1D01
1D01
超短パルスを用いたシアニン色素の多光子退色反応機構の解明（神戸大学） ○冬木正紀、和田昭英
Multiphoton photodegradation of indocyanine green in condensed phase(Kobe University)○Masanori Fuyuki, Akihide WADA

Ratio of |∆A||∆A|∆A

Carbocyanine dyes have been extensively studied and synthesized for more than a century [1].
Among them, indocyanine green (ICG) is one of the most intensively investigated dyes and thisnear-infrared (NIR) laser dye has quite low quantum yields of isomerization and degradation from theS1 state [2,3]. Thus, ICG molecules are used to detect specific cancers in human bodies [4].
Recently, we found that the quantum yield of ICG trans-to-cis photoisomerization was drasticallyincreased by femtosecond NIR multiphoton excitation [5,6]. However, the photodegradation
dynamics of ICG molecules has not been investigated thoroughly. In this paper, we reveal that themain photodegraded species are ICG fragments and the timescale of the photodegradation is 14 ps.
We also reveal that the optical order of NIR electronic excitation changes from two to four withincreasing the pump fluence.
Fig. 1 shows the transient absorption spectrum of ICG
0.0
pumped by NIR femtosecond pulse (800 nm, 70 fs, 402mJ/cm , 1 KHz) in ethanol. The details of experimental-0.3
setup are described elsewhere [5]. In Fig. 1, transient
signals observed around 560 nm, 790 nm, and 890 nm-0.6
are assigned as transient absorption (TA) by ICG leuco
forms, photobleaching (PB) of ICG in trans configuration,-0.9
and TA by photo-produced cis isomers, respectively [6].
500600700
800900Note that leuco forms and cis isomers were produced
Wavelength / nmwithin 2 ns [6] and the pump-probe delay for our Fig. 1. Transient absorption spectrum of
ICG in ethanol.
transient absorption measurement was 2.5 ns.
1Fig. 2 shows the NIR fluence dependence of-1slope = 2.0
10absorbance change by ICG photoproducts and PB. In-2
the fluence region lower than 3 mJ/cm2, the absolute of
10absorbance change by PB was proportional to the pump-3
10 slope= 3.0
fluence to the second power; in the leuco forms, it wasphotobleaching (785 nm)
leuco forms (500 nm)-4
proportional to the third power; and in the cis isomers, it
10
cis isomers (890 nm)
slope = 3.9
was proportional to the fourth power. These results
-510
11050
indicate that the main photodegraded species were2
NIR fluence / mJ / cm
produced by the NIR two-photon process and the species
Fig. 2. NIR fluence dependence ofwere not the leuco forms but the photo-fragments.
absorbance change by ICG photoproducts.
Fig. 3 shows the absolute ratios of absorbance
0.09
leuco forms vs photobleachingchanges in Fig. 2. ICG leuco forms and cis isomers were(500 nm)(785 nm)
cis isomers vs photobleachingproduced by NIR three-photon and four-photon processes,(890nm)(785 nm)0.06
respectively. The absolute ratio of absorbance change(open circles, leuco forms/photobleaching) increased
0.03
with the increase of the pump fluence to 4 mJ/cm2.
Then, the ratio decreased. On the other hand, theabsolute ratio of absorbance change (closed squares, cis0.00
010203040
isomers/photobleaching) increased with the increase of2NIR fluence / mJ / cm2
the pump fluence to 10 mJ/cm and leveled off thereafter. Fig. 3. Ratio of absorbance changes by ICG
These results indicated that the optical order of the photoproducts to that of photobleaching.

-∆A

-∆A

electronic excitation by the NIR pump pulse in the main photodegradation reaction changed as 2→3→4 with the increase of pump fluence. However, the results did not mean that the main
photodegraded species were ICG leuco forms or cis isomers in the high fluence region. The absoluteof the absorbance change by PB is much larger than those by the ICG photoproducts in Fig. 1 where
the fluence was 40 mJ/cm2. This result indicates that the main photodegraded species were ICG
fragments even in the high fluence region.
Fig. 4 shows the schematic diagram of the
newly found multiphoton reaction paths of ICG
molecules in ethanol. The timescales of therelaxation processes from the electronic excitedstates investigated by us and other groups arealso described [2,3,5,6]. The timescale of ICG
photodegradation from electronic excited stateshigher than S1 state has not been investigated.
In the following, we investigated the relaxation
4. Newly found ICG multiphoton reaction
process from high-order singlet states to the Fig.
pathways. Arrows with straight line represent NIR
ground state of the photodegraded species by pumping. Arrows awith dashedline drepresentbc
relaxation
processes.
ref.
5,ref.
2,ref.
6, ref. 3.
pump-probe measurement on PB.
Fig. 5 shows the absolute change by PB at 785 nm as (a) 1.2
a function of pump-probe delay. The pump fluence
Experiment
Model
Under the femtosecond NIR
was 10 mJ/cm2.
0.9
decay 1 (485 ps)
2decay 2 (14 ps)
irradiation of 10 mJ/cm , the fragments were mainly
decay 3 (1.4 ps)
0.6
produced by the NIR four-photon process as indicatedabove. With the increase of the delay time, the0.3
absolute of the absorbance change decreased. The
temporal profile was reproduced well by the model
0.0
function consisting of three exponential decay
0500
1000
1500
2000
Pump-probe
delay/pscomponents and a constant background. The time
constants of the three decay components were 1.4 ps, (b) 1.2
Experiment
14 ps, and 485 ps. The time constants of 1.4 ps andModel
0.9
decay 1 (485 ps)
485 ps were consistent with the lifetimes of ICG
decay 2 (14 ps)
decay 3 (1.4 ps)
molecules at the Sn and S1 states, respectively. The
0.6
lifetime of ICG molecules at singlet electronic excited
0.3states higher than the Sn state was shorter than that at
Sn state of 1.4 ps [6]. Thus, the decay component of 140.0ps was thought to represent the relaxation dynamics
0102030
4050
Pump-probe delay / psfrom the singlet system to other electronic systems or5. Absorbance change by ICGthe dynamics at electronic excited states outside of the Fig.
photobleaching at 785 nm as a function ofofsinglet system of trans configuration. In this study, we pump-probe delay. (b) Magnified view2(a). The pump fluence was 10 mJ/cm .
observed photoproduced ICG leuco forms, cis isomers,
and fragments. The timescales for leuco form production and cis isomer productions were < 10 ps,
and 1-2 ns, respectively [6]. Moreover, the singlet system for the ICG cis isomer is expected to besimilar to that for the ICG trans isomer [6,7]. Thus, newly found decay component of 14 ps shouldrepresent the relaxation dynamics in the multiphoton photodegradation process from the Sk state of
the trans singlet system.
References [1] W. West, Photograph. Sci. Eng. 18 (1974) 35. [2] S. H. Ashworth et al., J. Chem. Phys.
104 (1996) 5761. [3] H. Gratz et al., J. Photochem. Photobiol. A: Chem. 128 (1999) 101. [4] T. Sugieet al., Cancers 2 (2010) 713. [5] M. Fuyuki et al., Chem. Phys. Lett. 499 (2010) 121. [6] M. Fuyuki etal., J. Photochem. Photobiol. A: Chem. 252 (2013) 152; RSC Adv. 3 (2013) 7313. [7] M. Wittmann andA. Penzkofer, Appl. Phys. B 65 (1997) 761.