Luminescence of a new Ru(II) polypyridine complex controlled by a redox-responsive protonable anthra[1,10]phenanthrolinequinone

Article abstract of DOI:10.1135/cccc20051891

Redox-controlled luminescence quenching is presented for a new Ru(II)-bipyridine complex [Ru(bpy)₂(1)]²⁺ where ligand 1 is an anthra[1,10]phenanthrolinequinone. The complex emits from a short-lived metal-to-ligand charge transfer, ³MLCT state (τ = 5.5 ns in deaerated acetonitrile) with a low luminescence quantum yield (5 × 10 ^-4). The emission intensity becomes significantly enhanced when the switchable anthraquinone unit is reduced to corresponding hydroquinone. On the contrary, chemical one-electron reduction of the anthraquinone moiety to semiquinone in aprotic tetrahydrofuran results in total quenching of the emission.

Full text of DOI:10.1135/cccc20051891

New Ru(II) Polypyridine Complex
1891
LUMINESCENCE OF A NEW Ru (II) POLYPYRIDINE COMPLEX
CONTROLLED BY A REDOX-RESPONSIVE PROTONABLE
ANTHRA[1,10]PHENANTHROLINEQUINONE
a,
b
b1,
František HARTL *, Sandrine VERNIER and Peter BELSER
*
a
Van’t Hoff Institute for Molecular Sciences, Universiteit van Amsterdam,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands; e-mail: hartl@science.uva.nl
Department of Chemistry, University of Fribourg, Chemin du Musée 9, 1700 Fribourg,
Switzerland; e-mail: peter.belser@unifr.ch
b
1
Received Jun e 23, 2005
Accepted August 24, 2005
Redox-con trolled lum in escen ce quen ch in g is presen ted for a n ew Ru(II)-bipyridin e com plex
[Ru(bpy)2(1)]2+ wh ere ligan d 1 is an an th ra[1,10]ph en an th rolin equin on e. Th e com plex em its
from a sh ort-lived m etal-to-ligan d ch arge tran sfer, 3MLCT state (τ = 5.5 n s in deaerated ace-
ton itrile) with a low lum in escen ce quan tum yield (5 × 10–4). Th e em ission in ten sity be-
com es sign ifican tly en h an ced wh en th e switch able an th raquin on e un it is reduced to corre-
spon din g h ydroquin on e. On th e con trary, ch em ical on e-electron reduction of th e
an th raquin on e m oiety to sem iquin on e in aprotic tetrah ydrofuran results in total quen ch in g
of th e em ission .
Keyw ord s: Ruth en ium ; Polypyridin e com plexes; An th raquin on e; Ph en an th rolin es;
Bipyridin es; Electron tran sfer; Lum in escen ce quen ch in g; Electroch em istry; Redox switch ; Cy-
clic voltam m etry, UV-VIS spectroscopy.
Ph otoin duced en ergy- an d electron -tran sfer processes in supram olecular
system s with in corporated p-quin on es are a topical area of growin g in terest.
p-Quin on es are im portan t m olecules in biological electron tran sport an d
can be utilised as regulators of electron ic com m un ication in electron -
tran sfer system s based for exam ple on porph yrin s1,2. Th eir easy fun ction -
alisation an d well-defin ed reversible redox beh aviour m ake th em appealin g
can didates for con struction of m olecular electro-ph otoswitch in g devices3.
Am on g th e first studied in organ ic system s belon gs [Ru(bpy)3]2+ with pen -
dan t 1,4-ben zoquin on e covalen tly lin ked th rough a flexible h ydrocarbon
ch ain to on e of th e 2,2′-bipyridin e (bpy) ligan ds4. Th e lum in escen ce of th e
m etal com plex is a fun ction of th e quin on e redox state. Reversible in ter-
con version between th e 1,4-ben zoquin on e an d h ydroquin on e form s of th e
two-electron redox couple was foun d to switch th e lum in escen ce off an d
Collect. Czech. Chem. Commun. (Vol. 70) (2005)
doi:10.1135/cccc20051891

1892
Hartl, Vernier, Belser:
on , respectively. A sim ilar efficien t quen ch in g effect was observed for
electrogen erated ch em ilum in escen ce of [Ru(bpy)3]2+ in th e presen ce of
1,4-ben zoquin on es, h ydroquin on es, ph en ols an d catech ols5. A series of
ch rom oph ore-quen ch er dyads based on a 9,10-an th raquin on e acceptor
were studied by Meyer et al.6 More advan ced is a don or-acceptor triad with
th e 1,4-ben zoquin on e/h ydroquin on e redox couple lin kin g th e lum in escen t
[Ru(bpy)3]2+ un it with a secon dary [Co(bpy)3]3+ electron acceptor7. More
recen tly, a h ydroquin on e-fun ction alised 2,2′:6′,2″-terpyridin e (tpy) ligan d
coordin ated to an eth yn ylated Ru(II)-tpy fragm en t was studied as a
ch rom oph ore-quen ch er dyad, with th e h ydroquin on e/ben zoquin on e
on /off level of th e switch bein g tem perature-depen den t7.
In th is paper we presen t a n ew electron don or-acceptor dyad based on a
2+
ligh t-absorbin g {Ru(bpy)2} un it com plexed to a 1,10-ph en an th rolin e-type
(ph en ) ligan d with fused 9,10-an th raquin on e m oiety (com poun d 1 in
Ch art 1). Th e advan tage of th is rigid π-system is a fixed m etal–quin on e dis-
tan ce th at will facilitate a m ore rigorous description of th e ph otoph ysical
beh aviour of th e dyad8. Th e m ain goal of th e presen t study was to un der-
stan d th e redox beh aviour of th e an th raquin on e cen tre in [Ru(bpy)2(1)]2+
(2) an d to test its in fluen ce on th e lum in escen ce properties of th e Ru(II)-
polypyridin e ch rom oph ore. Th e experim en tal results are com pared with
th e literature data9 obtain ed for a series of structurally related referen ce
com plexes [Ru(bpy)2(L′)]2+, with L′ = n p, dppz, dcn p, pd depicted in Ch art 1,
an d bpy.
