Chemical Physics Letters 390 (2004) 433–439

www.elsevier.com/locate/cplett

Two-photon absorption properties of star-shapedmolecules containing peripheral diarylthienylamines

Sean Liu a, Kuen Shen Lin a, Victor M. Churikov a, Yi Zhen Su b,Jiann T’suen Lin b, Tzer-Hsiang Huang c, Chia Chen Hsu d,*

a Department of Physics, National Chung Cheng University, Ming Hsiung, Chia-Yi 621, Taiwanb Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan

c Department of Electronic Engineering, Wu Feng Institute of Technology, Ming Hsiung, Chia-Yi 621, Taiwand Department of Physics and Graduate Institute of Opto-Mechatronics, National Chung Cheng University, Ming Hsiung, Chia-Yi 621, Taiwan

Received 24 September 2003; in final form 5 March 2004

Available online 6 May 2004

Abstract

Two-photon absorption (TPA) properties of several novel donor–acceptor–donor (D–A–D) quadrupolar and octupolar star-

shaped molecules containing peripheral diarylthienylamines were investigated and are discussed in this Letter. The effects of donor–

acceptor strength, conjugation length, and molecular symmetry on the effective TPA cross-section of these novel molecules were

studied. It was found that incorporation of a triazine acceptor center in octupolar molecules can effectively enhance TPA response.

This provides a strategy for designing novel octupolar molecules with a large TPA response but without red-shifting their linear

absorption wavelengths.

� 2004 Published by Elsevier B.V.

1. Introduction

Research on two-photon absorption (TPA) prop-

erties of organic chromophores has become important

recently due to applications in optical power limiting

[1–4], three-dimensional optical data storage [5–7],

two-photon fluorescence microscopy [8–10], micro ornano structure fabrication [11,12], upconversion

[13,14], etc. These applications use two key aspects of

TPA: (1) the electronic transition from ground to

excited states takes place when a pair of photons with

photon energy that is about half of the transition

energy is absorbed, and (2) absorption is proportional

to the square of the incident intensity [15]. The former

aspect of TPA provides improved penetration into a

* Corresponding author. Present address: Department of Economics,

National Central University, Chung Li 32054, Taiwan. Fax: +886-3-

4222876.

E-mail address: cchsu@phy.ccu.edu.tw (C.C. Hsu).

0009-2614/$ - see front matter � 2004 Published by Elsevier B.V.

doi:10.1016/j.cplett.2004.03.050

sample, and the latter aspect allows a high degree of

spatial selectivity in three dimensions when chromo-

phores are excited by the use of a tightly focused laser

beam [16].

Unfortunately, TPA responses of most known or-

ganic chromophores are too small to allow the im-

plementation of the above-mentioned applications. Itis thus necessary to design and synthesize organic

molecules with highly efficient TPA. Knowledge of the

relationship between molecular structure and TPA

cross-section r2 can provide a guideline for this design

and synthesis. Previous research results showed that

some quadrupolar molecules with conjugated p-elec-tron donor–acceptor–donor (D–A–D) or acceptor–

donor–acceptor (A–D–A) structure possess very larger2, due mainly to their centrosymmetric charge re-

distribution [16–18].

Recently, TPA properties of molecules with octu-

polar and multi-branched structures have attracted

considerable attention. Prasad et al. [19] showed that

multi-branched structures exhibit large cooperative

434 S. Liu et al. / Chemical Physics Letters 390 (2004) 433–439

enhancement of TPA in comparison to its one-bran-

ched counterpart. Cho et al. [20–22] investigated the

TPA properties of 1,3,5-tricyano-2,4,6-tris(styryl)ben-

zene derivatives with octupolar structure. They found

that some of these molecules have very large r2, andthe maximum value of r2 increases as the donor

strength and conjugation length increase. A linear

relationship was observed between r2 and the first

hyperpolarizability b. This r2–b relationship served as

a useful synthetic strategy for the design of novel TPA

dyes with octupolar structure [20–22].

In this Letter, we report the TPA properties of several

novel D–A–D quadrupolar and octupolar star-shapedmolecules containing peripheral diarylthienylamines.

