, Institut de Chimie de Strasbourg,UMR7177 CNRS-Université de Strasbourg, 4 Rue Blaise Pascal, CS90032, F-67081 Strasbourg-Cedex, France. E-mail: c.daniel@unistra.frbUniversity of Tsukuba, Japan

This journal is © The Royal Society of Chemistry 2012 Dalton Trans.

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Slow trans to cis isomerization of the stilbene unit in [Re-(CO)3(bpy)(stilbene-amido-pyridine)]+ (bpy = 2,2′-bipyridine)has been observed after excitation to the MLCT band15 whereasefficient isomerization was found upon irradiation at 366 nm(ϕ > 0.5) of fac-[Re(CO)3Cl(stpy)2] (stpy = 4-styrylpyridine).16

Low isomerization quantum yields have been reported for anumber of fac-[Re(CO)3(stpy)(NN)]

+ or fac-[Re(CO)3(bpe)(NN)]complexes with NN = bpy or phen = 1,10-phenanthroline andbpe = 1,2-bis(4-pyridylethylene).17–19

More recently Iha et al.20–22 synthesized a series of Re(I)complexes with various isomerizable and antenna ligandscharacterized by UV/visible and NMR spectroscopies. Theseexperimental studies point to the dramatic influence of bothligands on the observed isomerization quantum yields that gen-erally increase with the wavelength of irradiation.

In order to improve the potential of application of this class ofmolecules, it is essential to understand the mechanism of photo-isomerization of stilbene- or azobenzene-like ligands coordinatedto transition metal atoms. Ultra-fast excited state dynamicsexperiments and quantum theory may help in deciphering thismechanism which differs from the one operating in organicmolecules by the participation of low-lying 1,3MLCT states.Indeed, these states may perturb the potential energy profiles ofthe 3IL (T1) and

1IL (S1) excited states governing photoisomeri-zation of stilbene-like compounds (Scheme 1).

Pioneering experiments combining time-resolved infra-red(IR) and resonance Raman (RR) spectroscopies with ultra-fastexcited state dynamics performed on various complexes inwhich different isomerizable ligands (stpy, papy = trans-4-phenyl-azopyridine, MeDpe = N-methyl-4-[trans-2-(4-pyridyl)ethenyl]-pyridinium) are coordinated to the ReI(CO)3(2,2′-bpy) chromo-phore, evidence of the crucial role of the low-lying 3MLCT(dRe → π*bpy) excited states in the isomerization process.23–25

The reactive 3IL (ππ*) excited state is populated via 3MLCT →3IL internal conversion in a ps time scale corresponding to anintramolecular energy transfer from the chromophore to the iso-merizable ligand. The trans to cis isomerization process is

preceded by a ∼90° rotation around the CvC double bond inthe 3IL state with a ∼10 ps time constant leading to a long-lived3IL state with a perpendicular conformation from which inter-system crossing occurs to the electronic ground state within a nstime scale.

The role of the low-lying 1,3MLCT excited states in the photo-isomerization process has been confirmed on the basis ofab initio calculations performed for [Re(CO)3(bpy)(stpy)]

+,representative of this class of molecules.26–28 It has been shownthat the photoisomerization of stilbene-like ligands coordinatedto Re(I) polypyridyl complexes at UV/visible wavelengths isbasically controlled by three elementary processes, namely the3MLCT → 3IL intramolecular energy transfer, the 1MLCT →1IL internal conversion at a conical intersection, the overcomingof the energy barrier in the 1IL state around a torsion angle ofthe CvC bond of ∼60°. From the shape of the potential energysurfaces calculated for the low-lying IL and MLCT states as afunction of a generalized coordinate combining six degrees offreedom it has been shown that three routes of isomerization arepotentially open upon irradiation between 313 nm and 400 nm(Scheme 2).

More recently the spectroscopic trends in the series [Re(CO)3-(phen)(stpy)]+, [Re(CO)3(phen)(bpe)]

+, [Re(CO)3(Me4phen)-(stpy)]+ (Me4phen = 3,4,7,8-tetramethyl-1,10-phenanthroline),[Re(CO)3(Me4phen)(bpe)]

+, [Re(CO)3(ph2phen)(stpy)]+ ph2phen

= 4,7-diphenyl-1,10-phenanthroline), [Re(CO)3(ph2phen)(bpe)]+

[Re(CO)3(Clphen)(stpy)]+ (Clphen = 5-chloro-1,10-phenanthro-

line), [Re(CO)3(Clphen)(bpe)]+ have been analyzed on the basis

of density functional theory (DFT) and time dependent-DFT(TD-DFT) calculations including solvent corrections.29 It isshown that the position of the ILstpy and Ilbpe states and thedegree of mixing with the low-lying MLCT is largely influencedby the nature of the antenna ligand.

The purpose of the present work is the rationalization of therole of the triplet states in the isomerization process and theinvestigation of the isomerization mechanism at the early stagearound the trans structure. To this end a state average complete

Scheme 1 Scheme 2

Dalton Trans. This journal is © The Royal Society of Chemistry 2012

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active space self consistent field (SA-CASSCF) multi state CASperturbation theory 2nd order (MS-CASPT2) study of the spin–orbit absorption spectra of the trans-[Re(CO)3(NN)stpy]

+ (NN =bpy 1a; NN = phen 1b; NN = ph2phen 1c) and [Re(CO)3(NN)-bpe]+ (NN = bpy 2a; NN = phen 2b; NN = ph2phen 2c)complexes has been performed followed by the determination ofthe associated potential energy surfaces calculated as a functionof a generalized coordinate describing the isomerization betweena CvC torsion angle of 0° and 60° for complexes 1a, 2a, 1band 2b.