CN
N
N
N
N
N
N
N
N
CN
dcnp
np
dppz
O
O
N
N
O
O
N
N
1
pd
CHART 1
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

New Ru(II) Polypyridine Complex
1893
RESULTS AND DISCUSSION
Syntheses of Ligand 1 and Complex 2
Ligan d 1 was prepared by Diels–Alder cycloaddition of 5,6-bis(brom o-
m eth yl)-1,10-ph en an th rolin e an d 1,4-n aph th oquin on e (Sch em e 1). Th e
m erely m oderate yield is probably caused by th e retro Diels–Alder reaction .
Th e ph otoch em ical radical brom in ation of th e m eth yl groups in th e 5,6-
dim eth yl-1,10-ph en an th rolin e precursor usin g N-brom osuccin im ide facili-
tates th e direct closure of th e arom atic rin g with out usin g an oxidisin g
agen t10. Com plex 2 was th en obtain ed by h eatin g poorly soluble crude
ligan d 1 with 1 equivalen t of [RuCl2(bpy)2] in eth ylen e glycol for 2 h .
O
O
N
N
N
N
Br
Br
+
O
O
1
N
Ru
N
N
N
Cl
1. ethylene glycol, reflux
2. NH4PF6
Cl
N
Ru
N
O
O
N
N
N
N
(PF6)2
2
SCHEME 1
Redox Behaviour of Ligand 1 and Complex 2
Cyclic an d Differen tial Pulse Voltam m etry
Voltam m etric studies of ligan d 1 an d com plex 2 were perform ed with th e
aim to provide a good in itial guess of th e localisation of th e in dividual re-
dox processes in th e com plex an d th e n ature of its fron tier orbitals. Th ese
results are of fun dam en tal im portan ce for th e discussion of th e electron ic
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1894
Hartl, Vernier, Belser:
absorption an d lum in escen ce properties of com plex 2. Th e series of in vesti-
gated com poun ds was com pleted with 9,10-an th raquin on e as a suitable
m odel for th e acceptor m oiety in ligan d 1. Th e electrode poten tials an d th e
localisation of th e redox processes are given in Table I. Th e assign m en t of
th e redox active cen tres h as been facilitated by com parison with literature
electroch em ical data for selected Ru(II) com plexes closely related to com -
plex 2.
Anthraquinone. Th e cyclic voltam m ogram of an th raquin on e in aceton i-
trile sh ows fully reversible reduction to th e correspon din g sem iquin on e
radical an ion at E1/2 = –1.36 V vs Fc/Fc+. Th e secon d cath odic step at E1/2
=
–2.02 V producin g th e dian ion is electroch em ically quasireversible (∆Ep =
120 vs 80 m V for Fc/Fc+) but n o coupled ch em ical reaction (proton ation )
was observed on th e sub-secon d tim e scale (Ip,a/Ip,c = 1 at v = 100 m V s–1). A
very sim ilar results was obtain ed in N,N-dim eth ylform am ide12
.
Ligand 1. Th e redox beh aviour of ligan d 1 is very close to th at of an th ra-
quin on e. Th e electrode poten tials of th e two cath odic steps localised on th e
p-quin on e m oiety are sligh tly sh ifted to less n egative values (Table I). Th e
quasireversible n ature of th e secon d cath odic step is m ain tain ed (∆Ep
=
90 vs 70 m V for Fc/Fc+). Th e reduction of th e n on -coordin atin g ph en an -
TABLE I
Electroch em ical data for ligan d 1, com plex 2 an d referen ce com poun dsa
Com poun d
RuII/III
L0/–1
L–1/–2
bpy0/–1 (1) bpy0/–1 (2) ph en 0/–1
9,10-An th raquin on eb
pdc
–1.36
–0.85
–1.31
–1.19
–1.23
–0.56
–1.15
–2.02
–1.65
–1.88
–1.73
–1.75
–1.29
1
2
2d
0.91
–1.81
–1.81
–2.02
–2.02
–2.43
–2.49
[Ru(bpy)2(N,N′–pd)]2+ c 0.95
[Ru(bpy)2(dcn p)]2+ e
[Ru(bpy)2(n p)]2+ e
[Ru(bpy)2(dppz)]2+ e
[Ru(bpy)3]2+ e
0.85
0.84
0.84
0.89
–1.70
–1.78
–1.84
–1.72
–1.90
–1.95
–2.07
–1.93
–2.07
–1.42
a
Con dition s: aceton itrile (un less stated oth erwise), 293 K, poten tials vs Fc/Fc+. For com pari-
son with literature data9a,11, E1/2 (Fc/Fc+) = 0.40 V vs SCE (aceton itrile). In th e com plexes,
L den otes th e arom atic acceptor m oiety in th e com posed 1,10-ph en an th rolin e (ph en )-type
b
c
d
e
ligan ds depicted in Ch art 1. Th is work. Ref.11 In THF. Ref.9a
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

New Ru(II) Polypyridine Complex
1895
th rolin e m oiety was n ot observed in th e available n egative poten tial win -
dow of th e aceton itrile electrolyte.
