Excimer formation in oligo[2,5-bis(hexadecyloxy)-1,4-phenylene]s followed by fluorescence spectroscopy

Article abstract of DOI:10.1135/cccc20011473

Using the steady-state and time-resolved fluorescence spectroscopy, the behavior of "hairy-rod" oligo- and poly[2,5-bis(hexadecyloxy)-1,4-phenylene]s in tetrahydrofuran solutions was investigated. The materials were prepared by the Yamamoto coupling reaction using zinc as a reducing metal, the nickel(II)/triphenylphosphine complex as a catalyst, and 2,2′-bipyridine as a coligand. The appropriate oligomer fractions were separated by fractional precipitation and characterized by GPC and end group analysis. The fluorescence quantum yield of oligomers and polymers increased with their increasing conjugation length. The fluorescence emission spectra of polymers and longer oligomers exhibited one emission maximum at 390 nm with a single-exponential decay and fluorescence lifetimes (τ) around 1 ns. The substitution in positions 2 and 5 forces the adjacent backbone benzene units out of the plane, which results in twist angles 60-80°, and the bulky substituents exclude the cofacial sandwich-type configuration necessary for excimer formation. However, with shorter oligomers, another emission band at 460 nm appeared. Fluorescence decays at 460 nm were found to be double-exponential with longer excited-state lifetimes [e.g. τ₁ = 6.9 ns (76%), τ₂ = 2.4 ns (24%)]. With shorter oligomers (dimer, trimer), we assume a sandwich-type configuration with sufficiently close interchain distance and hence the excimer can form. Hydrophobic interactions of long aliphatic side chains in a polar medium play an important role in the excimer formation.

Full text of DOI:10.1135/cccc20011473

Excimer Formation
1473
EXCIMER FORMATION IN OLIGO[2,5-BIS(HEXADECYLOXY)-
1,4-PHENYLENE]S FOLLOWED BY FLUORESCENCE SPECTROSCOPY
1,
2
3
Drahomír VÝPRACHTICKÝ *, Věra CIMROVÁ , Luꢀka MACHOVÁ
and Veronika POKORNÁ
4
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic,
1
Heyrovského nám. 2, 162 06 Prague 6, Czech Republic; e-mail: vyprach@imc.cas.cz,
2
3
4
cimrova@imc.cas.cz, machova@imc.cas.cz, pokorna@imc.cas.cz
Received May 10, 2001
Accepted August 8, 2001
Usin g th e steady-state an d tim e-resolved fluorescen ce spectroscopy, th e beh avior of
“h airy-rod” oligo- an d poly[2,5-bis(h exadecyloxy)-1,4-ph en ylen e]s in tetrah ydrofuran solu-
tion s was in vestigated. Th e m aterials were prepared by th e Yam am oto couplin g reaction us-
in g zin c as a reducin g m etal, th e n ickel(II)/triph en ylph osph in e com plex as a catalyst, an d
2,2′-bipyridin e as a coligan d. Th e appropriate oligom er fraction s were separated by fraction al
precipitation an d ch aracterized by GPC an d en d group an alysis. Th e fluorescen ce quan tum
yield of oligom ers an d polym ers in creased with th eir in creasin g con jugation len gth . Th e flu-
orescen ce em ission spectra of polym ers an d lon ger oligom ers exh ibited on e em ission m axi-
m um at 390 n m with a sin gle-expon en tial decay an d fluorescen ce lifetim es (τ) aroun d 1 n s.
Th e substitution in position s 2 an d 5 forces th e adjacen t backbon e ben zen e un its out of th e
plan e, wh ich results in twist an gles 60–80°, an d th e bulky substituen ts exclude th e cofacial
san dwich -type con figuration n ecessary for excim er form ation . However, with sh orter oligo-
m ers, an oth er em ission ban d at 460 n m appeared. Fluorescen ce decays at 460 n m were
foun d to be double-expon en tial with lon ger excited-state lifetim es [e.g. τ1 = 6.9 n s (76%), τ2
=
2.4 n s (24%)]. With sh orter oligom ers (dim er, trim er), we assum e a san dwich -type con figura-
tion with sufficien tly close in terch ain distan ce an d h en ce th e excim er can form . Hydroph o-
bic in teraction s of lon g aliph atic side ch ain s in a polar m edium play an im portan t role in
th e excim er form ation .
Keyw ord s: Tim e-resolved fluorescen ce spectroscopy; Fluorescen ce lifetim e; Excim er; Fluores-
cen ce quan tum yield; Con jugated polym ers; Poly(1,4-ph en ylen e)s; Polyph en ylen es;
Yam am oto couplin g; Suzuki couplin g.
Th e excim er form ation h as been foun d quite com m on for sm all organ ic
m olecules; th e studies1–3 h ave sh own th at th eir basic supram olecular struc-
tures are cofacial san dwich -type con figuration s with in terplan ar distan ces
of ≈0.3–0.4 n m . In con jugated polym ers, a pair of ch ain s m ay form an
excim er wh en on e of th em is excited by ligh t. Just as in th e case of an
excim er form ed by a pair of m olecules, th e stability of th e excim er arises
Collect. Czech. Chem. Commun. (Vol. 66) (2001)
doi:10.1135/cccc20011473

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Výprachtický, Cimrová, Machová, Pokorná:
from overlappin g π orbitals on th e two ch ain s4. It is th erefore a require-
m en t for excim er form ation th at th e two ch ain s m ust be parallel over a
len gth of at least several m on om er un its5, with an in terch ain distan ce of
≈0.3–0.4 n m . Th e distan ce m ust be sm all en ough for th eir π orbitals to
overlap but n ot so stron gly th at th e groun d states can form a n ew stable
m olecule. Sym m etry requires th at th e wavefun ction m ust be a quan tum -
m ech an ical superposition of th e state with excitation on on e ch ain an d th e
state with excitation on th e oth er ch ain . Em ission from an excim er h as a
lower frequen cy th an th at from on e of th e m olecules excited wh en isolated
because of th e lower en ergy in th e excim er state an d because in th e fin al
state, wh ere both ch ain s are in th eir groun d states, th ey repel each oth er.