Computational details

The geometries of the trans conformers of [Re(CO)3(NN)stpy]+

and [Re(CO)3(NN)bpe]+ complexes 1a, 2a, 1b, 2b, 1c and 2c

depicted in Fig. 1 have been optimized by means of densityfunctional theory (DFT) with the B3LYP functional30,31 underCs symmetry.29

Relativistic pseudopotentials of Stuttgart and associatedvalence basis sets for rhenium atom,32 Dunning double-ζ polar-ized (DZP) basis sets for second-row atoms, and Dunningdouble-ζ (DZ) basis sets for hydrogen atoms have been used.33

Electronic structures of the ground and excited states of 1a,2a, 1b, 2b, 1c, and 2c have been determined at the

SA-CASSCF/MS-CASPT2 level of theory.34,35 For 1a and 2a,twelve electrons (two πN,N, six 5d, and four πL electrons) havebeen correlated into eleven active orbitals including the sixdoubly occupied orbitals in the electronic ground state (one πN,N,three 5d, and two πL occupied orbitals), one π*N,N, three 5d′,and one π*L vacant orbitals. The eight lowest singlet and sevenlowest triplet states have been determined for 1a while the sevensinglet and six triplet states have been determined for 2a. Forcomplexes 1b, 1c, 2b, and 2c, one additional occupied πN,Norbital has been included, that is, fourteen electrons (four πN,N,six 5d, and four πL electrons) have been correlated into twelveactive orbitals including the seven doubly occupied orbitals inthe electronic ground state (two πN,N, three 5d, and two πL occu-pied orbitals), one π*N,N, three 5d′, and one π*L vacant orbitals.The nine lowest singlet and eight lowest triplet states have beendetermined for 1b and 1c while eight singlet and seven tripletstates have been determined for 2b and 2c.

A level shift correction of 0.2 hartree has been applied to theMS-CASPT2 calculations to avoid intruder-state problems.Spin–orbit couplings have been evaluated by Restricted-Active-Space State-Interaction Spin–Orbit method (RASSI-SO).36 TheANO-RCC (Atomic Natural Orbitals-relativistic correlation con-sistent) basis sets37–39 have been used for the SA-CASSCF/MS-CASPT2 calculations.

Fig. 1 [Re(CO)3(NN)stpy]+ 1a, 1b, 1c and [Re(CO)3(NN)bpe]

+ 2a, 2b, 2c.29

This journal is © The Royal Society of Chemistry 2012 Dalton Trans.

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The potential energy curves (PECs) associated to the low-lying singlet and triplet states along the isomerization processaround the trans structure varying the torsion angle from 0°(trans structure) to 60° have been calculated as a function of ageneralized isomerization coordinate Q. Here we have con-sidered five internal degrees of freedom (Scheme 3) which arethe driving coordinates of the isomerization reaction.

Table 1 shows five geometrical parameters that define the gen-eralized coordinate Q for the isomerization process of 1a and 2afrom trans structure to cis structure. The upper ring of the iso-merizable ligand is rotated by more than 30° from the transstructure to the cis structure. This additional molecular defor-mation is due to steric hindrance, and prevents the fold up of theligand.

The five internal degrees of freedom consist of the torsion ofthe ethylenic CvC bond (dihedral angle of C3–C1–C2–C4), therotations of rings (dihedral angle of C1–C2–C4–C6 and C5–C3–C1–C2), and angles opening around the ethylenic C–C bond(angle of C1–C2–C4 and C3–C1–C2). To compute the isomeri-zation PECs along Q deformations, we have linearly interpolatedthese five parameters between the trans and the cis values of theelectronic ground states.

All other geometrical parameters have been fixed to those ofthe trans conformer. The calculations of PECs have been per-formed at the SA-CASSCF/MS-CASPT2 level with active spaceincluding eight electrons (six 5d and two πL electrons) correlatedinto nine active orbitals, the four doubly occupied orbitals in theelectronic ground state (three 5d and one πL occupied orbitals),three 5d′, and one π*L vacant orbitals. Five singlet and fourtriplet states have been included.

The SA-CASSCF/MS-CASPT2 calculations reported in thepresent work have been performed with MOLCAS 7.4 QuantumChemistry software.40

Results

Electronic spectroscopy: spin-free and spin–orbit absorptionspectra

The MS-CASPT2 transition energies to the low-lying ILL, ILNN,MLCT and LLCT excited states of complexes 1a, 2a, 1b, 2b, 1c,

2c depicted in Fig. 1 are reported in Table 2 (L = stpy) andTable 3 (L = bpe). The corresponding spin-free and spin–orbitabsorption spectra are depicted in Fig. 2.

As illustrated in Fig. 2 the spin-free and spin–orbit absorptionspectra of the stpy substituted [Re(CO)3(NN)stpy]

+ complexes(1a, 1b, 1c) are nearly identical whereas some differences appearin the spectra of the [Re(CO)3(NN)bpe]

+ complexes (2a, 2b, 2c).The main effect of spin–orbit is the red shift of the lowest bandby ∼1300 cm−1 (L = stpy 1a 1b 1c) and ∼2000 cm−1 (L = bpe2a 2b 2c). This corresponds to the absorption of low-lyingMLCT states localized on the antenna NN ligands and is due tothe mixing between the 1MLCTNN and 3MLCTNN states. A con-tribution of the singlet state of 10% is enough to induce a signifi-cant oscillator strength, f = 0.020 – 0.025 in the bpy and phensubstituted complexes (1a, 1b, 2a, 2b), f = 0.042 – 0.052 in theph2phen substituted complexes (1c, 2c) (Tables 2 and 3). Thisred shift correlates with the singlet-triplet energy gap in the spin-free spectrum and the spin–orbit splitting of the triplet states.The induced oscillator strength is governed by the nature of thesinglet state that couples with the triplet state. In the case of theph2phen substituted complexes, where the effect is the most sig-nificant with a red shift of 1560 cm−1 and an oscillator strengthf = 0.052 for 1c and a red shift of 1975 cm−1 and an oscillatorstrength f = 0.042 for 2c, the strongly absorbing b1MLCT(5dyz → π*NN) state is involved in the spin–orbit coupling and isvery close to the lowest triplet state. Another general trend whenincluding the spin–orbit corrections is the lowering of the inten-sity and the broadening of the absorption spectra due to theincrease of the density of states. This effect has already beendemonstrated by ab initio studies of other organometallic com-plexes such as H2M(CO)4 (M = Fe, Os) or HM(CO)5 (H = Mn,Re).41,42