Complex 2. Th e cyclic voltam m ogram of com plex 2 in aceton itrile sh ows
a on e-electron oxidation of th e Ru(II) cen tre at E1/2 = 0.91 V vs Fc/Fc+,
wh ich is sligh tly m ore positive com pared to th e referen ce com plexes in
Table I. Th is sm all sh ift m ay reflect th e presen ce of th e acceptor an th ra-
quin on e com pon en t of ligan d 1. Th e first cath odic wave of com plex 2 at
E1/2 = –1.19 V is assign ed to on e-electron reduction of ligan d 1 largely local-
ised at th e an th raquin on e site. Com pared to free an th raquin on e an d 1, th is
cath odic step occurs less n egatively, wh ich is explain ed by stabilisation of
th e quin on e π-system due to coordin ation of ligan d 1 th rough th e ph en an -
th rolin e ch elate. Th is result reveals an in teraction between th e fused
ph en an th rolin e an d an traquin on e redox cen tres of ligan d 1 th at are n ot
fully in depen den t. Th e oth er cath odic waves of com plex 2 in aceton itrile
were stron gly affected by adsorption at th e electrode surface an d acceptable
resolution could on ly be ach ieved at a low con cen tration (~10–4 m ol l–1),
usin g th e differen tial pulse voltam m etry tech n ique (Table I). Th e cath odic
part of th e cyclic voltam m ogram of com plex 2 was th erefore recorded in
THF (Table I an d Fig. 1). Th e cyclic voltam m ogram sh ows th e well sepa-
rated in itial reduction of th e an th raquin on e m oiety to an th rasem iquin on e,
followed by two greatly overlappin g cath odic steps. Con siderin g on ly th e
values of th e electrode poten tials (Table I), obtain ed with a h igh er accuracy
FIG. 1
Cyclic voltam m ogram of com plex 2 recorded in THF at 293 K. Th e asterisk den otes th e refer-
en ce ferrocen e/ferrocen ium couple. Con dition s: Pt disk electrode, v = 100 m V s–1
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1896
Hartl, Vernier, Belser:
from th e correspon din g differen tial pulse voltam m ogram (Fig. 2), it is diffi-
cult to decide wh eth er th e secon d cath odic step E1/2 = –1.73 V is localised
on th e an cillary 2,2′-bipyridin e ligan ds or wh eth er th e an th rasem iquin on e
site in 2– reduces first. Th e an swer was sough t in a UV-VIS spectroelectro-
ch em ical experim en t (see below). Th e fourth reduction at E1/2 = –2.02 V
in volves th e secon d 2,2′-bipyridin e ligan d, as revealed by th e com parison
with th e referen ce com plexes (Table I). Th e fifth cath odic step th en occurs
with a h igh probability at th e coordin atin g ph en an th rolin e redox cen tre of
ligan d 12–. It lies m ore n egatively th an th e in itial reduction s of th e 2,2′-
bipyridin e ligan d, m ain ly as th e con sequen ce of th e precedin g two-electron
reduction of th e an th raquin on e m oiety. Th e voltam m etric respon se of
com plex 2 does n ot provide an y eviden ce for proton ation of reduced ligan d
1n– (n = 1–3), all cath odic steps bein g fully reversible in th e sub-secon d tim e
dom ain .
UV-VIS Spectroelectroch em istry
Anthraquinone. On e-electron reduction of th e acceptor buildin g block of
ligan d 1 produces th e stable sem iquin on e radical an ion . Its UV-VIS spec-
trum in aceton itrile at 293 K (Fig. 3) sh ows two structured absorption ban ds
cen tred at 400 an d 550 n m . Th e th ird, very broad, structured ban d lies in
th e n ear in frared region , below 700 n m . All th ese absorption features are
due to in traligan d (IL) electron ic tran sition s in volvin g th e sin gly occupied
FIG. 2
Differen tial pulse voltam m ogram of com plex 2 recorded in THF at 293 K. Th e asterisk den otes
th e referen ce ferrocen e/ferrocen ium peak. Con dition s: Pt disk electrode, v = 20 m V s–1, step in -
crem en t 3 m V. Th e sam e sam ple as sh own in Fig. 1
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

New Ru(II) Polypyridine Complex
1897
π* orbital. Furth er reduction leads to disappearan ce of th e n ear in frared
ban d. Th e ban d at 400 n m furth er grows an d ch an ges its sh ape, wh ile th e
ban d at 550 n m develops to a broad un resolved ban d cen tred at 520 n m
(Fig. 4). Reverse oxidation of th e two-electron reduction product is sh ifted
to a less n egative poten tial by ca. 1 V, as con firm ed by th e th in -layer cyclic
voltam m ogram recorded alon g with th e UV-VIS spectral ch an ges, an d re-
sults directly in th e full recovery of paren t an th raquin on e. Such beh aviour
FIG. 3
UV-VIS spectral ch an ges accom pan yin g on e-electron reduction of an th raquin on e to
an th rasem iquin on e. Con dition s: butyron itrile, 293 K, OTTLE cell
FIG. 4
UV-VIS spectral ch an ges accom pan yin g on e-electron reduction of an th rasem iquin on e to
an th rah ydroquin on e. Con dition s: butyron itrile, 293 K, OTTLE cell
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1898
Hartl, Vernier, Belser:
is ch aracteristic of proton ation of th e an th raquin on e-based dian ion to
an th rah ydroquin on e12. Figures 3 an d 4 serve as an im portan t diagn ostic
tool for th e discussion of th e cath odic respon ses of ligan d 1 an d com plex 2.
Ligand 1. Th e UV-VIS spectrum of ligan d 1 differs from th at of an th ra-
quin on e in th e presen ce of a weak absorption ban d cen tred at 595 n m ,
wh ich gives ligan d 1 its purple colour in solution (solid 1 is green ish gray).
As th is ban d is absen t in th e UV-VIS spectrum of com plex 2 (see below), it
m ay belon g to an electron ic tran sition in volvin g th e lon e pairs at th e n on -
coordin ated ph en an th rolin e n itrogen s an d directed toward th e adjacen t
π-acceptor an th raquin on e subun it. Th e ππ* ban d of th e quin on e m oiety is
red-sh ifted to 380 n m com pared to free an th raquin on e (325 n m ).
