Photophysics of cyanine dyes on surfaces: Laser-induced photoisomer emission of 3,3′-dialkylthiacarbocyanines adsorbed on microcrystalline cellulose

Article abstract of DOI:10.1135/cccc19990459

The photophysics of three thiacarbocyanine dyes, 3,3′-dimethylthiacarbocyanine iodide (DMTCC), 3,3′-diethylthiacarbocyanine iodide (DETCC), and 3,3′-dipropylthiacarbocyanine iodide (DPTCC) was studied when adsorbed on microcrystalline cellulose in the concentration range from 5.0 · 10^-4 to 10.0 μmol g^-1. Using ground-state diffuse reflectance absorption technique, only H aggregate formation was detected for all the probes. The amount of aggregate formed depends on the hydration degree of the sample, always decreasing with sample dryness. The fluorescence quantum yields for all the adsorbed dyes are one order of magnitude higher than those observed in nonviscous solvents, being 0.98 for DMTCC, 0.96 for DETCC, and 0.63 for DPTCC. Laser-induced fluorescence emissions were recorded (using an intensified-charge-coupled-device detection system) as a function of the laser power, showing that for dry concentrated samples irradiated with high laser intensity, a second fluorescence emission band (bathochromically shifted relative to the monomer emission) was detected. This emission shows a supralinear dependence on laser power. The new emissions here detected arise from fluorescent photoisomers formed via singlet monomers, by a two-photon absorption process.

Full text of DOI:10.1135/cccc19990459

Photophysics of Cyanine Dyes on Surfaces
459
PHOTOPHYSICS OF CYANINE DYES ON SURFACES: LASER-INDUCED
PHOTOISOMER EMISSION OF 3,3′-DIALKYLTHIACARBOCYANINES
ADSORBED ON MICROCRYSTALLINE CELLULOSE
b
a2
Anabela S. OLIVEIRAa1,*, Paulo ALMEIDA and Luis Filipe VIEIRA FERREIRA
a
Centro de Química Física Molecular – Complexo Interdisciplinar, Instituto Superior Técnico,
1
2
1096 Lisboa Codex, Portugal; e-mail: pcd739@alfa.ist.utl.pt, pcd748@alfa.ist.utl.pt
Departamento de Química da Universidade da Beira Interior, Rua Marques d’Ávila e Bolama,
6200 Covilhă, Portugal; e-mail: palmeida@alfa.ubi.pt
b
Received Jun e 1, 1998
Accepted August 17, 1998
Th e ph otoph ysics of th ree th iacarbocyan in e dyes, 3,3′-dim eth ylth iacarbocyan in e iodide
(DMTCC), 3,3′-dieth ylth iacarbocyan in e iodide (DETCC), an d 3,3′-dipropylth iacarbocyan in e
iodide (DPTCC) was studied wh en adsorbed on m icrocrystallin e cellulose in th e con cen tra-
tion ran ge from 5.0 · 10–4 to 10.0 µm ol g–1. Usin g groun d-state diffuse reflectan ce absorption
tech n ique, on ly H aggregate form ation was detected for all th e probes. Th e am oun t of aggre-
gate form ed depen ds on th e h ydration degree of th e sam ple, always decreasin g with sam ple
dryn ess. Th e fluorescen ce quan tum yields for all th e adsorbed dyes are on e order of m agn i-
tude h igh er th an th ose observed in n on viscous solven ts, bein g 0.98 for DMTCC, 0.96 for
DETCC, an d 0.63 for DPTCC. Laser-in duced fluorescen ce em ission s were recorded (usin g an
in ten sified-ch arge-coupled-device detection system ) as a fun ction of th e laser power, sh ow-
in g th at for dry con cen trated sam ples irradiated with h igh laser in ten sity, a secon d fluores-
cen ce em ission ban d (bath och rom ically sh ifted relative to th e m on om er em ission ) was
detected. Th is em ission sh ows a supralin ear depen den ce on laser power. Th e n ew em ission s
h ere detected arise from fluorescen t ph otoisom ers form ed via sin glet m on om ers, by a
two-ph oton absorption process.
Key w ord s: Cyan in e dyes; H an d J aggregates; Microcrystallin e cellulose; Diffuse reflectan ce;
Laser-in duced lum in escen ce; Ph otoisom ers; Two-ph oton absorption processes.
In th e last few years, we h ave been studyin g surface ph otoch em ical an d
ph otoph ysical processes of several probes, n am ely cyan in e dyes, adsorbed
on various opaque h eterogen eous powdered supports, particularly
m icrocrystallin e cellulose1–9
.
Th ese studies were possible due to th e recen t developm en t of diffuse
reflectan ce tech n iques an d fluorescen ce em ission experim en ts in fron t sur-
face geom etry com bin ed with powerful laser excitation sources1b,10–12
.
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

460
Oliveira, Almeida, Ferreira:
Th e above-m en tion ed processes are extrem ely depen den t on th e in terac-
tion of th e probes with solid supports, especially with th e powdered
m icrocrystallin e cellulose m atrix, wh ich in som e con dition s m ay provide a
particularly rigid en viron m en t stron gly affectin g th e ph otoph ysics of th e
probes1–6
.