The effects of donor–acceptor strength, conjugation

length, and molecular symmetry on TPA properties

were studied and are discussed here. Fig. 1 shows the

molecular structures of the molecules studied in this

work.

Due to theweak absorption of the incident radiation, it

is very difficult to determine r2 from measurement of adirect transmission. Two-photon absorption induced

(a)

(c)

(e)

P2SS2Ph

S

S S

S

S

S

NPh2

NPh2

Ph2N

Ph2N

S

NPh2

SNPh2SPh2N

NN N

S

S

SNPh(Tol)

NPh(Tol)

(Tol)PhN TRZ-Ph-Tol

P3SPh

Fig. 1. Structures of the star-shaped molecules studied in this work: (a) P2SS

(e) TRZ-Ph-Tol are octupolar, and (f) Rhodamine B is the reference sample

fluorescence (TPF) measurement is an alternative ap-

proach to the determination of r2. In theory, the time-

averaged TPF flux density hF ðtÞi measured by an ideal

detecting systemwith uniform spectrum-response is given

by [23]

F ðtÞh i � ð1=2Þgj/Cr2

8n P ðtÞh i2

pk

¼Z 1

0

h~F ðt;xÞidx; ð1Þ

where j is the fluorescence collection factor for the ideal

detecting system. Ideally, j should remain constant for

all samples when the same experimental configuration is

employed. g ¼ hI20 ðtÞi=hI0ðtÞi2is the normalized inten-

sity–intensity autocorrelation of the laser used. / is the

fluorescence quantum efficiency. C, n, and k are the

sample’s concentration, refractive index, and excitationwavelength, respectively. hP ðtÞi is the time-averaged

excitation power. h~F ðt;xÞi is the time-averaged TPF flux

density emitted from molecules at frequency x.In most experiments, a detecting system with a non-

uniform spectrum-response is used. Therefore, the time-

(b)

(d)

(f)

P2SS2PhNp

SS S

S

S NPhNp

NPhNp

NpPhN

S

NpPhN

S

S

S

SS

S

S

S

S

NPh2

Ph2N

NPh2

Ph2N

NPh2

NPh2

COH

O

O N+−CH2CH3CH3CH

2−N

CH2CH

3CH

2CH

3

P3SS2Ph

Rhodamine B

2Ph and (b) P2SS2PhNp are quadrupolar, (c) P3SPh, (d) P3SS2Ph and

.

eoctupolarmolecules(P3SPh,P3SS2Ph,TRZ-Ph-Tol)andthereference

molecule

RhodamineB

SS2PhNp

(c)P3SPh

(d)P3SS2Ph

(e)TRZ-Ph-Tol

(f)RhodamineB

373

432

424

545

438

547

552

572

0.493

0.603

0.678

0.107

740

862

825

813

120

170

2300

160

0.082

0.075

0.059

0.7

12

7.5

5.3

5.3

ence’and‘two-photonabsorption’.Stokes’shiftDEisdefined

asE(k

ð1Þ

max)–E(D

OPF

max).

fphotons.

ismeasured.

S. Liu et al. / Chemical Physics Letters 390 (2004) 433–439 435

averaged TPF flux density hF 0ðtÞi measured in an ex-

periment should be expressed as

F 0ðtÞ� �

� ð1=2Þgj0/Cr2

8n P ðtÞh i2

pk

¼Z 1

0

h~F ðt;xÞiRðxÞdx; ð2Þ

where j0 is the fluorescence collection factor for the real

experimental system and RðxÞ is the spectrum-responsefunction of the detecting system. A spectrum-response

correction factor f between the ideal and real experi-

mental systems is defined as

f � jj0 ¼

hF ðtÞihF 0ðtÞi ¼

R10h~F ðt;xÞidxR1

0h~F ðt;xÞiRðxÞdx

; ð3Þ

where f can be different for different samples because

they may have different h~F ðt;xÞi distributions. From

Eq. (2), we determine r2 of the sample studied bycomparing hF 0ðtÞi of the sample with that of the [24,25].