On e-electron reduction of ligan d 1 is accom pan ied by sim ilar spectral
ch an ges as observed in th e case of an th raquin on e. In th e visible region , a
structured ban d appears at 440 n m , wh ich h as an equivalen t in th e 400 n m
ban d of an th rasem iquin on e. Th e 550 n m IL ban d of an th rasem iquin on e is
also sh ifted to lower en ergies (720 n m ) in 1–; th e equivalen t of th e broad
an d weak NIR absorption below 700 n m was n ot observed down to 1000 n m
(Fig. 5). Th e secon d on e-electron cath odic step th en produced again th e
proton ated an th rah ydroquin on e form of 12– (H21), as con firm ed by th e ap-
pearan ce of th e absorption ban d at 515 n m with a sh oulder tailin g to
800 n m (Fig. 5). Th e lon g-wavelen gth sh oulder m ay belon g to a π–π* tran si-
tion in volvin g th e an th rah ydroquin on e an d ph en an th rolin e subun its of
H21 (ref.13). An oth er ch aracteristic feature in th e absorption spectra of 1–
FIG. 5
UV-VIS spectra of ligan d 1 (full lin e), 1– (dash ed lin e) an d proton ated 12– with an th rah ydro-
quin on e m oiety (H21; dotted lin e). Con dition s: butyron itrile, 293 K, OTTLE cell
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

New Ru(II) Polypyridine Complex
1899
an d H21 is th e fairly in ten se UV ban d (with a sh oulder) at 315 an d 320 n m ,
respectively, wh ich lies above 300 n m in th e case of an th rah ydroquin on e.
Complex 2. Accordin g to th e electroch em ical data in Table I, th e on e-
electron reduction of com plex 2 also largely resides on th e an th raquin on e
m oiety. Th is assign m en t is stron gly supported by th e appearan ce of th e
low-en ergy IL absorption ban d of 2– (in THF) at 900 n m (Fig. 6), wh ich is
furth er red-sh ifted com pared to th e correspon din g absorption of 1– (Fig. 5),
in agreem en t with th e less n egative reduction poten tial of com plex 2. Th e
equivalen t of th e 440 n m ban d of 1– is foun d in th e absorption spectrum of
2– at 470 n m ; h owever, in th is case, th e Ru-to-bpy MLCT absorption also
lies in th is region (see below). Th e sin gly reduced 2,2′-bipyridin e ligan d,
free14 or, for exam ple, in [Ru(bpy)3]n (n = +1, 0) (ref.15) or [Cr(bpy)(CO)4]–
(ref.16) absorbs stron gly between 500–600 n m but its π*π* IL absorption be-
low 700 n m h as a m uch lower in ten sity.
Th e secon d on e-electron reduction of com plex 2 at –1.75 V was studied
carefully in th e OTTLE cell usin g sm all n egative poten tial steps in order to
distin guish it from th e partly overlappin g th ird cath odic step (Figs 1 an d 2,
Table I). Th e result is depicted in Fig. 7. Clearly, th e observed spectral
ch an ges in th e visible spectral region n icely correspon d with th e absorption
spectrum of proton ated H21 (Fig. 5). Hen ce, th e proton ation of th e doubly
reduced an th raquin on e m oiety takes place also in th e doubly reduced com -
plex, 22–, alth ough th is process is n ot observable by cyclic voltam m etry on
th e sub-secon d tim e scale.
FIG. 6
UV-VIS spectra of com plex 2 (full line), 2– (dashed line) and the three-electron reduction product
(dotted lin e). Con dition s: THF, 293 K, OTTLE cell
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1900
Hartl, Vernier, Belser:
Th e on set of th e poorly separated th ird cath odic step at –1.81 V is in di-
cated by a structured absorption ban d at 530 n m . Th is diagn ostically im -
portan t ban d is also observed in th e UV-VIS spectrum of th e on e-electron -
reduced com plex [Ru(bpy)3]+ (ref.15); it belon gs to th e red-sh ifted Ru-to-bpy
CT ban d overlappin g with an IL absorption of th e radical an ion of
2,2′-bipyridin e. We can th erefore safely con clude th at th e first two cath odic
steps of com plex 2 are localised at th e an th raquin on e subun it of ligan d 1
an d on ly th e th ird an d fourth on e-electron reduction s occur at th e
2,2′-bipyridin e ligan ds, after double proton ation of 12–. In th is regard, com -
plex 2 differs from [Ru(bpy)2(dcn p)]2+ con tain in g th e com parably stron g
acceptor ligan d dcn p (Ch art 1 an d Table I) th at is on ly sin gly reduced prior
to th e followin g cath odic steps localised at th e 2,2′-bipyridin e ligan ds9a.
Th e voltam m etric an d spectroelectroch em ical m easurem en ts stron gly
support a substan tial electron ic separation between th e fused ph en an -
th rolin e an d acceptor an th raquin on e com pon en ts of ligan d 1. Th e facile
form ation of th e an th rah ydroquin on e system by th e con secutive two-
electron reduction of free an th raquin on e occurs also in ligan d 1 an d com -
plex 2, as docum en ted by UV-VIS spectroscopy. Th e stepwise two-electron
reduction of ligan d 1 in com plex 2 does n ot sign ifican tly destabilise th e
lowest un occupied π*(bpy) orbitals, wh ich causes a partial overlap of th e
an th rasem iquin on e- an d bpy-based cath odic waves. Th is would n ot be th e
case if th e ph en an th rolin e π-system of ligan d 1 were substan tially in volved
in th e in itial cath odic steps, in creasin g electron ic repulsion between re-
FIG. 7
UV-VIS spectral ch an ges accom pan yin g on e-electron reduction of 2–. Con dition s: THF, 293 K,
OTTLE cell
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

New Ru(II) Polypyridine Complex
1901
duced ligan d 1 an d th e an cillary 2,2′-bipyridin e ligan d an d, h en ce, separa-
tion of th e secon d an d th ird cath odic waves. On th e oth er h an d, th e elec-
tron ic com m un ication between th e fused an th raquin on e an d ph en an -
th rolin e un its is n ot com pletely n egligible, as in dicated by a n ew electron ic
tran sition of ligan d 1 at 595 n m (Fig. 5) an d by stabilisation (~0.1 eV) of
th e quin on e redox orbital upon coordin ation of ligan d 1. For com parison ,
th is stabilisation is m uch larger (~0.3 eV) for th e con den sed 1,10-
ph en an th rolin e-5,6-dion e (pd) ligan d (Ch art 1 an d Table I).