Microcrystallin e cellulose is a very pure form of cellulose obtain ed by
ch em ical treatm en t, wh ere its am orph ous region s are attacked an d tran s-
form ed in to a h igh ly crystallin e residue13. It is con stituted of glucose un its
join ed by β-1,4-glycosidic lin ks13, th e h ydroxy groups of wh ich h ave a
stron g affin ity for polar protic (alcoh ols) an d aprotic (e.g. aceton itrile, ace-
ton e) solven ts or probes wh ich can reach th em 5. If a solution of th e probe
in on e of th ese swellin g solven ts is added to m icrocrystallin e cellulose, th e
cellulose-to-cellulose h ydrogen bon ds are replaced by cellulose-to-solven t
bon ds an d th e m atrix swells to a degree wh ich depen ds on th e solven t used
for sam ple preparation 5. Probes can th en pen etrate in to subm icroscopic
pores of th e solid substrate an d rem ain stron gly en trapped with in th e cellu-
lose polym er ch ain s after solven t rem oval1–6
.
Cyan in e dyes are curren tly used as sen sitizers in ph otograph y an d
ph otodyn am ic th erapy, in dye lasers an d optical storage of data14,15. Such a
wide ran ge of relevan t application s supports an d justifies th e in creasin g in -
terest in th e study of th eir ph otoph ysical an d ph otoch em ical properties in
h eterogen eous m edia15b
.
In recen t works1–3 we verified th at th e deposition of several cyan in es
with in th e m icrocrystallin e cellulose ch ain s1a,2 usin g good swellin g sol-
ven ts, slows down th e n on radiative deactivation processes th at gen erally
determ in e th e cyan in e solution ph otoph ysics16, leadin g to an in crease in
th e fluorescen ce quan tum yields an d lifetim es. As a result of en trapm en t
in to cellulose, 2,2′-cyan in e, 2,2′-carbocyan in e1,3, oxacarbocyan in es, an d
oxadicarbocyan in e2 exh ibit fluorescen ce quan tum yields in som e cases
m ore th an on e order of m agn itude h igh er th an th ose observed in eth an olic
solution s. Th e sam e beh aviour was observed by rigidifyin g th e ch em ical
structure of th e cyan in e dye m olecule th rough its in clusion in rigid h osts
such as m icelles an d Lan gm uir–Blodgett film s17 or usin g cyan in es with a
rigid ch em ical structure in solution 16b. Groun d-state diffuse-reflectan ce
studies revealed th at on cellulose sam ples th e am oun t of aggregation is al-
ways greater in wet sam ples wh en com pared with th e dry on es1–3
.
In th is paper, we exten d our recen t studies with 2,2′-cyan in es1,3 an d sev-
eral oxacyan in es2 adsorbed on m icrocrystallin e cellulose to th ree
3,3′-dialkylth iacarbocyan in es with th e followin g structure.
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Photophysics of Cyanine Dyes on Surfaces
461
Sam ples were com pared usin g UV-VIS groun d-state diffuse-reflectan ce
m easurem en ts as well as steady-state an d laser excitation lum in escen ce in
order to get a close in sigh t in to th e in fluen ce of th e size of alkyl substitu-
en ts on th eir ph otoph ysics an d on th e tren ds of th eir ph otoisom er em is-
sion .
S
S
N
R
N
R
I
1
1a, R = Me 3,3'-Dimethylthiacarbocyanine iodide (DMTCC)
1b, R = Et 3,3'-Diethylthiacarbocyanine iodide (DETCC)
1c, R = Pr 3,3'-Dipropylthiacarbocyanine iodide (DPTCC)
With th ese studies we h ope to exten d our kn owledge on th e n ature of in -
teraction s of th ese cyan in es with polym er ch ain s an d on th e in fluen ce of
th e structure of th e guest cyan in e dye m olecule in th e im m obilization pro-
cess.
EXPERIMENTAL
Dyes Syn th esis, Materials an d Sam ple Preparation
3,3′-Dialkylth iacarbocyan in e iodides were syn th esized usin g th e followin g procedure18
.
A m ixture of equim olar proportion s of 2-m eth ylben zoth iazole an d of th e appropriate
alkyl 4-m eth ylben zen esulfon ate was h eated at 100 °C for 1–3 h un til an alm ost quan titative
con version in to th e quatern ary salt was obtain ed (TLC ch eck, Kieselgel 60F254, Merck). Th e
resultin g solid salt was con den sed with trieth yl orth oform ate in boilin g pyridin e (10 m l of
pyridin e per m ol of th e salt); th e boilin g period ran gin g from 1 to 3 h (TLC ch eck).
Th e dyes were isolated by addition of dieth yl eth er to th is m ixture, redissolvin g th e pre-
cipitate in th e m in im um am oun t of h ot m eth an ol an d con vertin g it in to th e iodide by
treatm en t with an excess of aqueous potassium iodide (1.4 g/10 m l). Gen erally, we used two
volum es of th e KI solution per on e volum e of th e m eth an olic solution .
Th e products were crystallized from m eth an ol or eth an ol un til TLC-pure dyes were ob-
tain ed. All th e dyes were ch aracterized by 1H NMR, 13C NMR, IR, an d UV-VIS spectra.