If hF 0ðtÞi of both the sample studied and the reference

are measured in identical experimental conditions,

8n=pk, g, and hP ðtÞi2 in Eq. (2) can be cancelled, leading

to

r2s ¼/rCrnrfshF 0ðtÞis/sCsnsfrhF 0ðtÞir

r2r; ð4Þ

where the subscripts s and r refer to the sample studied

and the reference, respectively. Once we know the valuesof all the physical parameters on the right hand side of

Eq. (4), the TPA cross-section r2s of the sample studied

can be determined.

Table

1

Spectroscopic

properties

ofthequadrupolarmolecules(P2SS2Ph,P2SS2PhNp),th

Compound

(a)P2SS2Ph

(b)P2

Linearabsorptionpeak,kð

1Þ

max(nm)

437

440

OPFspectralpeak,kO

PF

max(nm)

548

553

Stokes’shiftDE(eV)a

0.575

0.576

TPA

cross-sectionpeak,kð

2Þ

max(nm)

807

804

Effectivemaxim

alTPA

cross-section,b

r2;eff;m

ax(�

10�50cm

4s/#)

340

360

Fluorescence

quantum

efficiency,/

0.054

0.052

Excitationphotonfluxdensity

c

(�1027photoncm

�2s�

1)

5.3

5.3

aOPFandTPA

stand,respectively,for‘one-photonabsorptioninducedfluoresc

bThedata

listed

pertain

tothemaxim

alcross-section.#

standsforthenumber

ocExcitationphotonfluxdensity

isgiven

forthewavelength

atwhichther2;eff;m

ax

2. The experiment

Synthesis of P3SPh: The compound was synthesizedfrom 5-(N,N-diphenylamino)-2-(tributylstannyl)thioph-

ene, 1,3,5-tribromobenzene, and PdCl2(PPh3)2 follow-

ing the procedure described in an earlier report [26].

The pale yellow products were isolated with a 65%

yield.

Synthesis of P3SS2Ph: Orange powdery P3SS2Ph was

synthesized (37% yield) by the same procedure used for

P3SPh except that N5,N 5,N500 ,N500-tetraphenyl-50-tribu-tylstannyl-[2,20;30,200]terthiophene-5, 500-diamine [27] was

used instead of 5-(N,N-diphenylamino)-2-(tributylstan-

nyl)thiophene. The compounds P2SS2Ph and P2SS2-

PhNp were synthesized as described in [27].

Synthesis of TRZ-Ph-Tol: Yellow-orange powdery

TRZ-Ph-Tol was synthesized (89% yield) from 2,4,6-

tris-(4-bromo-phenyl)-[1,3,5]triazine, 5-(N,N-(m-tolyl)

phenylamino)-2-(tributylstannyl)thiophene [27], andPdCl2(PPh3)2 by a procedure similar to P3SPh. All the

sample molecular structures were identified by the NMR

technique.

436 S. Liu et al. / Chemical Physics Letters 390 (2004) 433–439

The molecules from the samples studied were dis-

solved in tetrahydrofuran (THF) solvent, whereas the

reference Rhodamine B molecules were dissolved in

methanol. The concentration of the studied and refer-

ence samples was about 1:0� 1016 and 7:5� 1014 cm�3,respectively. All the sample solutions were passed

through a 0.22 lm filter to eliminate the indissoluble

particles, and the concentrations of all the sample so-

lutions remained unchanged after filtration.

An optical parametric oscillation (OPO) laser (Con-

tinuum Mirage 500) with a 10 ns pulse width and a 10

Hz repetition rate was used as the light source in the

experiment. The excitation light was focused into thecenter of a cylindrical glass sample cell, and the TPF was

collected by a photo-multiplier tube (PMT, Hamamatsu

R928) in the direction perpendicular to the propagation

direction of the excitation beam. For the results shown

in Fig. 3 and r2;eff ;max in Table 1, a broad band-pass

filter instead of a monochromator was used in front of

the PMT so that all the TPF signals from the sample in

the wavelength range 400–700 nm were collected. InEq. (3) the denominator

R10h~F ðt;xÞiRðxÞdx was ob-

tained directly from the experiment, and the numerator

0.0

0.5

1.0

0

0

1

0.0

0.5

1.0

1.5

0

0

1

1

0.0

0.5

1.0

1.5

2.0

0

0

1

2

4

6

8

300 400 500 600 700 800

300 400 500 600 700 800

2

4

6

300 400 500 600 700 800

2

4

6

P3SPh

ab

sorb

ance

(A

.U.)