The MS-CASPT2 transition energies reported in Tables 2 and3 are overestimated by about ∼1500 cm−1 with respect to theexperimentally observed maxima in related compounds.17,43

However the main features of the experimental spectrum of[Re(CO)3(bpy)stpy]

+ (1a) are well reproduced with three mainbands17 observed at 334 nm, corresponding to the a1ILspty statecalculated at 321 nm (f = 1.113), at 312 nm corresponding to theb1MLCT state calculated at 316 nm (f = 0.349) and 266 nm cor-responding to the a1ILNN calculated at 257 nm (f = 0.986)(Table 2). Accordingly the main characteristics of the absorptionspectrum of [Re(CO)3(phen)bpe]

+ (2b), namely four bands at330 nm, 301 nm, 277 nm and 255 nm,43 are qualitatively repro-duced by the calculated spin-free transition energies withmaxima at 334 nm, 317 nm and 308 nm corresponding to the

Table 1 Geometrical deformations included in the generalizedcoordinate Q describing the trans to cis isomerization process in 1a and2a. The definition of the labels is shown in Scheme 3.

1a 2a

trans cis trans cis

C3–C1–C2–C4 0 170.2 0 171.8C1–C2–C4–C6 180.0 210.7 180.0 216.0C5–C3–C1–C2 0 23.70 0 25.71C1–C2–C4 127.4 131.8 126.6 130.7C3–C1–C2 125.5 131.0 125.7 130.6

Scheme 3

Dalton Trans. This journal is © The Royal Society of Chemistry 2012

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Table 2 SA-CASSCF/MS-CASPT2 spin-free and spin–orbit transition energies and oscillator strength [f] of [Re (CO)3(NN)stpy]+ (NN = bpy (1a);

phen (1b); ph2phen (1c))

[Re(CO)3(NN)-stpy]+

Spin-freestates

One electron excitation inthe main configuration

Spin-free transitionenergies in cm−1 (nm) f Spin–orbit states

Spin–orbit transitionenergies in cm−1 (nm) f

NN = bpy 100% a3ILstpy 25 250 (396)1a a3ILstpy πstpy → π*stpy 25 250 (396) 100% a3ILstpy 25 250 (396)

100% a3ILstpy 25 250 (396)73% a3MLCT 17%b3MLCT

28 305 (353)

a3MLCT 5dxz → π*N,N 29 070 (344) 73% a3MLCT 18%b3MLCT

28 310 (353)

80% a3MLCT 10%c3MLCT 10%b1MLCT

28 440 (352) 0.025

a1MLCT 5dxz → π*N,N 29 750 (336) 0.01 64% a1MLCT 25%b3MLCT 11%c3MLCT

28 745 (348) 0.007

42% b3MLCT 29%a3ILN,N 22% a3MLCT

30 830 (324)

b3MLCT 5dyz → π*N,N 30 850 (324) 44% b3MLCT 29%a3ILN,N 21% a3MLCT

30 830 (324)

65% b3MLCT 32%a1MLCT

31 350 (319) 0.002

a1ILstpy πstpy → π*stpy 31 170 (321) 1.113 53% a1ILstpy 23%b1MLCT 20% a3ILN,

N

31 320 (319) 1.006

39% a3ILN,N 38%a1ILstpy 11% a3MLCT

31 040 (322)

a3ILN,N πN,N → π*N,N 31 520 (317) 66% a3ILN,N 31%b3MLCT

31 780 (315)

66% a3ILN,N 31%b3MLCT

31 780 (315) 0.332

b1MLCT 5dyz → π*N,N 31 660 (316) 0.349 46% b1MLCT 38%a3ILN,N

32 100 (311) 0.112

86% c3MLCT 10%b3MLCT

32 930 (304)

c3MLCT 5dxy → π*N,N 32 450 (308) 86% c3MLCT 10%b3MLCT

32 930 (304)

56% c3MLCT 31%c1MLCT 10%b1MLCT

33 100 (302) 0.073

c1MLCT 5dxy → π*N,N 32 480 (308) 0.092 54% c1MLCT 22%c3MLCT 10%b1MLCT

32 870 (304) 0.008

100% b3ILstpy 36 100 (277)b3ILstpy πstpy → π*stpy 36 100 (277) 100% b3ILstpy 36 100 (277)

100% b3ILstpy 36 100 (277)1LLCT πstpy → π*N,N 36 620 (273) 99% 1LLCT 36 655 (273) 0.001

b1ILstpy πstpy → π*stpy 37 590 (266) 0.122 100% b1ILstpy 37 590 (266) 0.122100% 3LLCT 38 745 (258)

3LLCT πstpy → π*N,N 38 730 (258) 100% 3LLCT 38 750 (258)100% 3LLCT 38 750 (258)

a1ILN,N π N,N → π*N,N 38 910 (257) 0.986 100% a1ILN,N 38 910 (257) 0.987

NN = phen 100% a3ILstpy 26 680 (375)1b a3ILstpy πstpy → π*stpy 26 680 (375) 100% a3ILstpy 26 680 (375)

100% a3ILstpy 26 680 (375)63% a3MLCT 28%b3MLCT

28 180 (355) 0.027

a3MLCT 5dxz → π*N,N 29 030 (344) 63% a3MLCT 29%b3MLCT

28 200 (355)

85% a3MLCT 10%c1MLCT

28 500 (351)

68% b3MLCT 31%a3MLCT

29 950 (334)

b3MLCT 5dyz → π*N,N 29 540 (339) 68% b3MLCT 31%a3MLCT

29 950 (334)

68% b3MLCT 41%a1MLCT

30 180 (331) 0.003

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Table 2 (Contd.)