3,3′-Dieth ylth iacarbocyan in e iodide an d 3,3′-dipropylth iacarbocyan in e iodide were also
purch ased from Aldrich in th e h igh est purity available an d, after ch eckin g its purity by
UV-VIS spectroscopy an d TLC, were used with out furth er purification . Th e com m ercial
cyan in es produced th e sam e results as th ose syn th esized.
2-Meth ylben zoth iazole, m eth yl an d eth yl 4-m eth ylben zen esulfon ate, trieth yl
orth oform ate, pyridin e, an d potassium iodide were purch ased from Aldrich in th e h igh est
purity available. All th e solven ts used for syn th esis were of an alytical grade. Pyridin e was
dried before use. Propyl 4-m eth ylben zen esulfon ate was prepared from 4-m eth ylben zen e-
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

462
Oliveira, Almeida, Ferreira:
sulfon yl ch loride an d propan -1-ol in pyridin e at 0 °C, an d isolated as an oil by addin g an
excess of dilute h ydroch loric acid. Extraction was don e with dieth yl eth er, followed by dry-
in g, filterin g an d solven t evaporation , an d distillation of th e ester un der reduced pressure19
.
Eth an ol for sam ple preparation (Merck, Uvasol grade) was used as received. Molecular
sieves (3 an d 4 Å, 4–8 m esh , Aldrich , activated by slow h eatin g up to 250 °C un der vacuum )
were used sin ce very well dried eth an ol was required for sam ple preparation .
Microcrystallin e cellulose13 (Fluka DSO) with 50 µm average particle size was dried un der
vacuum (ca 10–1 Pa) at 60 °C for at least 24 h before sam ple preparation .
Th e sam ple preparation m eth od in volved deposition of probes on th e m icrocrystallin e
cellulose substrate by slow solven t evaporation an d is described in detail in refs1–6. Eth an ol
was used as a solven t. Th e sam ples were an alyzed un der dry (totally m oisture-free sam ples)
or wet con dition s (air-equilibrated sam ples, con tain in g som e m oisture).
Groun d-State Absorption Experim en ts in th e UV-VIS Region s
Groun d-state absorption studies of th e 3,3′-dialkylth iacarbocyan in es adsorbed on m icro-
crystallin e cellulose were perform ed usin g an OLIS 14 UV/VIS/NIR spectroph otom eter with a
diffuse reflectan ce attach m en t. Th e in tegratin g sph ere is 90 m m in diam eter coated in side
with a stan dard wh ite coatin g. Th e stan dard apparatus was m odified to in clude th e possibil-
ity of usin g sh ort-wave pass filters (Corion 550-S) wh ich exclude lum in escen ce of
th iacarbocyan in es from reach in g th e detector (Ham am atsu, Model R955). Furth er experi-
m en tal details an d description of th e system calibration used to obtain accurate reflectan ce
m easurem en ts are given in ref.6. Solution m easurem en ts were m ade usin g th e sam e appara-
tus in th e n orm al tran sm ission m ode.
Th e reflectan ce, R, from each sam ple was obtain ed by scan n in g th e excitation m on o-
ch rom ator in th e adequate wavelen gth region ; th e rem ission fun ction , F(R), was calculated
usin g th e Kubelka–Mun k equation , defin ed by Eq. (1):
F(R) = (1 – R)2/(2R) = K/S .
(1)
Th e Kubelka–Mun k equation applies to optically th ick sam ples, i.e., th ose wh ere an y fur-
th er in crease in th ickn ess does n ot affect th e experim en tally determ in ed reflectan ce R. Sym -
bols K an d S stan d for th e absorption an d scatterin g coefficien ts with dim en sion s (len gth )–1
(ref.10). For an ideal diffuser, wh ere th e radiation h as th e sam e in ten sity in all direction s,
K = 2εC (ε is th e Naperian absorption coefficien t, C is th e con cen tration )10. Sin ce th e sub-
strate usually absorbs at th e excitation wavelen gth , λe,
F(R)dye = F(R)tot – F(R)cell = Σi2εiCi/S ,
(2)
wh ere F(R)cell is th e blan k obtain ed with a cell con tain in g on ly m icrocrystallin e cellulose.
Th is equation predicts a lin ear relation for th e rem ission fun ction of th e probe as a fun ction
of con cen tration (for a con stan t scatterin g coefficien t) wh en ever th e probe is in th e form of
a m on om er.
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Photophysics of Cyanine Dyes on Surfaces
463
Steady-State an d Laser-In duced Fluorescen ce Em ission Experim en ts
Corrected steady-state fluorescen ce em ission spectra of th e cyan in es un der study adsorbed
on m icrocrystallin e cellulose were obtain ed usin g a h om e-m ade fluorim eter specifically de-
sign ed for fron t surface m easurem en ts, wh ich h as been previously described in detail11
.