Th e excim er em ission ch aracteristically h as a lon ger lifetim e because, as fol-
lows from th e sym m etry of th e electron ic wavefun ction in th e excim er
state, th e radiative tran sition to th e groun d state is forbidden 6. Electrolum i-
n escen ce of con jugated polym ers h as attracted a great deal of atten tion
sin ce th e first ligh t-em ittin g devices (LEDs) were fabricated7 on th e basis of
poly(1,4-ph en ylen evin ylen e) (PPV). Th e in terest in th e em ission from PPV
for LED reason s led to exten sive studies aim ed to determ in e wh at excita-
tion s are gen erated by ligh t in PPV an d a n um ber of its derivatives. An im -
portan t con tribution to th e problem of th e existen ce of excim ers or polaron
pairs in PPV was foun d in th e cyan o derivative, poly{[2,5-bis(h exyloxy)-
1,4-ph en ylen e]cyan ovin ylen e} (CN-PPV)8. Com parin g th e em ission from
film s with th at from dilute solution s, it was foun d th at th e form er is
red-sh ifted, with a m uch lon ger lifetim e (20 tim es), broad an d featureless.
Th ese properties an d th e lack of addition al low-frequen cy absorption in di-
cate th e form ation of excim ers in th e film . In th e studies of ph otoin duced
absorption
on
poly{[2-m eth oxy-5-(2-eth ylh exyloxy)-1,4-ph en ylen e]-
vin ylen e} (MEH-PPV), it was foun d th at th e lon g-lived excitation s are
excim ers even th ough th ey do n ot em it9. Detailed calculation s based on
th eir groun d-state geom etry sh owed th at excim ers probably decayed
n on radiatively.
Recen tly10,11 we h ave studied th e electrolum in escen ce (EL), ph oto-
lum in escen ce (PL), an d film m orph ology ch aracteristics of LEDs based on
poly[2,5-bis(isopen tyloxy)-1,4-ph en ylen e] (IP-PPP) an d its blen ds with
poly[m eth yl(ph en yl)silan ediyl] (PMPSi). Com pared with th e PL spectrum of
IP-PPP, th e EL spectrum was red-sh ifted an d, as th e tim e of LED operation
in creased, th e con tribution of yellow em ission in creased. LEDs prepared by
th e Lan gm uir–Blodgett tech n ique, wh ere well-organ ized film s with paral-
lel-ordered polym er ch ain s were used, proved th e aggregate form ation re-
spon sible for yellow em ission .
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Excimer Formation
1475
In th is paper we h ave syn th esized poly[2,5-bis(h exadecyloxy)-
1,4-ph en ylen e] (HD-PPP) an d its oligom ers as an exam ple of so-called
“h airy-rod” m acrom olecules with a lin ear rigid backbon e an d flexible h airy
alkyl side ch ain s. Tran sition electron m icroscopy of such polym ers revealed
a san dwich -type structure with layers of aliph atic side ch ain s perpen dicular
an d layers of th e backbon e parallel to th e substrate surface12,13. We believe
th at lon g alkyl side ch ain s could be an efficien t h in dran ce to possible ag-
gregate or excim er form ation in film s for LEDs. Here, th e steady-state an d
tim e-resolved fluorescen ce beh avior an d excim er form ation in tetra-
h ydrofuran (THF) solution s of HD-PPP an d its oligom ers was studied.
EXPERIMENTAL
Materials
2,5-Dibromohydroquinone. To a suspen sion of h ydroquin on e (82.6 g, 0.75 m ol) in acetic
acid (750 m l), a solution of brom in e (239.7 g, 1.5 m ol Br2) in acetic acid (75 m l) was added
dropwise (2 h ) keepin g th e tem perature below 40 °C. Th e reaction m ixture was stirred for
10 h an d th e precipitate form ed was filtered off. Th e volum e of th e reaction m ixture was re-
duced to on e h alf th us obtain in g an oth er part of th e product. Yield: 112 g (56%).
Recrystallization from water–eth an ol gave n early wh ite crystallin e product. M.p. 190–191 °C
(ref.14 190–191 °C, ref.15 186 °C). For C6H4Br2O2 (267.9) calculated: 26.90% C, 1.50% H,
59.65% Br; foun d: 26.91% C, 1.52% H, 59.41% Br.
2-Bromohydroquinone was prepared by a sim ilar procedure to th at for 2,5-dibrom o-
h ydroquin on e usin g 27.5 g (0.25 m ol) of h ydroquin on e an d 40 g (0.25 m ol) of brom in e in
th e reaction . Th e crude m aterial was recrystallized from ch loroform . Yield: 21.3 g (45%). M.p.
110 °C (ref.16 110.5 °C). For C6H5BrO2 (189.0) calculated: 38.13% C, 2.67% H, 42.28% Br;
foun d: 37.95% C, 2.61% H, 42.35% Br.
1,4-Dibromo-2,5-bis(hexadecyloxy)benzene. 2,5-Dibrom oh ydroquin on e (13.4 g, 0.05 m ol)
was dissolved in eth an olic potassium h ydroxide (7.0 g KOH in 150 m l of eth an ol) an d
refluxed. 1-Brom oh exadecan e (76.3 g, 0.25 m ol) was added slowly an d th e reaction m ixture
was refluxed for 10 h . After coolin g to room tem perature, th e resultin g precipitate was fil-
tered off, wash ed with water an d dried. Th e organ ic m aterial was dissolved in h ot toluen e,
filtered an d th e solven t was evaporated. Th e product was crystallized from aceton e–toluen e
(ch arcoal). Yield: 28.9 g (82%). M.p. 86–87 °C. For C38H68Br2O2 (716.8) calculated: 63.68% C,
9.56% H, 22.30% Br; foun d: 63.93% C, 9.60% H, 22.31% Br. 1H NMR (CDCl3): 0.87 (t, 6 H,
J = 6.6, 2 × CH3); 1.26 (s, 52 H, 26 × CH2); 1.78 (m , 4 H, J = 6.7, 2 × O-C-CH2); 3.93 (t, 4 H,
J = 6.6, 2 × OCH2); 7.07 (s, 2 H, H-arom .). FT IR (film on KBr): 2 917, 2 846 (C–H, stretch in g);
1 675 (C=C, stretch in g); 1 460 (CH2, scissorin g); 1 359 (CH3, ben din g); 1 264 (C–O, stretch -
in g); 1 211 (CH2, twistin g); 1 061 (C–C, stretch in g); 804, 721 (CH2, rockin g); 522 (C–Br).