[Re(CO)3(NN)-stpy]+

Spin-freestates

One electron excitation inthe main configuration

Spin-free transitionenergies in cm−1 (nm) f Spin–orbit states

Spin–orbit transitionenergies in cm−1 (nm) f

a1MLCT 5dxz → π*N,N 29 580 (338) 0.008 50% a1MLCT 41%b1MLCT

28 490 (351) 0.005

a1ILstpy πstpy → π*stpy 31 370 (319) 0.972 85% a1ILstpy 10%a3MLCT

31 370 (319) 0.835

b1MLCT 5dyz → π*N,N 31 990 (313) 0.664 72% b1MLCT 10%a3ILN,N 10% c3MLCT

31 870 (314) 0.672

c1MLCT 5dxy → π*N,N 32 550 (307) 0.019 42% c1MLCT 42%a3ILN,N

32 370 (309) 0.002

57% a3ILN,N 33%c3MLCT

32 490 (308)

a3ILN,N πN,N → π*N,N 32 810 (305) 60% a3ILN,N 39%c1MLCT

32 800 (305) 0.104

56% a3ILN,N 39%c1MLCT

33 180 (301) 0.001

83% c3MLCT 33 000 (303)c3MLCT 5dxy → π*N,N 32 820 (305) 53% c3MLCT 41%

a3ILN,N

33 320 (300)

64% c3MLCT 27%a3ILN,N 10% c1MLCT

33 380 (300) 0.016

93% b3ILN,N 34 650 (289)b3ILN,N πN,N → π*N,N 34 440 (290) 90% b3ILN,N 34 660 (289)

91% b3ILN,N 34 680 (288)

b1ILN,N πN,N → π*N,N 35 100 (285) 0.004 96% b1ILN,N 35 220 (284) 0.001

100% b3ILstpy 36 820 (272)b3ILstpy πstpy → π*stpy 36 820 (272) 100% b3ILstpy 36 820 (272)

100% b3ILstpy 36 820 (272)b1ILstpy πstpy → π*stpy 37 440 (267) 0.140 100% b1ILstpy 37 450 (267) 0.14a1ILN,N πN,N → π*N,N 38 120 (262) 0.951 96% a1ILN,N 38 130 (262) 0.95

NN = Ph2phen 100% a3ILstpy 26 400 (379)1c a3ILstpy πstpy → π*stpy 26 340 (380) 100% a3ILstpy 26 400 (379)

100% a3ILstpy 26 400 (379)78% b3MLCT 18%a3MLCT

27 310 (366)

b3MLCT 5dyz → π*N,N 27 570 (363) 79% b3MLCT 18%a3MLCT

27 310 (366)

92% b3MLCT 27 430 (365)84% a3MLCT 10%b1MLCT

28 170 (355) 0.052

a3MLCT 5dxz → π*N,N 28 530 (350) 76% a3MLCT 20%b3MLCT

28 570 (350)

76% a3MLCT 20%b3MLCT

28 570 (350)

b1MLCT 5dyz → π*N,N 29 730 (336) 0.579 87% b1MLCT 10%a3MLCT

29 790 (336) 0.506

a1MLCT 5dxz → π*N,N 30 120 (332) 0.003 80% a1MLCT 10%c3MLCT

29 770 (336) 0.003

a1ILstpy πstpy → π*stpy 31 680 (316) 1.189 81% a1ILstpy 13%a3ILN,N

31 720 (315) 0.964

59% a3ILN,N 16%a1ILstpy 11% c1MLCT

31 530 (317) 0.162

a3ILN,N πN,N → π*N,N 31 940 (313) 72% a3ILN,N 18%c3MLCT

31 580 (317)

70% a3ILN,N 10%c3MLCT 10%c1MLCT

31 625 (316) 0.092

c1MLCT 5dxy → π*N,N 32 765 (305) 0.022 49% c1MLCT 29%a3ILN,N 14% c3MLCT

33 120 (302) 0.007

73% c3MLCT 14%a1MLCT 11% b3ILN,N

32 960 (303)

c3MLCT 5dxy → π*N,N 32 780 (305) 64% c3MLCT 26%a3ILN,N

33 180 (301)

52% c3MLCT 24%a3ILN,N 15% c1MLCT

33 220 (301) 0.003

72% b3ILN,N 13%b1ILN,N 11% c1MLCT

34 410 (291)

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three low-lying 1MLCT states and two maxima at 290 nm corres-ponding to the a1ILbpe state (f = 1.032) and at 265 nm corres-ponding to the a1LNN state (f = 0.903). The discrepancy betweenthe calculated spectra and the experimental spectra recorded inacetonitrile is due to solvent effects not taken into account in thepresent calculations. Indeed the spin–orbit correction is of littleinfluence on the position of the bands maxima as illustrated forthe two complexes 1a and 2b for which experimental data areavailable. The interaction with the triplet states is too small toinduce a significant shift of the singlet absorbing states, the onlyconsequence being a decrease in intensity.

According to the conclusion of our previous work publishedrecently on [Re(CO)3(bpy)stpy]

+ an important key to the mecha-nism of isomerization at the first stage is the relative position ofthe two strongly absorbing a1ILstpy and b1MLCTNN states. In theseries of [Re(CO)3(NN)stpy]

+ complexes investigated here(Fig. 1), namely 1a (NN = bpy), 1b (NN = phen) and 1c (NN =ph2phen), the spin-free absorption spectra indicate two sets ofnearly degenerate a1ILstpy/b

1MLCTbpy states and a1ILstpy/b1MLCTphen states (Table 2). Spin–orbit corrections inducemixing of the a1ILstpy state with the b1MLCTbpy and

3ILbpy states(1a). This interaction is less important (<10%) in 1b and 1c.We may anticipate from the results obtained for the three stpysubstituted complexes 1a, 1b and 1c that the two upper routes 2and 3 via the 1MLCT and the 1IL states, depicted in Scheme 2,may be easily activated in a synergistic effect upon one uniquewavelength of irradiation in the UVor near-UV domain.