On a solid substrate, th e fluorescen ce in ten sity, IF, is related to th e con cen tration of th e
adsorbed dye th rough th e reflectan ce at th e excitation wavelen gth , accordin g to Eq. (3):
IF = GI0(1 – R)fdyeφF
,
(3)
wh ere G is a con stan t wh ich depen ds on th e apparatus geom etry, I0 is th e in ten sity of th e
excitation ligh t at th e excitation wavelen gth , R is th e reflectan ce m easured at th e excitation
wavelen gth , fdye is th e fraction of th e excitation ligh t absorbed by th e dye at th e excitation
wavelen gth (defin ed as fdye = F(R)dye/F(R)tot) an d φF is th e fluorescen ce quan tum yield.
Th e fluorescen ce quan tum yields, φF, of th e th iocarbocyan in es adsorbed on m icro-
crystallin e cellulose were determ in ed by th e m eth od reported in ref.6 for probes adsorbed in
powdered solids. Th e m eth ods uses Eq. (4):
(1 − Rλe )f
s
IFu
F
I0s(λ
)
e
φu = φs
,
(4)
s
F
IFs (1 − Rλe )f I0u(λ
)
e
u
u
wh ere u an d s refer, respectively, to un kn own an d stan dard sam ples; I0s(λe)/I0u(λe) is easily
obtain ed provided th at th e en ergy profile of th e system is accurately kn own . Rh odam in e
101 was used as a referen ce com poun d (φs = 1.0).
F
Fluorescen ce em ission spectra of powdered sam ples were also m easured in our laser-
in duced tim e-resolved em ission system . Th e excitation source is th e 337.1 n m pulse of a n i-
trogen laser (m odel PL2300, Ph oton Tech n ology In strum en ts) th at gives 1.6 m J/pulse an d
h as a pulse width of 600 ps. Th e ligh t em itted from th e powdered sam ples was m easured in
a fron t-surface arran gem en t by a gated in ten sified-ch arge-coupled-device (ICCD, m odel
In staspec V, Oriel). Neutral den sity filters were placed between th e laser an d th e sam ple to
reduce th a laser excitation in ten sity th at h its th e sam ple (en ablin g th e use of differen t exci-
tation in ten sities) an d also in fron t of th e ICCD cam era to preven t in ten se em ission ligh t
for saturatin g it.
RESULTS AND DISCUSSION
Ground-State Absorption Spectra of Thiacarbocyanines Adsorbed
on Microcrystalline Cellulose
Figure 1 displays groun d-state absorption spectra for diferen t loadin gs of
DMTCC adsorbed from eth an ol on m icrocrystallin e cellulose. Figure 1a
sh ows th e variation of th e percen tage reflectan ce as a fun ction of th e wave-
len gth . Figures 1b an d 1c sh ow th e correspon den t rem ission fun ction s n or-
m alized at th e absorption m axim um of th e dye. Figures 1a an d 1b presen t
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

464
Oliveira, Almeida, Ferreira:
th e results for air-equilibrated sam ples, con tain in g som e m oisture, wh ile
Fig. 1c refers to dry sam ples.
For low con cen tration s of th e DMTCC deposited on m icrocrystallin e cel-
lulose, th e dye absorbs from 400 to 650 n m , with th e m axim um at 564 n m
an d a vibration al sh oulder at 532 n m , in depen den tly of th e sam ple dry-
n ess. In spite of a bath och rom ic sh ift of 5 to 7 n m , th is spectrum resem bles
very m uch th e on e reported previously for th is dye in eth an ol16d an d for
1.00
F(R)
a.u.
100
R, %
a
1
2
b
dye
5
4
80
0.75
3
3
60
40
2
0.50
0.25
1
4
5
6
20
0
1
5
0.00
400
450
500
550
600
λ, nm
650
400
450
500
550
600
650
λ, nm
1.00
c
F(R) , a.u.
dye
0.75
3,4,5
0.50
1,2
0.25
0.00
5
1
400
450
500
550
600
λ, nm
650
FIG. 1
Reflectan ce spectra (a) an d rem ission fun ction values (n orm alized at th e m axim um of th e
m on om er) (b) an d (c) of 3,3′-dim eth ylth iacarbocyan in e iodide adsorbed on m icrocrystallin e
cellulose. a Air-equilibrated sam ples: 1 0, 2 2.3 · 10–8, 3 1.0 · 10–7, 4 5.8 · 10–7, 5 1.0 · 10–6
,
6 4.7 · 10–6 m ol of dye per gram of th e substrate; b air-equilibrated sam ples: 1 2.3 · 10–8,
2 1.0 · 10–7, 3 5.8 · 10–7, 4 1.0 · 10–6, 5 4.7 · 10–6 m ol of dye per gram of th e substrate; c dry
sam ples sh owin g m on om ers of H aggregates, con cen tration s correspon din g to b
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Photophysics of Cyanine Dyes on Surfaces
465
DETCC in alcoh ols (λm ax = 557 n m in m eth an ol16b; λm ax = 559 n m in eth a-
n ol20a with a vibration al sh oulder at 525 n m ) or in very dilute aqueous so-
lution s20b. Th is sim ilarity m akes it possible to in terpret th is absorption
ban d as th e ch aracteristic absorption of th e m on om er m olecules adsorbed
on th e m icrocrystallin e cellulose substrate. As sh own in Fig. 1, th e in crease
in th e DMTCC loadin g on m icrocrystallin e cellulose above 0.1 µm ol g–1
clearly produces a n eat in crease in th e absorption in ten sity at 532 n m , rela-
tive to th e m on om er absorption m axim um . A sim ilar beh aviour was al-
ready observed for DETCC in water20b or adsorbed on TiO2 (ref.20c). Th e
n ew ban d is due to th e presen ce of H aggregates (san dwich dim ers)21. As de-
scribed for oth er cyan in e dyes adsorbed on m icrocrystallin e cellulose, th e
dye aggregation is extrem ely depen den t on th e m oisture con ten t of th e
sam ple, bein g stron gly favoured by its presen ce1–3. As can be seen in Fig. 1,
absorption spectra are sim ilar for wet (Figs 1a an d 1b) an d dry (Fig. 1c) sam -
ples but for th e sam e dye loadin g, dry sam ples always sh ow sm aller
am oun ts of aggregates th an th e wet sam ples.