1,4-Dibromo-2,5-bis(decyloxy)benzene was prepared by a sim ilar procedure to th at for th e
h exadecyl derivative above, usin g 1-brom odecan e (0.25 m ol) as alkylation agen t. Yield:
18.10 g (66%). M.p. 67–69 °C. For C26H44Br2O2 (548.5) calculated: 56.94% C, 8.09% H,
29.14% Br; foun d: 57.13% C, 8.14% H, 28.92% Br. 1H NMR (CDCl3): 0.87 (t, 6 H, J = 6.6, 2 ×
CH3); 1.27 (s, 28 H, 14 × CH2); 1.78 (m , 4 H, J = 6.6, 2 × O-C-CH2); 3.93 (t, 4 H, J = 6.6, 2 ×
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1476
Výprachtický, Cimrová, Machová, Pokorná:
OCH2); 7.07 (s, 2 H, H-arom .). FT IR (film on KBr): 2 917, 2 848 (C–H, stretch in g); 1 680
(C=C, stretch in g); 1 463 (CH2, scissorin g); 1 360 (CH3, ben din g); 1 268 (C–O, stretch in g); 1 210
(CH2, twistin g); 1 061 (C–C, stretch in g); 804, 726 (CH2, rockin g); 521 (C–Br).
2-Bromo-1,4-bis(hexadecyloxy)benzene. Alkylation of 2-brom oh ydroquin on e (6.25 g, 0.033 m ol)
with 1-brom oh exadecan e (61.0 g, 0.20 m ol) in eth an olic potassium h ydroxide (5.0 g KOH in
100 m l EtOH) was perform ed sim ilarly to th at for th e dibrom o derivative above. Yield: 18.3
g (86%). M.p. 64–65 °C. For C38H69BrO2 (637.9) calculated: 71.56% C, 10.90% H, 12.53% Br;
foun d: 71.48% C, 10.64% H, 12.68% Br. 1H NMR (CDCl3): 0.87 (t, 6 H, J = 6.6, 2 × CH3); 1.26
(s, 52 H, 26 × CH2); 1.74 (m , 4 H, J = 6.7, 2 × O-C-CH2); 3.87 an d 3.93 (2 × t, 4 H, J = 6.6,
2 × OCH2); 7.08 (m , 3 H, J = 3.9, H-arom .). FT IR (film on KBr): 2 917, 2 846 (C–H,
stretching); 1 673 (C=C, stretching); 1 462 (CH2, scissoring); 1 359 (CH3, bending); 1 267 (C–O,
stretch in g); 1 217 (CH2, twistin g); 1 044 (C–C, stretch in g); 790, 722 (CH2, rockin g); 522 (C–Br).
2,2′,5,5′-Tetrakis(hexadecyloxy)biphenyl was prepared by th e m eth od developed for couplin g
of aryl ch lorides17. 2-Brom o-1,4-bis(h exadecyloxy)ben zen e (6.37 g, 10 m m ol), zin c powder
(2.20 g, 33.6 m m ol), n ickel(II) ch lorid e (0.13 g, 1 m m ol), trip h en ylp h osp h in e (1.04 g,
4 m m ol), 2,2′-bipyridin e (0.16 g, 1.01 m m ol) an d N,N-dim eth ylacetam ide (50 m l) were de-
gassed un der argon in a special glass reactor equipped with a m agn etic stirrer an d th ree-way
valve. Th e reactor was h eated (80 °C) with vigorous stirrin g for 18 h . After coolin g, th e reac-
tion m ixture was poured in to m eth an ol–water (800 m l, 1 : 1 by volum e) an d 5 m l con cen -
trated HCl was added. Th e organ ic precipitate (upper layer) was filtered off, dried, extracted
with h ot toluen e (250 m l) an d treated with ch arcoal. Th e toluen e solution was reduced
(100 m l); after addition of eth an ol, a wh ite product crystallized. Yield: 3.55 g (63%). M.p.
82–83 °C. For C76H138O4 (1 115.9) calculated: 81.80% C, 12.47% H; foun d: 81.87% C,
12.44% H. 1H NMR (CDCl3): 0.87 (t, 12 H, J = 6.6, 4 × CH3); 1.26 (s, 104 H, 52 × CH2); 1.72
(m , 8 H, J = 6.9, 4 × O-C-CH2); 3.88 (t, 8 H, J = 6.6, 4 × OCH2); 6.78 (m , 6 H, H-arom .). FT IR
(film on KBr): 2 917, 2 850 (C–H, stretch in g); 1 680 (C=C, stretch in g); 1 466 (CH2, scissor-
in g); 1 359 (CH3, ben din g); 1 268 (C–O, stretch in g); 1 216 (CH2, twistin g); 1 034 (C–C,
stretch in g); 826, 724 (CH2, rockin g).
Oligo[2,5-bis(hexadecyloxy)-1,4-phenylene]s (1, 2, an d 3, Sch em e 1). A special glass reactor
equipped with a m agn etic stirrer an d th ree-way valve was ch arged with 1,4-dibrom o-
2,5-bis(h exadecyloxy)ben zen e (7.16 g, 10 m m ol), zin c powder (2.20 g, 33.6 m m ol), n ickel(II)
ch loride (0.13 g, 1 m m ol), triph en ylph osph in e (1.04 g, 4 m m ol) an d 2,2′-bipyridin e (0.16 g,
R
R
O
O
NiCl2; Zn
Ph3P; 2,2'-bipyridine
Br
Br
Yamamoto coupling
m
O
O
R
R
1-6
R
m
n-C16H33
n-C16H33
n-C16H33
n-C16H33
n-C16H33
n-C10H21
1
2
3
4
5
6
≈ 4-6
≈ 2-4
≈ 2-3
≈ 10
≈ 15
≈ 15
SCHEME 1
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Excimer Formation
1477
1.01 m m ol). Th e reactor was evacuated several tim es, flush ed with argon an d 90 m l of
N,N-dim eth ylform am ide (DMF; bubbled with argon for 30 m in ) was added. Th e vacuum –
argon cycles were repeated several tim es, th en th e reactor was closed an d h eated (75 °C) for
72 h with vigorous stirrin g. After coolin g to room tem perature, th e solid was filtered off an d
extracted with THF. Th e extract was precipitated in to m eth an ol. Yield: 0.68 g of th e oligom er
fraction 1 (Mw = 2 280, Mn = 1 870). Th e DMF solution was precipitated in to m eth an ol.