Electronic Absorption and Luminescence of Complex 2
Th e UV-VIS spectrum of com plex 2 (Fig. 6) closely resem bles th at of
[Ru(bpy)3]2+, sh owin g th e stron g ππ*(bpy) ban d at 285 n m an d th e com -
posed Ru-to-bpy CT ban d cen tred at 450 n m . Th e CT tran sition to th e
ph en an th rolin e part of ch elated ligan d 1 probably also falls with in th is ab-
sorption . Th e m olar absorption coefficien t of th e MLCT ban d of com plex 2
is h igh er com pared to [Ru(bpy)3]2+ an d oth er referen ce com plexes9a in
Table II. Th e n ew ban d at 365 n m can be attributed to a π–π* tran sition
with in th e an th raquin on e m oiety (cf. Fig. 3). An oth er an th raquin on e-
cen tred absorption can be traced between 240–270 n m . Th ere is n o spectro-
scopic eviden ce for a CT tran sition from th e Ru(II) cen tre to th e acceptor
an th raquin on e m oiety.
TABLE II
Eletron ic absorption spectra an d em ission data of com plex 2 an d selected referen ce9a com -
poun dsa
λ
abs, n m
λem ,
n m
Φem × 10–3
Com plex
τ, n s
(εm ax, l m ol–1 cm –1
)
2
deaerated
451 (18300)
439 (14600)
610
700
5.5
0.5
0.5
2
[Ru(bpy)2(dcn p)]2+ aerated
deaerated
aerated
deaerated
[Ru(bpy)2(dppz)]2+ aerated
deaerated
aerated
deaerated
120
610b
610
610
667
610
611
611
1700, 185
170
[Ru(bpy)2(n p)]2+
452 (15100)
448 (15700)
450 (13000)
16
834
82
229
390
260
[Ru(bpy)3]2+
190
12
60
1000
a
b
Con dition s: aceton itrile, 298 K. Em ission m axim um of a m ulticom pon en t ban d.
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1902
Hartl, Vernier, Belser:
Th e differen ce tran sien t absorption spectrum of com plex 2 in deaerated
aceton itrile was recorded in th e n an osecon d tim e dom ain upon excitation
at 450 n m (Fig. 8). It sh ows th e bleach ed MLCT ban d at 450 n m wh ich de-
cays m on o-expon en tially with a lifetim e of 4.5 n s to restore th e pre-pulsed
baselin e. Th e tran sien t absorption between 500–820 n m probably belon gs
to a ch arge-tran sfer excited state with a n egative ch arge on an cillary
2,2′-bipyridin e ligan ds an d/or (given th e ban dwidth ) on th e exten ded
π-system of th e 1,10-ph en an th rolin e subun it of ligan d 1. Th e an th rasem i-
quin on e absorption in a ch arge-separated excited state can be excluded,
sin ce th e MLCT bleach would oth erwise be less developed an d differen tly
sh aped, an d th ere would be n o stron g tran sien t absorption startin g already
at 400 n m (see Fig. 6). Th e MLCT assign m en t is also supported by th e
ph otoph ysical properties of com plex 2 (see below).
Th e em ission spectrum of com plex 2 was recorded in aceton itrile at
293 K (Fig. 9). Th e ph otoph ysical data an d com parison with th e referen ce
com poun ds are presen ted in Table II. Th e lum in escen ce of 2 is observed in
deaerated aceton itrile at 610 n m , sim ilarly to th e em ission m axim um of
[Ru(bpy)3]2+. However, com pared to th e referen ce com plex, th e em ission is
very weak (Φem = 5 × 10–4) an d decays with a m uch sh orter lifetim e (5.5 n s),
wh ich is close to th e 4.5 n s value m easured for th e tran sien t in th e n an o-
secon d TA spectra (Fig. 8). In air-equilibrated aceton itrile, th e em ission in -
ten sity of com plex 2 furth er drops. Th is beh aviour agrees with population
FIG. 8
Differen ce tran sien t absorption spectra of com plex 2 in deaerated aceton itrile solution (λexc
=
450 n m , 2 n s FWHM, 1 n s in crem en t tim e delay)
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

New Ru(II) Polypyridine Complex
1903
of th e em ittin g Ru-to-bpy an d/or Ru-to-ph en 3MLCT excited states. Th e fast
decay an d very low em ission quan tum yield poin t to efficien t quen ch in g of
th e 3MLCT em ission by electron tran sfer to th e acceptor an th raquin on e
m oiety of ligan d 1 an d rapid recovery of th e groun d state by ch arge recom -
3
bin ation 6,8. By con trast, en ergy tran sfer occurs in th e MLCT excited state
of th e com plex [Ru(bpy)2(dcn p)]2+ to th e exten ded π-system of th e an th ra-
cen edicarbon itrile m oiety, resultin g in a two weakly em ittin g ch arge tran s-
fer excited states (Table II)9a. At low tem peratures, an d also in the other ref-
erence com plexes [Ru(bpy)2(dppz)]2+ and [Ru(bpy)2(np)]2+, th e 3MLCT state
is in a (solven t-depen den t) th erm al equilibrium with a sligh tly lower-lyin g
3IL state of th e acceptor subun it, wh ich furth er in creases th e em ission life-
tim e an d in ten sity com pared to [Ru(bpy)3]2+ (ref.9a). We h ave n o eviden ce
for such en ergy tran sfer to th e an th raquin on e m oiety in com plex 2.
Chemical Reduction of Ligand 1 and Luminescence of Complex 2
Com plex 2 was reacted with cobaltocen e an d Na2S2O4 in order to prove
wh eth er its weak em ission is affected by ch em ical reduction of th e
an th raquin on e site.