In contrast to the 1ILstpy states the1ILbpe states are well sepa-

rated of the low-lying MLCT absorbing states in [Re(CO)3(NN)-bpe]+ 2a (NN = bpy), 2b (NN = phen) and 2c (NN = ph2phen).They do not show any interaction with the other states andremain pure after spin–orbit correction, keeping nearly the sameintensity. These states will be populated exclusively uponmiddle-UV irradiation (∼290 nm) leading to isomerization via aroute not accessible in the experiments reported at 313 nm,365 nm and 404 nm. Indeed, even at 313 nm where the 1ILbpe

state absorbs, the energy is not sufficient to overcome the barrieraround a CvC torsion angle of 50° and activate routes 2 and 3as we shall discuss later. In the absence of IL/MLCT synergisticeffect the quantum yield of photoisomerization of the bpe substi-tuted complexes should not be influenced by the wavelength of

irradiation. This is in agreement with the determined quantumyields for complexes 2b and 2c which remain constant to ∼0.80and ∼0.40, respectively, in the 400–313 nm domain ofirradiation.7

Whereas the position and the degree of mixing of the singlet1ILL (L = stpy or bpe) states in all the complexes investigated inthis work depend on the nature of the antenna and on the isomer-izable ligand, the triplet 3ILL (L = stpy or bpe) states remainpure, are the lowest triplet states and are not affected by spin–orbit interactions. The role this state will play in the isomeriza-tion mechanisms is the same as the one observed in organicsystems (Scheme 1). Here, the 3ILL will be activated afterinternal conversion from the lowest 3MLCT state (route 1 inScheme 2). The efficiency of this process will depend on the3IL–3MLCT energy gap. It is worth noting that this energy gapdecreases significantly when replacing the stpy ligand by thebpe. In the NN series the substitution of bpy by phen andph2phen leads to nearly degenerate IL/MLCT states with ΔE(IL-MLCT) = 910 cm−1 in the stpy/ph2phen substituted complexand 610 cm−1 in the bpe/ph2phen substituted complex (Table 4)with the potential for an ultra-fast intramolecular energy transfer,less than 1 ps measured for the stpy/bpy complex.23

In contrast to the 3ILL (L = stpy or bpe) states, the 3ILNN

(NN = bpy, phen, ph2phen) states interact by spin–orbit couplingwith the ILL and MLCT states. In particular, they may acquiresignificant oscillator strengths through their spin–orbit interactionwith the 1MLCT or 1ILL states and should participate to theabsorption (see Tables 2 and 3). This emitting state is well abovethe 1,3MLCTNN and 1ILstpy states in the stpy complexes but inbetween in the bpe complexes.

In order to further investigate the mechanism of isomerizationin this class of Re(I) complexes we present in the next sectionthe potential energy curves associated to the low-lying excitedstates discussed above.

Potential energy profiles

The potential energy curves describing the early stage of thetrans to cis photoisomerization of the stpy and bpe ligands in[Re(CO)3(NN)stpy]

+ and [Re(CO)3(NN)bpe]+ complexes with

Table 2 (Contd.)

[Re(CO)3(NN)-stpy]+

Spin-freestates

One electron excitation inthe main configuration

Spin-free transitionenergies in cm−1 (nm) f Spin–orbit states

Spin–orbit transitionenergies in cm−1 (nm) f

b3ILN,N πN,N → π*N,N 34 010 (294) 83% b3ILN,N 16%c1MLCT

34 440 (290)

85% b3ILN,N 14%c1MLCT

34 460 (290)

b1ILN,N πN,N → π*N,N 34 230 (292) 0.012 74% b1ILN,N 14%b3ILN,N 10%c3MLCT

34 620 (289) 0.016

1LLCT πstpy → π*N,N 36 460 (274) 0.305 100% 1LLCT 36 470 (274) 0.328100% b3ILstpy 36 750 (272)

b3ILstpy πstpy → π*stpy 36 690 (273) 100% b3ILstpy 36 750 (272)100% b3ILstpy 36 750 (272)

a1ILN,N πN,N → π*N,N 37 080 (270) 0.451 99% a1ILN,N 37 130 (269) 0.427b1ILstpy πstpy → π*stpy 37 480 (267) 0.127 100% b1ILstpy 37 480 (267) 0.127

This journal is © The Royal Society of Chemistry 2012 Dalton Trans.

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Table 3 SA-CASSCF/MS-CASPT2 spin-free and spin–orbit transition energies and oscillator strength [f] of [Re (CO)3(NN)bpe]+ (NN = bpy (2a);

phen (2b); ph2phen (2c))

[Re(CO)3(NN)-bpe]+

Spin-freestates

One electronexcitation in the mainconfiguration

Spin-freetransition energiesin cm−1 (nm) f Spin–orbit states

Spin–orbittransition energiesin cm−1 (nm) f

NN = bpy 100% a3ILbpe 25 265 (396)2a a3ILbpe πbpe → π*bpe 25 260 (396) 100% a3ILbpe 25 265 (396)

100% a3ILbpe 25 265 (396)74% a3MLCT 17%b3MLCT

27 985 (357)

a3MLCT 5dxz → π*N,N 28 720 (348) 75% a3MLCT 16%b3MLCT

28 000 (357)

82% a3MLCT 10%b1MLCT

28 130 (355) 0.020

a1MLCT 5dxz → π*N,N 30 105 (322) 0.013 53% a1MLCT 34%b3MLCT 13% c3MLCT

28 810 (347) 0.008

41% b3MLCT 34% a3ILN,N20% a3MLCT

30 510 (328)

b3MLCT 5dyz → π*N,N 30 590 (327) 41% b3MLCT 34% a3ILN,N19% a3MLCT

30 510 (328)

60% b3MLCT 40%a1MLCT

31 360 (319) 0.005

65% a3ILN,N 20%b1MLCT 10% a3MLCT10% c3MLCT

30 790 (325)

a3ILN,N πN,N → π*N,N 31 175 (321) 62% a3ILN,N 35%b3MLCT

31 490 (318)