TRZ-Ph-Tol

P2SS2Ph

wavelengt

Fig. 2. Linear absorption spectra (solid lines) and two-photon fluorescence (T

were detected with a monochromator/PMT system with a spectral resolution

R10h~F ðt;xÞidx was determined by removing the con-

tribution of the spectrum-response function RðxÞ of thedetecting system from the TPF spectra of all the samples

in Fig. 2. The spectrum-response function RðxÞ of the

detecting system was acquired from the manufacturers’data sheets.

The TPA cross-sections r2s of the samples were de-

termined from Eq. (4) based on the values of r2r and

/r(/r ¼ 0:7) of Rhodamine B reported in the literature

[23,28] and the refractive indices ns ¼ 1:407 and

nr ¼ 1:328. Here, we assume that TPF and one-photon

absorption induced fluorescence (OPF) have the identi-

cal fluorescence quantum efficiency, which remainsconstant in the excitation wavelength range used in this

work. Support for this assumption is given below.

3. Results and discussions

Linear absorption spectra of all samples studied are

shown, along with the TPF spectra caused by the831 nm radiation, in Fig. 2. The TPF spectra of all the

samples, except that of P3SS2Ph, remain unchanged as

.0

.5

.0

.0

.5

.0

.5

.0

.5

.0

2

4

6

2

4

6

300 400 500 600 700 800

300 400 500 600 700 800

300 400 500 600 700 800

2

4

6

8

P2SS2PhNp

P3SS2Ph

Rhodamine B

inten

sity (A.U

.)

h (nm)

PF) spectra (dotted solid lines) of the molecules studied. TPF spectra

of 10 nm.

S. Liu et al. / Chemical Physics Letters 390 (2004) 433–439 437

excitation wavelength varied from 740 to 990 nm. This

shows that their TPF emission states, and thus their

quantum yields, were unchanged in this excitation

wavelength range. For the P3SS2Ph sample in the 740–

890 nm excitation wavelength range, the TPF peaked at550 nm, as shown in the figure. When the excitation

wavelength increased to the 900–990 nm region, the TPF

peak shifted to 580 nm (not shown in the figure). This

indicates that the P3SS2Ph sample may be a hybrid-

aggregate comprising two different species, each of

which fluoresces independently. It was also found that

the TPF and OPF spectra were quite similar for all the

700 800 900 1000

1.4

1.6

1.8

2.0

700 800 900 1000

700 800 900 1000

1.4

1.6

1.8

2.0

1.4

1.6

1.8

2.0

P2SS2Ph

TRZ-Ph-Tol

wavelengt

po

wer

-dep

end

ence

P3SPh

Fig. 3. The power dependence of TPF intensity on incident photon-flux-den

termined for incident photon flux density falling in the range �1027–1028 ph

samples studied. This shows that both TPF and OPF

processes emit fluorescence photons from the same ex-

cited state. Consequently, both processes should have

the same quantum efficiency /s (or /r).

Ideally, the TPF intensity should follow the power-squared dependence on the excitation intensity. We

measured the power dependence of all the samples in the

excitation wavelength range from 740 to 990 nm with

excitation photon flux densities �1027–1028 photon

cm�2 s�1. As shown in Fig. 3, power dependence at long

excitation wavelengths was close to the power-squared

dependence, whereas that at short excitation wavelengths

700 800 900 1000

700 800 900 1000

700 800 900 1000

1.4

1.6

1.8

2.0

1.4

1.6

1.8

2.0

1.4

1.6

1.8

2.0 Rhodamine B

P3SS2Ph

P2SS2PhNp

h (nm)

sity plotted against excitation wavelength. Power dependence was de-

oton cm�2 s�1.