Accordin g to data in Table I, cobaltocen e (E1/2 = –1.34 V vs Fc/Fc+) is ca-
pable of reducin g com plex 2 exclusively to 2–. Addition of cobaltocen e
(1 equivalen t or in excess) to a solution of com plex 2 in dry aceton itrile led
to com plete disappearan ce of th e em ission ban d observed for n on -reduced
FIG. 9
Em ission spectrum of com plex 2 in deaerated aceton itrile (λexc = 450 n m , A = 0.1, l = 1 cm )
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1904
Hartl, Vernier, Belser:
2 at 610 n m . Th is result can be ascribed8 to quen ch in g of th e *{RuIII(bpy• –)}
an d/or *{RuIII(ph en • –)} ch arge tran sfer excited states by electron tran sfer
from th e reduced an th rasem iquin on e m oiety to Ru(III). Th is process is re-
versible an d th e em ission of com plex 2 could be restored by addition of an
excess of FcBF4.
In con trast to th e reduction with cobaltocen e, addition of 2 equivalen ts
of Na2S2O4 in a sm all am oun t of water to a solution of com plex 2 in aceto-
n itrile led to a substan tial in crease in th e em ission in ten sity (Fig. 10). Th e
UV-VIS absorption spectrum of th e resultin g oran ge solution sh owed n ew
features at 330 n m an d aroun d 450–500 n m , but n o absorption below
750 n m , wh ich is con sisten t with th e form ation of th e doubly proton ated
form of 22– con tain in g th e an th rah ydroquin on e12 m oiety (Figs 5 an d 7). For
com parison , addition of th e sam e am oun t of water con tain in g n o reducin g
agen t caused m uch less pron oun ced en h an cem en t of th e lum in escen ce in -
ten sity (Fig. 10). Apparen tly, th e an th rah ydroquin on e m oiety does n ot act
as a quen ch er an d th e em ittin g excited state becom es lon ger-lived, result-
in g in im proved lum in escen ce properties.
Furth er reduction with m ore th an 2 equivalen ts of Na2S2O4, wh ich is
localised at th e an cillary 2,2′-bipyridin e ligan ds, led again to dim in ish ed lu-
m in escen ce, sim ilar to th e quen ch in g of em ission by electron tran sfer
caused by th e on e-electron reduction of th e an th raquin on e m oiety in 2
with cobaltocen e.
FIG.10
Steady-state em ission spectra of 4.1 × 10–5 M 2 a in deaerated dry aceton itrile (4 m l), b after ad-
dition of 2 equivalen ts of Na2S2O4 in water (20 µl) an d c after addition of water (20 µl) (λexc
450 n m , A = 0.74, l = 1 cm )
=
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

New Ru(II) Polypyridine Complex
1905
Conclusions
Th e Ru-to-bpy an d/or Ru-to-ph en ph otoexcitation of com plex 2 in aceton i-
trile results in th e population of a relatively sh ort-lived an d weakly em it-
3
tin g MLCT state com pared to th e ph otoph ysical properties of th e referen ce
com plex [Ru(bpy)3]2+. Th ere is n o eviden ce for an equilibrium between th e
3MLCT state an d a 3IL state localised on th e an th raquin on e m oiety in
ligan d 1, in differen ce from th e situation en coun tered9a in th e oth er refer-
en ce com plexes [Ru(bpy)2(L′)]2+ (L′ = dppz, n p, dcn p; see Ch art 1). Th e lu-
m in escen ce quen ch in g is probably caused by in tram olecular electron tran s-
3
fer in th e MLCT state to th e adjacen t an th raquin on e receptor an d con com -
itan t ch arge recom bin ation . Sim ilarly to th e ligh t-in duced reduction of th e
an th raquin on e m oiety, on e-electron ch em ical reduction producin g th e
an th rasem iquin on e don or results in fully quen ch ed em ission due to elec-
tron tran sfer to th e ph otoexcited *{RuIII(bpy• –)} an d/or *{RuIII(ph en • –)}
lum in oph ores. Th e reductive quen ch in g is switch ed off by proton ation of
th e an th raquin on e-dian ion m oiety in 22– form ed by addition of sodium
dith ion ite in water, resultin g in a substan tial gain of lum in escen ce in ten -
sity com pared to th at of n on -reduced com plex 2. Th e detailed description
of th ese processes requires addition al studies at variable tem peratures an d
pH, wh ich are curren tly un der way.
EXPERIMENTAL
Materials an d Syn th eses
All syn th eses were perform ed un der an in ert atm osph ere of dry n itrogen , usin g stan dard
Sch len k tech n iques.
Toluen e, aceton itrile an d butyron itrile (all Acros) were fresh ly distilled from P2O5, an d
THF (Acros) from Na/ben zoph en on e un der n itrogen .
Th e followin g com poun ds were prepared accordin g to literature procedures:
(i) ferrocen ium tetrafluoroborate, FcBF4, from HBF4 an d ferrocen e17, (ii) 5,6-bis(brom o-
m eth yl)-1,10-ph en an th rolin e ph otoch em ically10 from 5,6-dim eth yl-1,10-ph en an th rolin e
an d N-brom osuccin im ide (both Fluka), an d (iii) [RuCl2(bpy)2] from RuCl3·3H2O (JM)18. Th e
supportin g electrolyte tetrabutylam m on ium h exafluoroph osph ate, Bu4NPF6 (Aldrich ), was
recrystallised twice from eth an ol an d dried un der vacuum at 80 °C overn igh t. Cobaltocen e,
ferrocen e (both Aldrich ) an d sodium dith ion ite, Na2S2O4·H2O (BDH), were used as received.
Alum in um oxide for colum n ch rom atograph y was purch ased from Fluka (type 507 C n eu-
tral, Brockm an n grade 1, particle size 0.05–0.15 m m , pH 7.0 ± 0.5).
Elem en tal an alyses were con ducted at th e Un iversity of Applied Scien ces of Western Swit-
zerlan d, Fribourg.