Yield: 2.85 g of th e oligom er fraction 2 (Mw = 1 410, Mn = 1 160). Oligom er fraction 2
(1.0 g) was dissolved in 60 m l THF an d 40 m l of m eth an ol was added. Th e precipitated
m aterial was filtered off an d th e solution was evaporated yieldin g th e oligom er fraction 3
(0.76 g, Mw = 1 210, Mn = 1 080). All th e th ree fraction s were ch aracterized by GPC (Fig. 1)
an d elem en tal an alysis. Foun d: 78.73% C, 11.87% H, 3.92% Br for 1; 76.82% C, 11.54% H,
6.63% Br for 2; 75.92% C, 11.25% H, 7.60% Br for 3. 1H NMR (deuterated THF) an d FT IR
spectroscopy con firm ed th e structure of m on om er un its in oligom ers. FT IR of 3 (film on
KBr): 2 920, 2 852 (C–H, stretch in g); 1 679 (C=C, stretch in g); 1 466 (CH2, scissorin g); 1 390
(CH3, ben din g); 1 272 (C–O, stretch in g); 1 216 (CH2, twistin g); 1 043 (C–C, stretch in g); 827,
724 (CH2, rockin g); 520 (C–Br). Th e vibration ban d at 520 cm –1 for oligom ers 2 an d 1 de-
creases as a con sequen ce of in creasin g m olecular weigh t. UV (THF): λm ax 304 n m , ε304 3 900
l m ol–1 cm –1 for 1; λm ax 297 n m , ε297 3 020 l m ol–1 cm –1 for 2; λm ax 296 n m , ε296 2 760
l m ol–1 cm –1 for 3.
Poly[2,5-bis(hexadecyloxy)-1,4-phenylene]s (4 an d 5, Sch em e 1). A glass reactor was ch arged in
th e sam e way as above for oligom ers. Th e reaction m ixture was h eated (105 °C) for 12 days
with vigorous stirrin g. After coolin g to room tem perature, th e reaction m ixture was poured
in to m eth an ol (700 m l) an d acidified with con cen trated HCl (10 m l). Th e crude m aterial
was filtered off, dried an d dissolved in THF (200 m l at 50 °C). Th e solution was filtered,
evaporated to a volum e of ca 30 m l an d precipitated in to m eth an ol (300 m l). Yield: 4.25 g
(76%) of polym er 4 (Mw = 3 890, Mn = 1 980). Polym er 4 (1.0 g) was dissolved in 60 m l of
5
4
3
2
1
0
2.5
3.0
3.5
4.0
log M
FIG. 1
Norm alized weigh t distribution fun ction s, w(M), determ in ed by GPC an alyses in tetrah ydro-
furan for oligom ers 1 (dash ed lin e), 2 (dash ed an d dottted lin e), an d 3 (solid lin e)
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

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Výprachtický, Cimrová, Machová, Pokorná:
THF (50 °C) an d left to precipitate in a fridge (5 °C). Th e solid was filtered off an d dried.
Yield: 0.54 g of polym er 5 (Mw = 5 940, Mn = 3 970). Both polym ers were ch aracterized by
GPC, 1H NMR, FT IR, an d elem en tal an alysis. Foun d: 80.15% C, 11.81% H, 1.92% Br for 4
an d 81.26% C, 12.13% H, 1.42% Br for 5. UV (THF): λm ax 315 n m , ε315 4 900 l m ol–1 cm –1
for 4; λm ax 325 n m , ε325 5 800 l m ol–1 cm –1 for 5. 1H NMR of 5 (CDCl3): 0.86 (t, 6 H, 2 ×
CH3); 1.24 (s, 52 H, 26 × CH2); 1.72 (m , 4 H, 2 × O-C-CH2); 3.87 (m , 4 H, 2 × OCH2); 6.80
(m , 2 H, H-arom .). FT IR of 5 (film on KBr): 2 920, 2 852 (C–H, stretch in g); 1 677 (C=C,
stretch in g); 1 469 (CH2, scissorin g); 1 390 (CH3, ben din g); 1 273 (C–O, stretch in g); 1 220
(CH2, twistin g); 1 040 (C–C, stretch in g); 825, 721 (CH2, rockin g).
Poly[2,5-bis(decyloxy)-1,4-phenylene] (6, Sch em e 1). A glass reactor was ch arged un der argon
(see above) with 1,4-dibrom o-2,5-bis(decyloxy)ben zen e (8.23 g, 15 m m ol), zin c powder
(3.30 g, 50 m m ol), n ickel(II) ch lorid e (0.20 g, 1.5 m m ol), trip h en ylp h osp h in e (1.56 g,
6 m m ol) an d 2,2′-bipyridin e (0.24 g, 1.52 m m ol). Th e reaction m ixture was h eated (80 °C)
for 10 days with vigorous stirrin g. After coolin g to room tem perature, th e reaction m ixture
was poured in to m eth an ol (600 m l) an d acidified with con cen trated HCl (10 m l) to rem ove
excess of zin c. Th e polym er was filtered off an d dried (4.13 g). Th e crude m aterial was dis-
solved in THF, filtered, an d precipitated in to m eth an ol. Yield: 3.14 g (54%) of polym er 6.
Th e polym er was ch aracterized by 1H NMR, FT IR, GPC (Mw = 3 750, Mn = 2 580), an d ele-
m en tal an alysis. Foun d: 80.10% C, 11.02% H, 0.30% Br. 1H NMR (deuterated THF): 0.87 (t,
6 H, 2 × CH3); 1.26 (s, 28 H, 14 × CH2); 1.70 (m , 4 H, 2 × O-C-CH2); 3.86 (m , 4 H, 2 ×
OCH2); 6.74 (m , 2 H, H-arom .). FT IR (film on KBr): 2 922, 2 855 (C–H, stretch in g); 1 678
(C=C, stretch in g); 1 470 (CH2, scissorin g); 1 382 (CH3, ben din g); 1 270 (C–O, stretch in g);
1221 (CH2, twistin g); 1 062 (C–C, stretch in g); 822, 790, 722 (CH2, rockin g); very weak 524
(C–Br). UV (THF): λm ax 315 n m , ε315 4 500 l m ol–1 cm –1
.