We also studied groun d-state absorption of DETCC an d DPTCC on
m icrocrystallin e cellulose an d we verified th at both of th em beh ave sim i-
larly to th e m eth yl derivative (see above). On cellulose, for th e ran ge of
con cen tration s un der study, th ere is n o eviden ce for th e form ation of
3,3′-dialkylth iacarbocyan in e H aggregates larger th an dim ers, con trary to
wh at was observed on th e TiO2 surface20c or in water20b wh ere th e H aggre-
gate form ation process progresses beyon d aggregates with two species.
Steady-State Fluorescence Spectra of Thiacarbocyanines Adsorbed
on Microcrystalline Cellulose
Figure 2 sh ows th e corrected steady-state fluorescen ce em ission spectra of
in creasin g loadin gs of DMTCC adsorbed on m icrocrystallin e cellulose. Th e
experim en ts were perform ed at 20 ± 1 °C, irradiatin g th e sam ple with a xe-
n on lam p at 500 n m .
Th ese results were obtain ed with dry cellulose sam ples, but th ey are very
sim ilar to th ose obtain ed with wet sam ples in th e overall con cen tration
ran ge. Wh en adsorbed on m icrocrystallin e cellulose, DMTCC fluoresces in -
ten sely in con trast to th e gen erally weak fluorescen ce of carbocyan in es in
n on viscous solven ts16. Th e fluorescen ce in ten sity in creases with th e dye
loadin g on th e substrate up to 0.2 µm ol g–1. For h igh er con cen tration s, th e
fluorescen ce em ission in ten sity decreases. A com parison of curves 4 an d 5
in Fig. 2 clearly shows an extinction process starting at about 0.2–0.4 µm ol g–1,
wh ich is th e con cen tration ran ge wh ere H aggregates start to be form ed. In
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

466
Oliveira, Almeida, Ferreira:
fact, it is th e presen ce of aggregates th at prom otes th e observed fluores-
cen ce extin ction process: as th eir con cen tration grows, th e fraction of th e
in ciden t ligh t th at is absorbed by th e em issive m on om ers decreases.
Th e ch an ges in th e fluorescen ce in ten sity of DMTCC on m icrocrystallin e
cellulose with a con cen tration param eter clearly illustrate th e above-
explain ed beh aviour; th ey are presen ted in Fig. 3.
Th e fluorescen ce em ission depen den ce on th e con cen tration of th e ad-
sorbed dye was also in vestigated for DETCC an d DPTCC deposited on cellu-
lose; th e experim en ts were perform ed un der iden tical con dition s. Th e
obtain ed results are very sim ilar to th ose with DMTCC, presen tin g th e sam e
fluorescen ce in ten sity depen den ce with in creasin g of th e dye loadin g. In
spite of th e fact th at DETCC fluoresces with approxim ately th e sam e
in ten sity as DMTCC, we foun d th at for DPTCC, fluorescen ce in ten sity is
clearly sm aller. Th e variation in th e fluorescen ce in ten sity of DETCC an d
DPTCC on m icrocrystallin e cellulose as a fun ction of con cen tration param -
eter is also plotted in Fig. 3.
1.00
4
I , a.u.
F
0.75
5
FIG. 2
3
Corrected steady-state fluorescen ce em is-
0.50
sion spectra of 3,3′-dim eth ylth iacarbo-
cyan in e iodide adsorbed on m icro-
crystallin e cellulose (dry sam ples excited at
6
2
0.25
0.00
500 n m ). Th e con cen tration s: 1 5.0 · 10–9
2 2.3 · 10–8, 3 1.0 · 10–7, 4 5.8 · 10–7, 5 1.0 · 10–6
,
,
1
7
6 2.3 · 10–6, 7 4.7 · 10–6 m ol of dye per
550
600
650
700
750
λ, nm
gram of th e substrate
120
I , a.u.