62% a3ILN,N 35%b3MLCT

31 490 (418) 0.037

b1MLCT 5dyz → π*N,N 31 530 (317) 0.185 51% b1MLCT 33% a3ILN,N10% a1MLCT

31 930 (313) 0.115

86% c3MLCT 32 740 (305)c3MLCT 5dxy → π*N,N 32 230 (310) 86% c3MLCT 10%

a1MLCT32 760 (305) 0.001

46% c3MLCT 28%c1MLCT18% b1MLCT

32 780 (305) 0.002

c1MLCT 5dxy → π*N,N 32 440 (308) 0.082 59% c1MLCT 30%c3MLCT

32 945 (304) 0.107

a1ILbpe πbpe → π*bpe 34 190 (293) 1.125 100% b1ILbpe 34 200 (292) 1.112100% b3ILbpe 35 200 (284)

b3ILbpe πbpe → π*bpe 35 200 (284) 100% b3ILbpe 35 200 (284)100% b3ILbpe 35 200 (284)

b1ILbpe πbpe → π*bpe 37 070 (270) 0.207 100%b1ILbpe 37 070 (270) 0.207a1IL N,N πN,N → π*N,N 38 600 (259) 0.955 100% a1ILN,N 38 600 (259) 0.956

NN = phen 100% a3ILbpe 26 470 (378)2b a3ILbpe πbpe → π*bpe 26 470 (378) 100% a3ILbpe 26 470 (378)

100% a3ILbpe 26 470 (378)63% a3MLCT 29%b3MLCT

27 700 (361)

a3MLCT 5dxz → π*N,N 28 540 (350) 63% a3MLCT 29%b3MLCT10% c1MLCT

27 710 (361)

86% a3MLCT 10%b1MLCT

28 035 (357) 0.021

62% b3MLCT 30%a1MLCT

28 300 (353) 0.005

b3MLCT 5dyz → π*N,N 29 020 (345) 68% b3MLCT 31%a3MLCT

29 440 (340)

68% b3MLCT 31%a3MLCT

29 440 (340)

a1MLCT 5dxz → π*N,N 29 955 (334) 0.014 56% a1MLCT 37%b3MLCT

30 110 (332) 0.008

b1MLCT 5dyz → π*N,N 31 575 (317) 0.377 51% b1MLCT 22% a3ILN,N15% c3MLCT11 %a3MLCT

31 300 (320) 0.192

64% a3ILN,N 27%c3MLCT

31 900 (313)

a3ILN,N πN,N → π*N,N 32 200 (311) 66% a3ILN,N 21%c1MLCT

31 920 (313) 0.001

61% a3ILN,N 34%b1MLCT

32 335 (309) 0.146

Dalton Trans. This journal is © The Royal Society of Chemistry 2012

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Table 3 (Contd.)

[Re(CO)3(NN)-bpe]+

Spin-freestates

One electronexcitation in the mainconfiguration

Spin-freetransition energiesin cm−1 (nm) f Spin–orbit states

Spin–orbittransition energiesin cm−1 (nm) f

80% c3MLCT 14%a1MLCT

32 670 (306) 0.002

c3MLCT 5dxy → π*N,N 32 340 (309) 59% c3MLCT 35% a3ILN,N 32 825 (305)41% c3MLCT 35%c1MLCT17% a3ILN,N

32 960 (303) 0.03

c1MLCT 5dxy → π*N,N 32 460 (308) 0.011 29% c1MLCT 32% a3ILN,N29% c3MLCT

32 845 (304) 0.005

b 3ILN,N πN,N → π*N,N 33 930 (295) 91% b3ILN,N 10%c3MLCT

34 175 (293)

91% b3ILN,N 10%c3MLCT

34 180 (293)

90% b3ILN,N 10%c1MLCT

34 180 (293)

a1ILbpe πbpe → π*bpe 34 510 (290) 1.032 100% a1ILbpe 34 510 (290) 1.033100% b3ILbpe 34 740 (288)

b3ILbpe πbpe → π*bpe 34 740 (288) 100% b3ILbpe 34 740 (288)100% b3ILbpe 34 740 (288)

b1ILN,N πbpe → π*bpe 35 230 (284) 0.066 97% b1ILN,N 35 330 (283) 0.055b1ILbpe πN,N → π*N,N 36 875 (271) 0.218 100% b1ILbpe 36 880 (271) 0.218a1ILN,N πN,N → π*N,N 37 665 (265) 0.903 100% a1ILN,N 37 670 (265) 0.904

NN = Ph2phen 100% a3ILbpe 26 290 (380)2c a3ILbpe πbpe → π*bpe 26 290 (380) 100% a3ILbpe 26 290 (380)

100% a3ILbpe 26 290 (380)81% b3MLCT 16%a3MLCT

26 900 (372)

b3MLCT 5dyz → π*N,N 27 200 (368) 81% b3MLCT 16%a3MLCT

26 910 (372)

95% b3MLCT 27 040 (370)86% a3MLCT 8%b1MLCT

27 865 (359) 0.042

a3MLCT 5dxz → π*N,N 28 270 (354) 78% a3MLCT 18%b3MLCT

28 230 (354)

78% a3MLCT 18%b3MLCT

28 230 (354)

b1MLCT 5dyz → π*N,N 29 840 (335) 0.558 83% b1MLCT 10%a3MLCT

29 840 (335) 0.463

a1MLCT 5dxz → π*N,N 30 440 (329) 0.007 74% a1MLCT 14%c3MLCT (14%)

29 890 (335) 0.006

74% a3ILN,N 16%c3MLCT

31 090 (322)

a3ILN,N πN,N → π*N,N 31 500 (317) 76% a3ILN,N 12%c1MLCT

31 110 (321) 0.002

76% a3ILN,N 10%c3MLCT 10% b1MLCT

31 270 (320) 0.047

71% c3MLCT 21%a1MLCT 8% b3ILN,N

32 780 (305)

c3MLCT 5dxy → π*N,N 32 470 (308) 66% c3MLCT 24% a3ILN,N 32 820 (305)56% c3MLCT 23% a3ILN,N14% c1MLCT