Anthra[2,3-f][1,10]phenanthrolin-10,15-quinone (1). 5,6-Bis(brom om eth yl)-1,10-ph en an th ro-
lin e (0.15 g, 0.41 m m ol) an d 1,4-n aph th oquin on e (0.26 g, 1.63 m m ol) were suspen ded in
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1906
Hartl, Vernier, Belser:
dry toluen e (250 m l) in a 500-m l roun d-bottom flask. Th e grey suspen sion was refluxed un -
der argon for 12 h . Durin g th is tim e, th e suspen sion colour turn ed oran ge-red an d fin ally
dark purple. After coolin g to room tem perature an d filtration , th e filtrate was divided in to
th ree fraction s of equal volum es an d 6 M HCl (75 m l per fraction ) was added. Th e resultin g
suspen sion was stirred for 1 day. Th e purple precipitate was th en filtered off, wash ed succes-
sively with water, toluen e an d dieth yl eth er an d dried overn igh t un der reduced pressure at
45 °C. Yield 44 m g (60 %). 1H NMR (DMSO-d6, 300 MHz; see Ch art 2): 8.00 (2 H, HB); 8.10
(2 H, HE); 8.30 (2 H, HE); 9.23 (2 H, HC); 9.50 (2 H, HD); 9.64 (2 H, HA). MS (ESI), m/z: 361
(M+).
Due to its poor solubility in com m on organ ic solven ts, crude ligan d 1 was furth er used
with out purification for th e syn th esis of com plex 2.
HB
HA
HC
HD
O
O
HE
N
N
HF
CHART 2
[Ru(bpy)2(1)][PF6]2 (2). [RuCl2(bpy)2] (39 m g, 0.08 m m ol) an d ligan d 1 (23 m g, 0.07 m m ol)
were suspen ded in eth ylen e glycol (10 m l) in a 50-m l roun d-bottom flask an d h eated un der
stirrin g at 150 °C for 2 h . Eth ylen e glycol was rem oved un der reduced pressure an d replaced
by water (25 m l). Am m on ium h exafluoroph osph ate (70 m g) was added in th e n ext step an d
th e suspen sion was stirred slowly for 1.5 h . Th e precipitate was isolated by suction filtration
an d wash ed with dieth yl eth er. Th e crude com plex was extracted with aceton e an d purified
on SiO2 plates, usin g MeCN/MeOH/H2O/saturated aqueous KNO3 (40:10:10:1, volum e ratios)
as th e m obile ph ase. After dryin g, fin al purification of th e oran ge product was accom plish ed
by colum n ch rom atograph y over n eutral alum in ium oxide, usin g aceton e/water (99:1) as
eluen t. Yield 38 m g (45%). 1H NMR (CD3CN, 300 MHz; see Ch arts 2 an d 3): 7.25 (t, 2 H,
H8); 7.47 (t, 2 H, H8′); 7.67 (d, 2 H, H7); 7.84 (m , 2 H, H2/7′); 7.92 (dd, 2 H, H6); 8.05 (t, 2 H,
H9); 8.12 (t, 2 H, H9′); 8.17 (d, 2 H, H1); 8.32 (dd, 2 H, H5); 8.53 (t, 4 H, H10/10′); 9.38 (d,
2 H, H3); 9.60 (s, 2 H, H4). 13C NMR (CD3CN, 75 MHz): 125.33, 125.68, 128.38, 128.50,
128.81, 129.00, 131.12, 133.32, 134.39, 135.06, 136.22, 139.27, 150.23, 153.27, 153.51,
154.59, 158.29, 158.48. MS (ESI), m/z: 919 (M+), 387 (M2+). For C44H28F12N6O2P2Ru (1063.7)
calculated: 49.68% C, 2.65% H, 7.90% N; foun d: 50.28% C, 2.45% H, 6.88% N.
9
8
10
10'
7'
2
7
3
9'
8'
N
Ru
N
O
O
1
5
4
N
N
N
6
(PF6)2
N
2
CHART 3
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

New Ru(II) Polypyridine Complex
1907
Ph ysical Measurem en ts
Spectroscopy. ESI (electron spray ion isation ) m ass spectra were obtain ed with a Bruker
FTMS 4.7 T Bio APEXII spectrom eter.
Electron ic absorption spectra were recorded usin g Perkin –Elm er Lam bda 18 an d Hewlett–
Packard 8453A diode-array spectroph otom eters, an d 1H an d 13C NMR spectra on a Varian
Gem in i 300 spectrom eter. Th e m olar absorption coefficien t for th e MLCT ban d of 2 was de-
term in ed for four differen t con cen tration s of th e com plex in th e ran ge 10–6–10–5 m ol l–1
.
Con tin uous-wave em ission spectra were m easured on a Spex Fluorolog 1681 spectrofluori-
m eter equipped with a Xe arc ligh t source, a Ham am atsu R928 ph otom ultiplier tube detec-
tor an d double excitation an d em ission m on och rom ators. Em ission spectra were corrected
for source in ten sity an d detector respon se usin g stan dard correction curves. Deaerated solu-
tion s were prepared by th e freeze-pum p-th aw tech n ique on a h igh -vacuum lin e. Lum in es-
cen ce quan tum yields (Φem ) were m easured in optically dilute solution s (A ~ 0.1), usin g
stan dard [Ru(bpy)3]2+ (Φem = 0.012 an d 0.060 in aerated an d deaerated aceton itrile9a, respec-
tively), accordin g to Eq. (1) (ref.19
)
Φsem = Φer m (Is/Ir)(Ar/As)(ηs/ηr)2
(1)
wh ere subscripts s an d r den ote th e sam ple an d referen ce, respectively. Th e sym bol I stan ds
for th e in tegrated em ission in ten sity, A is th e absorban ce at th e excitation wavelen gth an d
η th e refractive in dex of th e solven t.