Poly[2,5-bis(isopentyloxy)-1,4-phenylene] (7, Sch em e 2). Th is polym er was prepared by th e
aryl–aryl couplin g reaction of bis(cyclic ester) of 2,5-bis(isopen tyloxy)ben zen e-1,4-diboron ic
acid with propan e-1,3-diol (8) an d 1,4-dibrom o-2,5-bis(isopen tyloxy)ben zen e in th e
presen ce of tetrakis(triph en ylph osph in e)palladium accordin g to th e described procedure18
.
Th e prepared m aterial was ch aracterized by 1H an d 13C NMR, FT IR, GPC (Mw = 35 000, Mn
=
21 000) an d elem en tal an alysis. Th e spectral m eth ods con firm ed th e structure of 7. Th e de-
gree of polym erization (Pn ) was estim ated from 1H NMR to be 37, wh ich correspon ds with
GPC results. 1H NMR (CDCl3): 0.77–0.92 (m , 12 H, 4 × CH3); 1.49 (t, 4 H, 2 × O-C-CH2);
1.65 (m , 2 H, 2 × CH); 3.88 (m , 4 H, 2 × OCH2); 6.90–7.03 (m , 2 H, H-arom .). 13C NMR
(CDCl3): 20.77 (4 C, CH3); 23.41 (2 C, alif. C-3); 36.64 (2 C, alif. C-2); 66.70 (2 C, alif. C-1);
116.49 (2 C, arom . 3,6-C); 126.14 (2 C, arom . 1,4-C); 148.71 (2 C, arom . 2,5-C). UV-VIS an d
fluorescen ce data are sum m arized in Tables I an d II.
O
O
O
O
O
O
O
(Ph3P)4Pd
Br
Br
+
B
B
Suzuki
coupling
m
O
O
O
8
7
SCHEME 2
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Excimer Formation
1479
TABLE I
Relative fluorescen ce quan tum yield (Q) an d m axim um wavelen gth (λm ax) of absorption , ex-
citation , an d em ission spectra for oligom ers 1–3 an d polym ers 4–7 in tetrah ydrofuran
λ
m ax, n m
Oligom er
Q
or polym er
absorption spectrum
excitation spectrum
em ission spectrum
1
2
2
3
4
5
6
7
304
297
297
296
315
325
315
335
330
330
340
345
335
340
335
340
390
390
460
465
390
395
390
400
0.08
0.04
0.03
0.02
0.38
0.46
0.25
0.59a
a
Th e value was taken from ref.27 an d verified with a carbazole stan dard.
TABLE II
Fluorescen ce lifetim es an alyzed by sin gle-expon en tial (τ) or double-expon en tial (τ1, τ2) fit
for oligom ers 1–3 an d polym ers 4–7 in tetrah ydrofuran (λex, excitation wavelen gth ; λem
,
em ission wavelen gth )
Oligom er
or polym er
λ
ex, n m
λem , n m
τ1, τ2, n s (rel B1, B2, %)
τ, n s
a
a
1
2
2
3
4
5
6
7
335
335
335
338
335
338
338
338
390
390
460
465
390
400
400
400
1.02
1.00
(4.63)
(5.69)
1.02
6.03 (66), 2.11 (34)
6.88 (76), 2.35 (24)
1.01 (99.5), 35.7 (0.5)
a
0.82
a
a
1.03
0.77
a
Th e decay was virtually sin gle-expon en tial.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1480
Výprachtický, Cimrová, Machová, Pokorná:
Meth ods an d Procedures
Steady-state fluorescen ce spectra were m easured with a Hitach i Perkin –Elm er MPF-2A spec-
trom eter in L-form at an d corrected19 for th e calculation of th e fluorescen ce quan tum yields.
Th e spectral sen sitivity coefficien t was determ in ed usin g a tun gsten lam p h avin g a kn own
spectral distribution an d stan dard fluorescen ce spectra20 of 2-n aph th ol an d N,N-di-
m eth yl-3-n itroan ilin e.
Th e relative fluorescen ce quan tum yield (Q) of oligom ers 1–3 an d polym ers 4–6 was cal-
culated21 from
I(λ )dλ
AS n 2
∫
λ
Q = QS
,
(1)
A nS2 IS(λ )dλ
∫
λ
wh ere subscript S den otes a stan dard com poun d, A is th e absorban ce of a solution at th e ex-
citation wavelen gth , n th e refractive in dex of th e solven t an d I(λ) th e in ten sity of fluores-
cen ce at wavelen gth λ. Th e “in n er filter effect” was elim in ated usin g solution s of th e sam e
absorban ce (A = 0.4). Polym er 7 in dich lorom eth an e22 (QS = 0.59) an d carbazole in cyclo-
h exan e23 (QS = 0.38) were used as stan dard com poun ds. Th e area un der th e corrected em is-
sion spectrum was evaluated by n um erical in tegration . Th e relative fluorescen ce quan tum
yields are given in Table I.