32 835 (305) 0.002

c1MLCT 5dxy → π*N,N 32 700 (306) 0.002 51% c1MLCT 19%c3MLCT 17% a3ILN,N

33 030 (303) 0.002

85% b3ILN,N 13%c3MLCT

34 090 (293)

b3ILN,N πN,N → π*N,N 33 700 (297) 85% b3ILN,N 13%c3MLCT

34 095 (293)

81% b3ILN,N 17%c1MLCT

34 100 (293) 0.004

a1ILbpe πbpe → π*bpe 34 330 (291) 0.843 97% a1ILbpe 34 370 (291) 0.973b1ILN,N πbpe → π*bpe 34 870 (287) 0.308 92% b1ILN,N 35 070 (285) 0.175

100% b3ILbpe 34 810 (287)b3ILbpe πbpe → π*bpe 34 810 (287) 100% b3ILbpe 34 810 (287)

100% b3ILbpe 34 810 (287)a1ILN,N πN,N → π*N,N 36 530 (274) 0.722 100% a1ILN,N 36 540 (274) 0.722b1ILbpe πbpe → π*bpe 36 780 (272) 0.145 100% b1ILbpe 36 790 (272 0.145

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NN = bpy (1a, 2a), phen (1b, 2b) and ph2phen (1c) are depictedin Fig. 3.

These curves show the spin-free SA-CASSCF/MS-CASPT2energy profiles of the 1,3ILstpy (1a, 1b, 1c),

1,3ILbpe (2a, 2b) and1,3MLCTNN (1a, 2a, 1b, 2b, 1c) excited states along Q defor-mations represented as a function of the CvC bond torsionangle from 0° (trans) to 60°. These potential energy curves havebeen obtained with a limited 8e9a CASSCF active space (seecomputational details) as compared to the 12e11a (1a, 2a) or14e12a (1b, 1c, 2b) active spaces used for the calculation of theabsorption spectra described in the previous section. Thisrestricted CASSCF active space leads to a biased description ofthe Franck–Condon region, especially of the lowest 3IL states.The curves have been shifted systematically in order to take intoaccount this artificial discrepancy that amounts to 600 cm−1 forthe 1,3MLCT states and ∼3000 cm−1 for the 3IL states. This cor-rection does not modify drastically the shape of the potentialenergy curves as illustrated by the comparison of the non-shiftedand shifted curves for [Re(CO)3(bpy)stpy]

+ 1a (Fig. 3). Themain difference is the relative position of the lowest 3IL statewith respect to the 3MLCT states, the 3IL states being artificiallydestabilized with the limited CASSCF active space.

The potential energy profiles of the excited states of [Re(CO)3-(bpy)stpy]+ (1a), [Re(CO)3(phen)stpy]

+ (1b) and [Re(CO)3-(ph2phen)stpy]

+ (1c) are very similar with a nearly flat

3ILstpy state well separated from the upper states followed by ahigh density of 1,3MLCT states accessible upon visible/near-UVirradiation. The 3ILstpy state will be accessible via 3MLCT →3ILstpy internal conversion, more probably around the trans con-formation. The 3MLCT/3ILstpy energy gap in the Franck–Condon region decreases when going from bpy to phen and toph2phen. When the spin–orbit coupling is taken into accountthese states become nearly degenerate in the ph2phen substitutedcomplex (see previous section) leading to an ultra-fast decay tothe 3ILtrans, below the ps time-scale. The 1ILstpy potential ischaracterized by several crossings with the manifold of MLCTstates in both bpy and phen substituted complexes 1a and 1b.The 1ILstpy state of [Re(CO)3(bpy)stpy]

+ (1a) and [Re-(CO)3(phen)stpy]

+ (1b) is potentially accessible after irradiationinto the MLCT band, opening the upper route 3 of isomerization(Scheme 2). In contrast and because of the stabilization of thelowest MLCT states by substitution of the phen ligand byph2phen in 1c the 1ILstpy is less accessible upon near-UVirradiation in [Re(CO)3(ph2phen)stpy]

+. The occurrence ofseveral 1ILstpy/

1MLCT conical intersections and 1ILstpy/3MLCT

crossings between 10° and 60° torsion angles will increase non-adiabatic transitions probability via vibronic and spin–orbitcouplings in bpy and phen complexes 1a and 1b at the earlystage of the isomerization process. This effect will be less impor-tant in the mechanism of isomerization of the ph2phen substi-tuted complex 1c.

The shape of the 1,3MLCT states and 3IL energy profiles in[Re(CO)3(bpy)bpe]

+ (2a) and [Re(CO)3(phen)bpe]+ (2b) com-

plexes follow the same trends as in the stpy complexes 1a and1b. The only difference is the destabilization of the 1ILbpe withrespect to the MLCT states. This key state of the isomerizationprocess remains pure in the Franck–Condon region as indicatedby the absorption spectra analysis, well above the MLCT statesand is not accessible upon visible/near-UV irradiation into theMLCT band. Moreover there is no conical intersection or mixing

Fig. 2 Calculated spectra of 1a, 2a, 1b, 2b, 1c and 2c by SA-CASSCF/MS-CASPT2 method. Blue dotted line and red solid line show spin-free andspin–orbit spectra, respectively. The calculated spectra are obtained by convoluting the theoretical line spectrum by Gaussians of 1000 cm−1 FWHM.

Table 4 SA-CASSCF/MS-CASPT2 spin–orbit ILL–MLCTNN energygap [cm−1] in [Re (CO)3(NN)L]

+ (L = stpy and NN = bpy (1a); phen(1b); ph2phen (1c) or L = bpe and NN = bpy (2a); phen (2b); ph2phen(2c))

stpy bpe

Bpy 3050 (1a) 2720 (2a)Phen 1500 (1b) 1230 (2b)Ph2phen 910 (1c) 610 (2c)

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with the low-lying 1,3MLCT states until a torsion angle of 50°that coincides with the maximum of the energy barrier whichcharacterizes the 1IL (so-called S1 state in stilbene-like systems)energy profile. Consequently, the activation of the upper route 3of photoisomerization in [Re(CO)3(NN)bpe]

+ complexes is prob-ably a minor process. This hypothesis is corroborated by thesmall influence of the wavelength of irradiation on the isomeriza-tion quantum yields observed in [Re(CO)3(NN)bpe]

+ (NN =phen, ph2phen).