Tim e-resolved em ission m easurem en ts were perform ed at sin gle wavelen gth , usin g a con -
tin uously tun able (420–710 n m ) Coh eren t In fin ity XPO laser as excitation source. Th e em it-
ted ligh t was collected in an Oriel m on och rom ator, detected with a Ham am atsu P28 ph oto-
m ultiplier tube an d recorded with a Tektron ix TDS3052 (500 MHz) oscilloscope. A ph oto-
diode was used as extern al trigger source. Optically dilute (A < 0.3) sam ple solution were
used.
Nan osecon d tran sien t absorption (n s TA) spectra of deaerated solution s were obtain ed by
irradiatin g th e sam ples with 2-n s pulses of th e 450 n m lin e (typically 4 m J pulse–1) of th e
Coh eren t In fin ity XPO laser workin g at 10 Hz. Th e probe ligh t from a low-pressure, h igh -
power EG&G FX-504 Xe lam p was passed th rough th e sam ple cell an d dispersed with an
Acton Spectra-Pro-150 spectrograph equipped with 150 or 600 m m –1 gratin g an d a 400 µm
slit, resultin g in a 6 or 1.2 n m m axim um resolution , respectively. Th e data collection system
con sisted of a gated in ten sified CCD detector (Prin ceton In strum en ts ICCD-576EMG/RB), a
program m able pulse gen erator (PG-200), an d an EG&G Prin ceton Applied Research Model
9650 digital delay gen erator.
Electrochemistry. Cyclic an d differen tial pulse voltam m ogram s of 10–3 M sam ples in 10–1
M
Bu4NPF6 electrolyte solution were recorded in a gas-tigh t, sin gle-com partm en t cell equipped
with a platin um disk workin g electrode (apparen t surface area 0.42 m m 2), a coiled platin um
wire auxiliary electrode an d a silver wire pseudoreferen ce electrode. Th e cell was con n ected
to a com puter-con trolled PAR Model 283 poten tiostat. All electrode poten tials are reported
again st th e ferrocen e/ferrocen ium (Fc/Fc+) redox couple used as an in tern al stan dard20. Con -
version to oth er referen ce electrode scales can be foun d elsewh ere21
.
Spectroelectroch em ical experim en ts were perform ed with an optically tran sparen t th in -
layer electroch em ical (OTTLE) cell22 equipped with a Pt m in igrid workin g electrode (32 wires
per cm ) an d CaF2 win dows. Th e poten tial durin g th e th in -layer electrolyses was con trolled
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1908
Hartl, Vernier, Belser:
with a PA4 (EKOM, Czech Republic) poten tiostat. Th e con cen tration of th e spectroelectro-
ch em ical sam ples was 1 × 10–3 m ol l–1
.
We thank the National Swiss Science Foundation for funding through the NFP 47 programme. Pro-
fessor Luisa De Cola (Universität Münster) is acknowledged for valuable discussions and support of
this work. Mr R. T. F. Jukes (Universiteit van Amsterdam) kindly helped with measurements of some
of the emission spectra.
REFERENCES
1. Crossley M. J., Johnston L. A.: Chem. Commun. 2002, 1122.
2. Wasielewski M. R.: Chem. Rev. 1992, 92, 435.
3. Yan P., Holman M. W., Robustelli P., Chowdhury A., Ishak F. I., Adams D. M.: J. Phys.
Chem. B 2005, 109, 130.
4. Goudle V., Harriman A., Lehn J.-M.: J. Chem. Soc., Dalton Trans. 1993, 1034.
5. McCall J., Alexander C., Richter M. M.: Anal. Chem. 1999, 71, 2523.
6. Opperman K. A., Mecklenburg S. L., Meyer T. J.: Inorg. Chem. 1994, 33, 5295.
7. Borgström M., Johansson O., Lomoth R., Berglund Baudin H., Wallin S., Sun L.,
Åkermark B., Hammarström L.: Inorg. Chem. 2003, 42, 5173.
8. Benniston A., Chapman G. M., Harriman A., Rostron S. A.: Inorg. Chem. 2005, 44, 4029.
9. a) Albano G., Belser P., De Cola L., Gandolfi M. T.: Chem. Commun. 1999, 1171;
b) Albano G., Belser P., Daul C.: Inorg. Chem. 2001, 40, 1408; c) Batista E. R., Martin
R. L.: J. Phys. Chem. A 2005, 109, 3128.
10. Barton J. W., Shepherd M. K., Willis R. J.: J. Chem. Soc., Perkin Trans. 1 1986, 967.
11. Fujihara T., Okamura R., Wadu T., Tanaka K.: Dalton Trans. 2003, 3221.
12. Babaei A., Connor P. A., McQuillan A. J.: J. Chem. Educ. 1997, 74, 1200.
13. Keyes T. E., Jayaweera P. M., McGarvey J. J., Vos J. G.: J. Chem. Soc., Dalton Trans. 1997,
1627.
14. Krejčík M., Vlček A. A.: J. Electroanal. Chem. 1991, 313, 243.
15. Heath G. A., Yellowless L. J., Braterman P. S.: J. Chem. Soc., Chem. Commun. 1981, 287.
16. Víchová J., Hartl F., Vlček A., Jr.: J. Am. Chem. Soc. 1992, 114, 10903.
17. Hendrickson D. N., Sohn Y. S., Gray H. B.: Inorg. Chem. 1971, 10, 1559.
18. Hua X: Ph.D. Thesis. University of Fribourg, Fribourg 1993.
19. Demas J. N., Crosby G. A.: J. Phys. Chem. 1971, 75, 991.
20. Gritzner G., Kůta J.: Pure Appl. Chem. 1984, 56, 461.
21. Pavlishchuk V. V., Addison A. W.: Inorg. Chim. Acta 2000, 298, 97.
22. Krejčík M., Daněk M., Hartl F.: J. Electroanal. Chem. Interfacial Electrochem. 1991, 317,
179.
Collect. Czech. Chem. Commun. (Vol. 70) (2005)