438 S. Liu et al. / Chemical Physics Letters 390 (2004) 433–439

deviated more from the power-squared dependence. It is

possible that higher excited states were reached with

shorter excitation wavelength (higher photon energy).

As a result, extra non-fluorescent energy dissipation

processes, such as excited-state absorption, intersystemcrossing, and stimulated emission, involved. These dis-

sipation processes competed with TPF (i.e., use up in-

cident photon energy without producing TPF) and

hence resulted in deviation from the power-squared

dependence.

Thus, the r2 values determined in this work cannot

fully represent the true TPA property of the molecules

studied. Nonetheless, it is useful to introduce the effec-tive TPA cross-section r2;eff to qualitatively describe the

TPA response of the samples. r2;eff depends critically on

the excitation conditions used and the energy dissipation

processes involved. It will be equal to r2 only when no

non-TPF energy dissipation processes are involved. As

suggested in Fig. 3, the power dependence of the P3SPh

molecule does not follow the general trend of a decrease

in power dependence with the decrease of the excitationwavelength. This may be because the transition energy

of P3SPh molecules is higher than twice the excitation

photon energy used in Fig. 3. Therefore, the above-

mentioned non-TPF energy dissipation processes are

insignificant under short wavelength excitations shown

in Fig. 3.

The results we obtained for all samples are summa-

rized in Table 1. The fluorescence quantum efficiencies /listed in the table were determined by the OPF tech-

nique. The errors of the r2;eff ;max values are approxi-

mately 20%.

We first look at the effect of conjugation length and

electron donation. As shown in Table 1, the effective

maximal TPA cross-section r2;eff ;max and Stokes’ shift

DE of the octupolar P3SS2Ph were larger than those of

octupolar P3SPh, which has a less extended conjugationlength than that of P3SS2Ph. The change of peripheral

electron donor from NPh2 in the quadrupolar P2SS2Ph

to NPhNp in the quadrupolar P2SS2PhNp resulted in a

small increase in r2;eff ;max. Since the difference is within

the 20% experimental uncertainty range, we cannot as-

certain the trend of the r2;eff ;max increase solely from the

present measurement.

Next, let us examine the effect of changing the electronacceptor center from benzene to triazine on TPA re-

sponse. Comparing the octupolar star-shaped molecules

P3SS2Ph (with a benzene at the center) and TRZ-Ph-Tol

(with a triazine at the center), we found that the latter had

a DE that was slightly larger, a r2;eff ;max that was more

than ten times larger, and a kð1Þmax that was not red-shifted,

as is usually the case. Since the molecule TRZ-Ph-Tol has

a strong central acceptor, triazine, this comparison hintsat a plausible strategy for designing novel octupolar

molecules with a large TPA response but without red-

shifting their linear absorption wavelength.

As shown in Table 1, the peak wavelength positions

of the effective TPA cross-section kð2Þmax of the star-

shaped octupolar molecules are generally twice their

linear absorption peak positions kð1Þmax. This indicates

that in these non-centrosymmetric structures both one-and two-photon transitions to the same electronic

state are allowed [20]. On the contrary, the kð2Þmax of the

star-shaped quadrupolar molecules are relatively

smaller than twice their linear absorption peaks kð1Þmax.

This means the final state of TPA in these quadru-

polar molecules is higher than the zero-vibration level

of the first excited electronic state, which is the final

state of one-photon absorption, because of their cen-trosymmetric structures [17].

4. Summary

Using a tunable wavelength OPO laser, we investi-

gated the TPA properties of several novel D–A–D

quadrupolar and octupolar star-shaped molecules con-

taining peripheral diarylthienylamines. Experimental

results showed that the use of a triazine electron–acceptorcenter in star-shaped octupolar molecules provides an

effective way to enhance TPA efficiency without in-

creasing the absorption peak wavelength. The locations

of the one- and the two-photon states were found to be

related to inversion symmetry: in non-centrosymmetric

octupolar molecules both one- and two-photon transi-

tions to the same electronic state were allowed, but in

centrosymmetric quadrupolar molecules the two-photonstate was above the lowest one-photon state.

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