7

Trans–cis photoisomerization mechanism

According to the results obtained for the series of complexesinvestigated in the present work and in our previous studies26–28

we may propose the following mechanisms at the early stage ofphotoisomerization in rhenium(I) complexes:

Upon irradiation into the visible at ∼400 nm only the lowestweakly absorbing 1MLCT state will be populated. The only oper-ative channel of isomerization will be intersystem crossing to the3IL state localized on the isomerizable ligand following the sen-sitized stilbene-like mechanism along the corresponding poten-tial energy profile until the perpendicular conformation put inevidence by ultra-fast spectroscopy and excited states dynamicsexperiments.23 One way to improve the probability and kinetics

of this reaction pathway is to reduce the 3MLCT/3IL energy gapby stabilizing the lowest 3MLCT state either by substituenteffects or/and by spin–orbit splitting. This is illustrated here bythe antenna effect when replacing the bpy ligand by a phen orph2phen substituent.

Irradiation into the near-UVor middle-UVenergy domain mayincrease the quantum yield of isomerization by opening tworoutes:

(i) population of the 1IL ligand via vibronic or spin–orbitcouplings with the 1,3MLCT states, followed by isomerizationalong the 1IL pathway involving CvC bond stretching and pyra-midalization to overcome the energy barrier (∼5000 cm−1);

(ii) activation of the sensitized stilbene-like channel via thelow-lying 3MLCT/3IL states by population of the upper stronglyabsorbing 1MLCT/1IL states.

The branching ratio between these two routes, namely theupper one via the 1IL state and the sensitized stilbene-likepathway via the triplet states, will be controlled by the relativeposition of the IL and MLCT states in the Franck–Condondomain, the occurrence of 1IL/1MLCT conical intersections atthe early stage of the isomerization and the degree of IL/MLCTmixing by spin–orbit interactions.

Whereas the stpy complexes investigated here have a greatpotential for following the upper channel of isomerization via

Fig. 3 Potential energy profiles (spin-free) associated to the 1,3ILL states (black solid and dashed lines for the singlet and triplet, respectively)(L = stpy or bpe), to the 1MLCTNN states (red solid lines) and to the 3MLCTNN states (blue dashed lines) of 1a, 2a, 1b, 2b, and 1c along Q deformation,as a function of the torsion angle of the ethylenic CvC bond.

This journal is © The Royal Society of Chemistry 2012 Dalton Trans.

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the 1IL state showing a significant dependence of the quantumyield on the wavelength of irradiation, in contrast the bpe com-plexes should decay only via the low-lying triplet states afterirradiation in the near-UV/UVenergy domain.

Conclusion

The mechanism of photo-induced trans–cis isomerization ofstilbene-like ligands coordinated to Re(I) polypyridyl has beendeciphered on the basis of the accurate determination of the low-lying MLCT and IL excited states participating to the processand of the calculation of the associated potential energy profilesas a function of the CvC double bond torsion angle. Fivedegrees of freedom have been taken into account via thedefinition of a generalized coordinate. The spin-free and spin–orbit absorption spectra show a high density of ILL (L = stpy orbpe), ILNN (NN = antenna ligand) and MLCT excited statesbetween 400 nm and 250 nm. It is shown that the sensitizedstilbene-like mechanism via the 3ILL state is activated afterirradiation at about 400 nm via the low-lying weakly absorbingMLCT state. The 3MLCT/3ILL energy gap which is modulatedby the nature of the antenna ligand and by spin–orbit stabiliz-ation of the lowest 3MLCT state controls the kinetics andefficiency of this isomerization pathway. The stilbene-like mech-anism via the 1ILL state is activated either directly by UVabsorp-tion like in organic molecules or by near-UV absorption towardsthe strongly absorbing MLCT state via vibronic and spin–orbitcouplings. The presence of several 1ILL/MLCT conical inter-sections along the potential energy profiles of [Re(CO)3(bpy)-stpy]+ and [Re(CO)3(phen)stpy]

+ points to a very complicateddynamics with ultra-fast concurrent processes of deactivationstrongly dependent on the wavelength of irradiation. In contrast,the photoisomerization process observed in [Re(CO)3(bpy)bpe]

+

and [Re(CO)3(phen)bpe]+ is hardly occurring via the 1ILL state

which does not interact with the low-lying MLCT states. Indeedwhatever the wavelength of irradiation above 313 nm thequantum yield remains nearly constant in agreement with asingle deactivation channel via the 3MLCT/3ILL route as pro-posed by Iha et al.20

A quantitative study of the excited states dynamics in thisclass of molecules would need more accurate potential energysurfaces taking into account other important nuclear relaxationand solvent effects. This would be the price for a realistic ration-alization of the observed quantum yields. This is beyond thescope of the present study. An open question is the highquantum yield (∼0.8) observed experimentally for [Re(CO)3-(phen)(bpe)]+ upon irradiation at 404 nm. Another aspect of ourcurrent studies is devoted to the reversible cis–trans isomeriza-tion process concurrent to the luminescence of the cis-struc-ture.21 For this purpose we currently investigate in detail theelectronic structure of the cis [Re(CO)3(N,N(L)]

+ complexes.

Acknowledgements

M. K. is grateful to the ANR program Blanc (Projects HeterocopANR-09-BLAN-0183-0 and Photobiomet ANR-09-BLAN-0191-01) for financial support. The quantum chemical calcu-lations have been performed at the national IDRIS (Paris) and

CINES (Montpellier) computer centres through a grant ofcomputer time by GENCI, and at the regional HPC centre(Strasbourg).

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