Th e fluorescen ce lifetim e (τ) was obtain ed with a tim e-resolved fluorim eter FL 900 CDT
(Edin burgh An alytical In strum en ts, U.K.) usin g th e tim e-correlated sin gle-ph oton coun tin g
(TCSPC) m eth od19,24. TCSPC is a digital tech n ique, coun tin g ph oton s, wh ich are tim e-
correlated in relation to th e excitation pulse. Sum m in g over m an y pulses, a probability h is-
togram of coun ts versus tim e ch an n els was built up. Experim en ts were con tin ued un til
10 000 coun ts in th e peak ch an n el were collected (Figs 5 an d 6). Excitation of th e sam ples
was carried out by a th yratron -con trolled pulse lam p n F 900 h avin g th e repetition frequen cy
rate 40 kHz. Th e lam p was filled with n itrogen (>99.995%) at 150 kPa. Th e in ten sity of th e
em ission beam was adjusted so th at 800 fluorescen ce ph oton s or less were observed per sec-
on d (ca 2% of repetition frequen cy). Th e tim e-resolved an d lam p profile m easurem en ts were
perform ed in L-form at of a cen trally illum in ated cuvette (1 × 1 × 4 cm ). Th e lam p pulse
(profile) was m easured usin g a scatterin g solution of LUDOX (DuPon t) closely spaced in
tim e to a particular fluorescen ce decay. Th e th eoretical fluorescen ce decay, F(t), an alyzed by
decon volution of th e lam p pulse with th e im pulse respon se of th e sam ple, is given by
F(t ) = C + B exp (−t / τ i ),
(2)
i
∑
i
wh ere Bi is a preexpon en tial factor represen tin g th e fraction al con tribution to th e tim e-
resolved decay of th e com pon en t with a lifetim e τi [rel Bi = (Bi τi/ΣBiτi) × 100%], C is back-
groun d an d t is tim e. Least-square an alysis of th e observed tim e-resolved decay, R(t), was
used for estim ation of th e im pulse respon se fun ction F(t). Th e goodn ess-of-fit param eter, χ2,
was calculated from
2
[R(t ) − RC(t )]2
R(t )
R(t ) − RC(t )
σ(t )
χ 2
=
=
=
u 2(t ),
(3)
∑
∑
∑
t
t
t
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Excimer Formation
1481
wh ere RC(t) is th e calculated decay of fluorescen ce, σ(t) th e stan dard deviation of each data
poin t an d u(t) th e weigh ted residuals. From Poisson statistics th e stan dard deviation is
kn own to be th e square root of th e n um ber of ph oton coun ts, σ(t) = [R(t)]1/2. In TCSPC it is
straigh tforward relation sh ip between χ2 an d residuals. Th e value of χ2 is th e sum of th e
squared deviation s between th e m easured values R(t) an d th e expected values RC(t), each di-
vided by squared deviation s expected for th e n um ber of detected ph oton s. Th e values χ2
<
1.7 are typical of a good fit. Th e residuals are th e differen ce between th e m easured values
R(t) an d th e expected values RC(t), weigh ted by th e stan dard deviation of each data poin t. A
good fit sh ould give a residual curve th at on ly con tain s ran dom n oise distributed aroun d
zero (Fig. 5). Th e excitation an d em ission wavelen gth s an d th e m easured fluorescen ce life-
tim es are sum m arized in Table II.
Molecular weigh ts of th e polym ers an d oligom ers were determ in ed usin g
a GPC
ch rom atograph (Laboratory In strum en ts, Czech Republic) equipped with RI an d UV 254 n m
detectors, a colum n 8 × 600 m m (PSS 10 000, PSS, Germ an y), an d data collection an d treat-
m en t (Data Mon itor, Watrex, Czech Republic). Polystyren e stan dards (PL Laboratories, U.K.)
were used for calibration . Th e syn th esized 2,2′,5,5′-tetrakis(h exadecyloxy)biph en yl (dim er
m odel) was used as an extern al stan dard to verify th e polystyren e calibration at low m olecu-
lar weigh ts. Th e sam ples were m easured in THF, wh ich was dried over m olecular sieve 4A
an d distilled.
UV-VIS spectra were taken on a Perkin –Elm er Lam bda 20 spectrom eter. Molar absorption
coefficien ts (ε) of oligom ers an d polym ers are expressed per m on om er un it. 1H an d 13C NMR
spectra were taken on a Bruker ACF-300 spectrom eter at 300.1 MHz in CDCl3 or deuterated
THF usin g h exam eth yldisiloxan e as an in tern al stan dard. Ch em ical sh ifts are given in ppm
(δ-scale), couplin g con stan ts (J) in Hz. FT IR spectra (ν in cm –1) were m easured usin g a
Perkin –Elm er Paragon 1000 PC Fourier tran sform in frared spectrom eter in KBr pellets or as a
film on KBr.
RESULTS AND DISCUSSION
Procedures for th e preparation of structurally defin ed oligo- an d poly-
(1,4-ph en ylen e)s were developed on th e basis of n um erous syn th etic prin ci-
ples of organ ic an d organ om etallic ch em istry. Th e availability of m ore
effective m eth ods for th e aryl–aryl couplin g of sm all m olecules is an im por-
tan t drivin g force in th e developm en t of n ew syn th etic strategies for substi-
tuted poly(1,4-ph en ylen e)s an d oth er poly(arylen e)s. In particular, th e
Pd(0)-catalyzed aryl–aryl couplin g accordin g to Suzuki25 (arylboron ic acid
plus aryl h alide or tosylate) an d th e n ickel(0)- or n ickel(II)-m ediated cou-
plin g accordin g to Yam am oto26 (aryl h alide or tosylate plus aryl h alide or
tosylate) h ave been m ost successfully em ployed. Both syn th etic m eth ods
h ave been used very frequen tly in th e last decade.
Th e Suzuki m eth od is used m ain ly in th e cases, wh ere h igh -m olecular-
weigh t m aterials are to be prepared. Typical degrees of polym erization (Pn )
ran ge from 30 to 100. We used th is m eth od for th e preparation of poly-
m er 7 (Sch em e 2), wh ich served as th e stan dard m aterial for th e fluores-
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1482
Výprachtický, Cimrová, Machová, Pokorná:
cen ce quan tum yield (Q) determ in ation (Eq. (1)). Usin g an oligom er
approach , MALDI-TOF m ass spectrom etry an d HPLC, th e “effective con ju-
gation len gth ” of IP-PPP was determ in ed27 as 11. Th e syn th esized polym er
7 (Pn ≈ 37, Mw = 35 000, Mn = 21 000) h ad average m olecular-weigh t param -
eters substan tially h igh er an d its use as fluorescen ce stan dard was th us con -
firm ed. Th e publish ed value22 for IP-PPP (QS = 0.59) was verified for
polym er 7 usin g an oth er fluorescen ce stan dard (carbazole in cycloh ex-
an e)23. The measured values of Q for oligomers and polymers 1–6 are shown in
Table I. Th e fluorescen ce quan tum yield in creases with th e average m olecu-
lar weigh t of th e fraction s or, m ore specifically, with in creasin g con jugation
len gth of th e sam ples.
On the other hand, we have used the Yamamoto method for the preparation
of oligomers 1–3 and polymers 4–6 (Scheme 1). For nickel-mediated coupling of
1,4-dibrom oben zen e derivatives, we used th e n ickel(II)/triph en ylph osph in e
com plex as a catalyst, zin c as th e reducin g m etal, an d 2,2′-bipyridin e as a
coligan d. Th is m eth od is con ven ien t for th e syn th eses of m aterials with
lower m olecular weigh ts; th e average values of Pn usually ran ge from 10 to
15. Th e determ in ed m olecular-weigh t param eters of polym ers 4–6 (see Ex-
perim en tal) were in agreem en t with th e expected values. Reducin g th e reac-
tion tim e an d tem perature, oligom er fraction s 1–3 were prepared. Th eir
GPC ch aracteristics (n orm alized distribution fun ction s) are sh own in Fig. 1.