F
100
80
60
FIG. 3
Fluorescen ce in ten sity of 3,3′-dim eth yl-
th iacarbocyan in e iodide (■), 3,3′-dieth yl-
th iacarbocyanine iodide (✴), and 3,3′-dipropyl-
thiacarbocyanine iodide (▲) adsorbed on mi-
crocrystallin e cellulose, excited at 500 n m
an d m easured as th e total area un der th e
corrected em ission spectra as a fun ction of
(1 – R)fdye
40
20
0
0
0.2
0.4
0.6
0.8
(1 – R)f
1.0
dye
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Photophysics of Cyanine Dyes on Surfaces
467
Fluorescence Quantum Yield of Thiacarbocyanines Adsorbed on
Microcrystalline Cellulose
Usin g th e m eth od previously described in ref.6, we determ in ed th e fluores-
cen ce quan tum yield, φF, for th e 3,3′-dialkylth iacarbocyan in es on m icro-
crystallin e cellulose. Th e obtain ed results are sh own in Fig. 3. Values of
0.98 ± 0.02, 0.96 ± 0.02, an d 0.62 ± 0.02 were foun d, respectively, for th e
fluorescen ce quan tum yield of th e m eth yl, eth yl, an d propyl derivatives of
3,3′-dialkylth iacarbocyan in e on cellulose. Th e quan tum yields refer to th e
m on om er em ission an d are determ in ed for low loadin gs. Wet, dry or argon --
purged sam ples gave th e sam e φF with in th e experim en tal error. Th ese values
sh ow an in crease in th e fluorescen ce quan tum yield for th ese adsorbed dyes
of about on e order of m agn itude h igh er th an th at determ in ed for th e sam e
dyes in eth an olic solution s, wh ere φF n ever exceeds 0.02–0.05 (refs16b,20a).
Alth ough DMTCC an d DETCC reach approxim ately a value of th e quan -
tum yield, φF 1, DPTCC sh ows a con siderably lower quan tum yield. Th is
beh aviour is attributed to th e in fluen ce of th e volum e of th e alkyl substitu-
en ts at th e 3 an d 3′ position s of th e th iacarbocyan in e structure; th e results
m ake clear its im portan t role on th e fluorescen ce em ission properties of
th ese th iacarbocyan in es, even wh en dyes are adsorbed on substrates th at
can in duce an in crease in th e rigidity of th e ch rom oph oric structures, as in
th e case of th e adsorption on m icrocrystallin e cellulose. If th e 3 an d 3′ sub-
stituen ts are sm all, as it is th e case of m eth yl an d eth yl, th e in clusion of th e
dye in th e cellulose m atrix im ports th e dye m olecule h igh rigidity an d pla-
n arity, wh ich secures a h igh fluorescen ce quan tum yield. Th e n on radiative
deactivation processes, usually dom in ated by th e ph otoisom erization pro-
cess wh ich prevails in n on viscous m edia, are alm ost com pletely
elim in in ated, as in dicates th e n early un it fluorescen ce quan tum yields. Th is
also sh ows th at DMTCC an d DETCC on m icrocrystallin e cellulose are ad-
sorbed m ain ly as th e all-trans con form er, con sidered to be respon sible for
th e fluorescen ce em ission of th is dye16b,22. However, th e presen ce of a
sligh tly bulkier substituen t, such as propyl, already prom otes a stereo-
ch em ical h in dran ce wh ich leads to a distortion of th e plan ar, stron gly
em issive form of th iacarbocyan in e derivatives. Th e observed decrease in th e
fluorescen ce quan tum yield of th e DPTCC sh ows th at in th is case, in clusion
in a rigid m edium such as m icrocrystallin e cellulose, is n ot sufficien t to pre-
ven t th e large departure from plan arity of th e ch rom oph oric ch ain , in -
duced by th e presen ce of bulky substituen ts on th e n itrogen atom s of
term in al h eterocyclic rin gs.
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

468
Oliveira, Almeida, Ferreira:
It is im portan t to com pare th ese results with th ose obtain ed with an an a-
logue of th iacarbocyan in e, structurally rigidified by th e in clusion of th e
polym eth in e ch ain in fused h eterocyclic rin gs wh ich lin k both th e n itrogen
atom s, th us in h ibitin g th e rotation about th e con jugated ch ain . Th e quan -
tum yield of th e rigid an alogue in m eth an ol is on ly 0.52 (ref.16b).
Carbocyan in es structurally rigidified usually exh ibit h igh fluorescen ce
quan tum yields wh ich depen d on th e in crease in rigidity an d plan arity.
From th is com parison it can be seen th at th e large in crease in th e fluores-
cen ce quan tum yield for th e cyan in e dyes on cellulose is related to th e de-
crease in m obility of th e polym eth in e ch ain s with in m icrocrystallin e
cellulose. In th e specific case of th e 3,3′-dialkylth iacarbocyan in e dyes, it
can be con cluded th at th e decrease in th e participation of n on radiative de-
activation m ech an ism s is m ore efficien t by in clusion of th e n on rigidified
dye with in th e m icrocrystallin e cellulose substrate th an by structural
rigidification of th e com poun d. In fact, th e value of 0.52 exh ibited by th e
rigidified th iacarbocyan in e in m eth an ol sh ows th at its structure still experi-
en ces a h igh degree of torsion wh ich is larger th an th at in th e 3,3′-dipropyl
derivative with in m icrocrystallin e cellulose.
Laser-Induced Fluorescence Spectra of Thiacarbocyanines Adsorbed
on Microcrystalline Cellulose
Figure 4 presen ts th e corrected laser-in duced fluorescen ce em ission spectra
for in creasin g loadin gs of DMTCC (dry sam ples) on m icrocrystallin e cellu-
lose. Th e spectra were produced usin g a pulsed N2 laser as excitation source
(337 n m an d 1.6 m J per pulse, 600 ps pulse width ).
For dilute sam ples down to 0.1 µm ol g–1, a sin gle em ission ban d was re-
corded, wh ich coin cides with th e typical em ission of th e m on om eric form
of th e dye adsorbed on cellulose, obtain ed previously with a con ven tion al
xen on excitation lam p (cf. Fig. 2 an d curves 1–3 of Fig. 4a). However, for
m ore con cen trated sam ples, a secon d em ission ban d, n ot detected for
steady-state fluorescen ce m easurem en ts, appears at wavelen gth s h igh er
th an th e m on om er em ission wavelen gth s. Th is secon d em ission , wh ich
peaks at 630–632 n m , is especially in ten se for 1.0 µm ol g–1 an d decreases
con siderably for h igh er con cen tration s, bein g produced on ly if h igh excita-
tion in ten sities per un it of area are used (full laser power beam of 1.6 m J,
focused to a spot with an area of 2–3 m m 2). For wet sam ples, th e secon d
em ission is con siderably sm aller.
We furth er in vestigated th e laser-in duced fluorescen ce of differen t load-
in gs of DETCC an d DPTCC adsorbed on m icrocrystallin e cellulose; a sim ilar
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Photophysics of Cyanine Dyes on Surfaces
469
dual fluorescen ce em ission was observed. In parallel with th e above re-
ported fluorescen ce quan tum yield results for th e m on om eric form of th ese
dyes, DMTCC an d DETCC, h ave approxim ately th e sam e dual fluorescen ce
em ission in ten sity, wh ile for DPTCC it is con siderably sm aller. Th is sug-
gests th at th e volum e of th e alkyl group also affects th e yield of th e secon d
em ission .
Th e dual fluorescen ce em ission s are sim ilar to th ose already reported for
oth er cyan in es on cellulose, viz. for 2,2′-carbocyan in e1a an d for th e
3,3′-dieth yloxadicarbocyan in e2, th eir origin bein g th e em issive ph oto-
isom ers. Th ese isom ers can be already form ed in th e groun d state (as is th e
case of th e 3,3′-dieth yloxadicarbocyan in e)2 an d/or be form ed un der h igh
in ten sity laser excitation (like for th e 2,2′-carbocyan in e)1a. Alth ough a care-
ful an alysis of th e groun d-state absorption an d steady-state em ission results
from th e th ree 3,3′-dialkylth iacarbocyan in es on m icrocrystallin e cellulose
did n ot reveal th e presen ce of groun d-state con form er an d/or ph otoisom er
em ission ban d. Its existen ce can n ot be disregarded, sin ce for th e DETCC in
solution , both th e all-trans isom er absorbin g at 560 n m (ref.23) an d th e cis
m etastable ph otoisom er absorbin g at 545 n m (refs16c,24) were reported. For
th e wet sam ples, th e observation of a con siderably lower ph otoisom er em is-
sion is associated with th e existen ce of a h igh con cen tration of aggregated
species wh ich determ in es th e decrease in th e am oun t of free m on om ers
wh ich can un dergo ph otoisom erization .
30
6
I , a.u.
F
5
20
10
7
4
2
3
9
8
1
0
550
600
650
600
650
700
λ, nm
FIG. 4
Corrected fluorescen ce em ission spectra of 3,3′-dim eth ylth iacarbocyan in e iodide adsorbed
on m icrocrystallin e cellulose, obtain ed after pulsed laser excitation (337 n m , 1.6 m J per
pulse) an d recorded with an ICCD. Th e con cen tration s: 1 5.0 · 10–9, 2 5.0 · 10–8, 3 1.0 · 10–7
,
4 2.5 · 10–7, 5 7.5 · 10–7, 6 1.0 · 10–6, 7 2.5 · 10–6, 8 5.0 · 10–6, 9 10.0 · 10–6 m ol of dye per
gram of th e substrate
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

470
Oliveira, Almeida, Ferreira:
Th e depen den ce of th e in ten sity of both fluorescen ce em ission ban ds
with th e in ten sity of th e excitation source for DMTCC, DETCC an d DPTCC
adsorbed on m icrocrystallin e cellulose was furth er studied. Figure 5 pres-
en ts th e laser-in duced corrected em ission spectra of 0.5 µm ol g–1 of DMTCC
on m icrocrystallin e cellulose obtain ed after several laser excitation in ten si-
ties. Th ese results are sim ilar to th ose obtain ed for th e oth er two dialkyl-
th iacarbocyan in es on cellulose in th e sam e con cen tration ran ge. For dilute
sam ples, wh ich sim ply sh ow m on om er em ission , th e spectra are in depen d-
en t of th e laser in ten sity. In th is case, th e results were iden tical to th ose ob-
tain ed with th ese sam ples usin g a xen on lam p or a pulsed laser as
excitation sources. On th e con trary, for m ore con cen trated sam ples, wh ere
th e presen ce of both em ission ban ds is revealed for full laser excitation in -
ten sity (see Fig. 5), a progressive decrease in th e laser in ten sity leads to a
parallel decrease in th e in ten sity of th e secon d em ission ban d. For suffi-
cien tly low laser in ten sities, fluorescen ce em ission spectra are sim ilar to
th ose obtain ed with station ary excitation . Th e ch an ges in th e ph otoisom er
em ission in ten sity as fun ction of th e laser excitation in ten sity were quan ti-
fied. In th e in set of Fig. 5, for 0.5 µm ol g–1 of DMTCC on m icrocrystallin e
cellulose, th e in ten sity of th e ph otoisom er em ission , IN*, was plotted as
6
12
1
I
I , a.u.