In fraction 1, h exam ers an d tetram ers prevail, som e dim ers bein g also pres-
en t. Th e ratio of h exam er : tetram er : dim er can be rough ly estim ated from
th e peak h eigh ts as 2.5 : 2.5 : 1. Sim ilarly for fraction 2, th e h exam er :
tetram er : dim er ratio can be estim ated as 1 : 2 : 5 an d fin ally for fraction 3,
th e tetram er : dim er ratio was approxim ately 1 : 5 if th e presen ce of
h exam ers was foun d to be n egligible (sh oulder). In spite of th e fact th at th e
GPC ch rom atograph used was calibrated usin g polystyren e stan dards, th e
m olecular weigh ts of th e oligom er peaks fit well keepin g in m in d th e m o-
lecular weigh t of th e m on om er un it bein g 557. Th e syn th esized biph en yl
derivative (dim er m odel) was used as an extern al stan dard, wh ich con -
firm ed th e assum ed dim er position s in Fig. 1 (i.e., th e peaks with m olecular
weigh t aroun d 1 000). Th e results of elem en tal an alysis supported th e aver-
age com position of fraction s 1–3. Th e con ten t of brom in e (en d group) de-
creases (3 > 2 > 1) with in creasin g average Pn . Also for polym ers 4–6 is th e
brom in e con ten t (an d, con sequen tly, calculated average Pn ) in good agree-
m en t with GPC an alyses.
Fluorescen ce em ission spectra of oligom ers 1 an d 2 in THF are sh own in
Fig. 2. Oligom er fraction 1 h as on e em ission m axim um at 390 n m an d sim i-
lar m axim a were observed for polym ers 4–6 (Table I). Un substituted
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Excimer Formation
1483
poly(1,4-ph en ylen e) possesses a 23° twist between th e adjacen t ben zen e
un its due to ortho-h ydrogen in teraction . Th e substitution in position s 2 an d
5 forces th e adjacen t ben zen e un its even furth er out of th e plan e, result-
in g28 in twist an gles 60–80°. Such twist an gles an d lon g h exadecyloxy sub-
stituen ts exclude th e cofacial san dwich -type arran gem en t with in terplan ar
ch ain distan ces of ≈0.3–0.4 n m n ecessary for excim er form ation . Th at is
wh y we did n ot observe an excim er em ission from polym ers 4–6. Even
tetram ers an d h exam ers of 1 are too lon g to form an excim er structure. On
th e oth er h an d, oligom ers 2 sh ow two em ission m axim a at 390 an d 460 n m
(Fig. 2). Th e m axim um at 460 n m in creases with in creasin g con cen tration
of 2 in solution . Fraction 2 con tain s m ain ly dim ers an d we assum e th at th e
excim er form ation is respon sible for th e em ission at a lon ger wavelen gth .
Th e m axim um of th e excitation spectrum for em ission at 460 n m is red-
sh ifted in com parison with th e excitation spectrum m axim um for em ission
at 390 n m (Table I). If th e excim er is excited in oligom ers 2 at 360 n m , its
em ission at 460 n m prevails an d th e sh oulder on ly is observed at 390 n m
(Fig. 3). An oth er in crease in th e excim er form ation was observed with
oligom er fraction 3, wh ere th e con ten t of lon g oligom er ch ain s was furth er
decreased by fraction al precipitation . Th e em ission of th e excim er at 465 n m
was observed as a m ain peak an d th e em ission at 390 n m was foun d to be
n egligible. Th e excitation an d em ission spectra of th e excim er in oligom ers
3 were com pared with th ose of polym ers 4, wh ere th e excim er was n ot
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
λ, nm
FIG. 2
Fluorescen ce em ission spectra (λex = 330 n m ) of oligom ers 1, c = 0.20 g l–1 (dotted lin e); 2, c =
0.20 g l–1 (dash ed lin e) an d c = 0.84 g l–1 (solid lin e) in tetrah ydrofuran
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1484
Výprachtický, Cimrová, Machová, Pokorná:
form ed (Fig. 4). Th e excim er form ation causes a red sh ift both in em ission
(from 390 to 460 n m ) an d excitation (from 335 to 345 n m ) spectra. On th e
con trary, th e absorption spectra are red-sh ifted with in creasin g degree of
con jugation , i.e. with in creasin g Pn (Table I). Oligom ers 2 an d 3 did n ot
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
λ, nm
FIG. 3
Fluorescen ce em ission spectra of oligom er fraction 2 (c = 0.84 g l–1) at excitation wavelen gth
330 n m (solid lin e) or 360 n m (dash ed lin e) in tetrah ydrofuran
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
λ, nm
FIG. 4
Fluorescen ce excitation (λem = λm ax) an d em ission (λex = λm ax) spectra of polym er 4 (solid
lin e) an d oligom er fraction 3 (dash ed lin e) in tetrah ydrofuran , c = 0.04 g l–1
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Excimer Formation
1485
sh ow addition al absorption at lon ger wavelen gth s, wh ich could in dicate
th e groun d state in teraction . Con sequen tly, th e em ission at 460–465 n m
was assign ed rath er to excim ers th an to aggregates (polaron pairs).
Th e excim er em ission sh ould exh ibit a lon ger fluorescen ce lifetim e be-
cause th e radiative tran sition to th e groun d state is forbidden . Oligom ers
an d polym ers 1–7 were studied in THF by tim e-resolved fluorescen ce spec-
troscopy (Table II). Th e em ission decay curves at 390–400 n m were foun d
to be practically sin gle-expon en tial (Eq. (2)) with fluorescen ce lifetim es
aroun d 1 n s (Fig. 5 sh ows typical data). Th e values of χ2 (Eq. (3)) were less
th an 1.3 for sin gle-expon en tial fit an d were n ot im proved by double-
expon en tial fit. Wh en such fit was tested, th e con ten t of th e lon ger com po-
n en t was 0.5% or less (Table II, polym er 4). Fluorescen ce decays at 460–465
n m were double-expon en tial, exh ibitin g lon ger lifetim es th an th ose at
390–400 n m . A com parison of th e em ission decay curves at 460 an d 390 n m
is shown in Fig. 6. In decays at 460 nm, the longer component (τ1 ≈ 6.0–6.9 ns)
prevails [Eq. (2); rel B1 ≈ 70%] over th e sh ort com pon en t (τ2 ≈ 2.1–2.4 n s;
rel B2 ≈ 30%). Th e sin gle-expon en tial values sh own in Table II (in brackets)
4
10
3
10
1
2
10
2
1
10
0
10
0.0
0
0
10
10
20
30
30
40
40
Lifetime, ns
4
0
–4
20
FIG. 5
Time-resolved fluorescence decay of polymer 4 (τ = 1.02 ns, λex = 335 nm, λem = 390 nm, χ2 = 1.23)
in tetrah ydrofuran (1), th e m easured lam p pulse (2) an d th e weigh ted residuals
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1486
Výprachtický, Cimrová, Machová, Pokorná:
for decays at 460–465 n m are average values (≈5 n s) just for an approxim ate
com parison with th ose for decays at 390 n m . Oth erwise th eir χ2 values are
too h igh (>1.7), th e double-expon en tial fit bein g n ecessary to use to calcu-
late th e fluorescen ce lifetim es.