N*
F
2
8
4
0
4
3
4
5
0
20 40 60 80 100
2
0
I , a.u.
0
6
550
600
650
700
750
λ, nm
FIG. 5
Corrected fluorescen ce em ission spectra of 3,3′-dim eth ylth iacarbocyan in e iodide adsorbed
on m icrocrystallin e cellulose, obtain ed after pulsed laser excitation (337 n m , 1.6 m J per
pulse) an d recorded with an ICCD. Th e spectra are n orm alized at th e m axim um of th e
m on om er em ission . Th e con cen tration of th e sam ple is 5.0 · 10–7 m ol of dye per gram of
th e substrate. 1 100% is th e m axim um laser excitation en ergy, i.e., 1.6 m J per pulse; 2
79.8%, 3 49.2%, 4 20.8%, 5 5.9%, 6 2.2%. In set: In ten sity of th e ph otoisom er em ission as
fun ction of th e laser excitation in ten sity for 5.0 · 10–7 m ol of 3,3′-dim eth ylth iacarbocyan in e
iodide per gram of m icrocrystallin e cellulose
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Photophysics of Cyanine Dyes on Surfaces
471
fun ction of th e laser excitation in ten sity, I0. IN* was m easured th rough sub-
traction of th e m on om er em ission in ten sity from th e total laser-in duced
em ission in ten sity at th e m axim um ph otoisom er em ission wavelen gth . A
supralin ear depen den ce on laser excitation in ten sity was observed. For
DETCC an d DPTCC, th e sam e n on lin ear depen den ce of th e ph otoisom er
em ission on th e excitation in ten sity was foun d. Th is result is con sisten t
with ph otoisom er em ission arisin g from isom ers form ed after an d/or before
laser excitation . If th e isom ers are form ed after laser excitation , th ey are
origin ated in a two-ph oton absorption process, th e first ph oton bein g
n eeded to create th e isom er an d th e secon d on e to excite it; if th ey are al-
ready form ed before laser excitation , a sin gle ph oton is n eeded for th e
ph otoisom er em ission to occur.
CONCLUSIONS
For th e th ree 3,3′-dialkylth iacarbocyan in e dyes adsorbed on m icro-
crystallin e cellulose (alkyl = m eth yl, eth yl, propyl), H dim ers are th e on ly
aggregates form ed with th e in crease in dye loadin g. As observed previously
for oth er cyan in es on th is n atural polym er, th e decrease in th e h um idity
con ten t of th e sam ples reduces th e am oun t of th e H aggregates.
DMTCC an d DETCC exh ibit h igh fluorescen ce quan tum yields on cellu-
lose, sh owin g th at th e in clusion of th ese m olecules with in th e polym er
ch ain s by th e use of eth an ol leads to an alm ost com plete elim in ation of th e
n on radiative deactivation m ech an ism s.
Th e presen ce of a bulky substituen t such as propyl in th e 3 an d 3′ posi-
tion s of th e th iacarbocyan in e prom otes a clear in crease in th e n on radiative
deactivation processes, in h ibitin g th e m olecule to becom e en trapped
with in th e polym er ch ain s, an d in th is way, to h ave a plan ar structure.
Fluorescen ce em ission studies of wet an d dry cellulose sam ples of th e
th ree th iacarbocyan in es usin g a xen on lam p as an excitation source pro-
duced on ly m on om er em ission in th e wh ole ran ge of con cen tration s un der
study. However, un der in ten se laser excitation , dry sam ples exh ibit a sec-
on d em ission ban d located at h igh er wavelen gth s th an th e n orm al m on o-
m er em ission . Th is secon d em ission is very in ten se an d yields a supralin ear
depen den ce on excitation in ten sity. In fact, after in ten se laser excitation ,
ph otoisom erization of n on -aggregated m on om ers occurs an d th ey m ay
reach a h igh er electron ic state by absorption of a secon d ph oton . Th is ex-
cited ph otoisom er is respon sible for th e above-m en tion ed secon d em ission .
With wet sam ples, aggreagate form ation greatly in h ibits th e ph oto-
isom erization process.
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

472
Oliveira, Almeida, Ferreira:
One of the authors (A. S. O.) thanks FCT for her Post-Doc grant (BPD Praxis XXI 14182/97). This
work was financed by Project Praxis/2/2.1/QUI/22/94.
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