From fluorescen ce beh avior of oligom ers 2 in solven ts of differen t polar-
ity, we believe th at h ydroph obic in teraction s of h exadecyloxy side ch ain s
play an im portan t role in th e excim er form ation . Th e fluorescen ce em ission
spectrum of 2 in THF (m edium -polarity solven t) was com pared (Fig. 7) with
th ose in eth an ol (polar solven t) or h exan e (n on polar solven t). In polar eth -
an ol, h ydroph obic in teraction s support th e excim er form ation , th e excim er
em ission at 460 n m bein g observed as th e m ain peak. Th e in solubility of
h igh er oligom ers in th is solven t explain s n early total absen ce of th e em is-
sion at 390 n m . In n on polar h exan e are oligom ers 2 com pletely soluble but
th e m ain em ission at 390 n m in dicates th at excim ers were n ot form ed. Hy-
droph obic in teraction s of side ch ain s are n ot effective in th is solven t an d
th e m olecules are too separated by side ch ain s to form th e san dwich struc-
ture. Th e m axim a of excitation spectra in eth an ol an d h exan e were foun d
to be at 340 an d 330 n m , respectively, wh ich is in agreem en t with th e dis-
cussion above. Th e m easured fluorescen ce lifetim es in h exan e for em ission
at 390 n m (τ = 0.85 n s) or in eth an ol for em ission at 460 n m [τ1 = 6.45 n s
(76%), τ2 = 1.68 n s (24%)] supported th e steady-state results. Hydroph obic
4
10
3
10
2
10
1
10
0
10
0
10
20
30
40
Lifetime, ns
FIG. 6
Tim e-resolved fluorescen ce decay of oligom er fraction 2 (λex = 335 n m ) in tetrah ydrofuran
for em ission at 460 n m (τ1 = 6.03 n s (66%), τ2 = 2.11 n s (34%), χ2 = 1.19) (solid lin e) or at
390 n m (τ = 1.00 n s, χ2 = 1.05) (dash ed lin e) an d th e m easured lam p profile (dots)
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Excimer Formation
1487
in teraction s of aliph atic side ch ain s of polym ers or low-m olecular-weigh t
alkan e m odels in polar m edium were described29,30
.
In con clusion , we h ave syn th esized an d ch aracterized “h airy-rod” poly-
m ers, poly[2,5-bis(h exadecyloxy)-1,4-ph en ylen e] an d poly[2,5-bis(decyl-
oxy)-1,4-ph en ylen e], by th e Yam am oto couplin g reaction . In solution , th e
lon g side substituen ts created structural h in dran ce resultin g in an
out-of-plan e twist an gle between th e adjacen t ben zen e rin gs. For steric rea-
son s, polym er ch ain s are n ot able to give rise to san dwich con figuration s
with distan ces suitable for excim er form ation . Con sequen tly, we h ave
foun d out a sin gle fluorescen ce em ission ban d at 390 n m with th e sin gle-
expon en tial lifetim e close to 1 n s. However, in th e case of sh ort oligom ers,
we observed an oth er em ission ban d at 460 n m with lon ger double-
expon en tial lifetim e [6.9 n s (76%), 2.4 n s (24%)]. Such beh avior was ob-
served in th e oligom er fraction s, wh ere dim ers prevail. We assum e th at in
th e case of dim er or, possibly, trim er, th e san dwich -type arran gem en t with
sufficien tly close in terch ain distan ces can arise an d an excim er can be
form ed. Hydroph obic in teraction s of aliph atic side ch ain s in polar m edia
were foun d to be favorable for th e excim er form ation . Th e in vestigation of
polym ers will con tin ue in th e solid state (film s) for utilization in th e or-
gan ic LED con struction . Because th e excim er form ation is an un favorable
process, th e polym ers will be fraction ally precipitated before use to rem ove
sh ort oligom ers.
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
λ, nm
FIG. 7
Fluorescen ce em ission spectra (λex = λm ax) of oligom er fraction 2 in solven ts of differen t po-
larity: h exan e (solid lin e), tetrah ydrofuran (dotted lin e), eth an ol (dash ed lin e)
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1488
Výprachtický, Cimrová, Machová, Pokorná:
We thank the Grant Agency of the Czech Republic (grant No. 102/98/0696) and the Volkswagen
Foundation for supporting this work.
SYMBOLS
A
B
C
c
absorban ce
preexpon en tial factor
backgroun d con stan t
con cen tration , g l–1
F
I
J
M
Mn
Mw
n
th eoretical tim e-resolved fluorescen ce in ten sity, a.u.
steady-state fluorescen ce in ten sity, a.u.
NMR couplin g con stan t, Hz
m olecular weigh t
n um ber-average m olecular weigh t
weigh t-average m olecular weigh t
refractive in dex
Pn
Q
R
t
degree of polym erization
fluorescen ce quan tum yield
observed tim e-resolved fluorescen ce in ten sity, a.u.
tim e, s
u
weigh ted residuals
w
χ2
ε
weigh t distribution fun ction
goodn ess-of-fit param eter
m olar absorption coefficien t, l m ol–1 cm –1
wavelen gth , n m
λ
λem
λex
λm ax
σ
em ission wavelen gth , n m
excitation wavelen gth , n m
wavelen gth of th e m axim um of a spectrum , n m
stan dard deviation
τ
fluorescen ce lifetim e, n s
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