Comparison between Theoretically and Experimentally Determined

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Comparison between Theoretically and Experimentally Determined Electronic Properties: Applications to Two-Photon Singlet Oxygen Sensitizers. Christian Benedikt Orea Nielsen, Henning Osholm Sørensen, and Jacob Kongsted J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp5122849 • Publication Date (Web): 03 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

February, 2015

Comparison

between

Theoretically

and

Experimentally Determined Electronic Properties: Applications

to

Two-Photon

Singlet

Oxygen

Sensitizers. Christian B. O. Nielsena,b,c Henning Osholm Sørensen,d and Jacob Kongstede a

Polymer Department, Risø National Laboratory, Frederiksborgvej 399, DK-4000 Roskilde,

Denmark b

Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C,

Denmark c

Present address: Sun Chemical A/S, Københavnsvej 112, DK-4600 Køge, Denmark

d

Nano-Science Center, Department of Chemistry, Universitetsparken 5, DK-2100 Copenhagen Ø,

Denmark

ACS Paragon Plus Environment

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e

Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej

55, 5230 Odense M, Denmark Correspondence should be addressed to [email protected] Running title: Electronic structure of oligo-phenylene vinylene molecules.

Abstract A series of oligo-phenylene vinylenes are investigated with spectroscopic measurements and calculations with the aim to rationalize the correlation between the electronic structure of the molecules and their efficiency as two-photon singlet oxygen sensitizers. The band-shape functions of selected two-photon absorption processes are analyzed and the corresponding spectra are deconvoluted. This analysis allows for a comparison between calculated and measured twophoton absorption cross sections where a reasonable agreement is found.

Introduction Predictions of two-photon absorption (TPA) properties and in particular the TPA cross section of molecules have recently received significant interest in materials applications such as imaging and optical limiters,1-5 3D-microfabrication,6-8 lasing materials9-11 and two-photon sensitized production of singlet oxygen.12-19 Seminal work within the area of two-photon absorption processes originates from Göppert-Meyer20 who not only defined the TPA cross-section, but also derived selection rules for electronic transitions in a TPA process: Molecules possessing centrosymmetric symmetry obey selection rules for which excited electronic states can be populated. In a one-photon absorption process only electronic states with opposite parity to the initial state can


2

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be populated. This is in contrast to the TPA process where only electronic states of the same parity as the initial state can be populated. Experimentally observed TPA cross-sections can be rationalized according to the expression by Göppert-Meyer,20

2 e 4 2  2 2 2  c 0 h

 i

 f  i

i   g

i  

2

g  2 Eq.(1)

where e is the charge of the electron, c is the speed of light, ε0 is the vacuum permittivity, h is Planck’s constant, ω is the energy of the incident photons, g(2ω) is the line width function of the final state,  is the dipole operator,  f is the final electronic state, Y i is an intermediate virtual state, g is the ground state, and ωi is the energy separation between the ground state and the intermediate states. The TPA cross section has the unit of cm4·s but is often expressed in the Göppert-Meyer unit (GM): 1 GM = 10-50 cm4·s. It is important to emphasize that Eq. (1) is only valid when the two-photon excitation process occurs with two photons of the same wavelength. Otherwise, an additional term is needed that take into account the different wavelengths of the two photons.20 Another important aspect is the energies of the excited states which in a twophoton processes is the sum of energies of the two photons needed to populate the excited state. The implementation of the response methodology in for example the Dalton quantum chemistry program21,22 allows a direct calculation of the sum-over-states term in Eq. (1), but it is not possible to calculate the line width function using ab-initio methods. Often, the calculated TPA cross sections reported in the literature uses a fixed value for the band-shape function at the absorption maximum.23-24 One of the purposes of the present work is to quantify the band shape function for a variety of molecules in order to probe whether meaningful comparisons can be


3


made using a fixed value for the band-shape function at the absorption maximum for different molecules. TPA cross sections can be measured using a variety of experimental techniques such as Zscan,25 two-photon fluorescence26 and photoacoustic measurements.27,28 The two latter techniques use either fluorescence or an acoustic signal as the probe for the optical process. Another probe useful for measuring TPA cross sections is singlet oxygen formed in a cascade reaction upon relaxation of the excited state of the molecule studied (hereafter referred to as the singlet oxygen sensitizer), where energy is transferred between the singlet oxygen sensitizer and oxygen itself (Figure 1).

Sn Sm IC ISC

S1 virtual Fluorescence

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T1 Oxygenation and/or Oxidation of M

IC

O 2(a1Δ g) ISC Energy Transfer

S0

Two One Photon Photon

a-X emission and physical deactivation

M

O 2(X 3Σ g- )

Figure 1. Illustration of one- and two-photon triplet-state photosensitized production of singlet oxygen, O2(a1g). Depending on the sensitizer, the simultaneous absorption of two photons may or may not populate the same state as that created upon the absorption of a single higher-energy photon. ISC denotes intersystem crossing and IC denotes internal conversion.

The label “a-X emission” refers to the 1270 nm

phosphorescence of singlet oxygen. M denotes the singlet oxygen sensitizer.

Obtaining high TPA cross sections on its own has been the focus of much work and has led to molecular design rules, briefly summarized in the following. For example, narrowing the


4

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excitation bandwidth of the line width function, g(2ω) in Eq. (1), will result in an increase of the line width function at its maximum value since this function should be normalized. Decreasing the width of the line width function is done by reducing the density of states function for the final electronic state, i.e. reducing the number of vibrational states in the vicinity of the excited state. Also, by increasing the conjugation length of a π-conjugated system an increase in the density of states for all the electronic states is obtained.29 Another design rule focusses on having a onephoton transition close to the two-photon laser frequency. This will create a singularity in Eq. (1) as a i- difference close to zero would be obtained. One way to achieve this is to have a large conjugation length. On the other hand this could impose solubility problems. The most common approach in the molecular design of molecules possessing large TPA cross sections is to introduce chemical functional groups in the molecules that give rise to significant charge relocalization upon excitation, i.e. significant changes in ground-to-excited state dipole moments.3033

A variety of molecular templates have been used along this line including distyrylbenzenes and

naphthalene derivatives.30-37 Most of these compounds are centrosymmetric and are designed for use as fluorescence probes. Significant changes of the dipole moment, however, is not always necessary to obtain molecules with large TPA cross sections. Polyfluorenes, for example, exhibit a large cross section of approximately 70,000 GM. This large value has been suggested to be the result of the high molecular weight and therefore larger conjugation compared to low molecular weight molecules.38 Similar findings have been obtained for certain dendrimers.39 Optimization of both a large TPA cross section and a large singlet oxygen quantum yield has been the focus of some recent works.12-19 The present work aims at quantifying the electronic properties that will lead to high TPA cross sections and also a high singlet oxygen quantum yield.


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These studies also include the particulars of the band-shape function for the two-photon transitions. Experimental and computational details Voltammetry. In the cyclic voltammetric experiments a three electrode setup was employed with a glassy carbon (o.d.=1 mm) serving as working electrode. The reference electrode applied was Ag/AgI ( C I- =0.1 M) but all the potentials were also referenced against the ferrocenium/ferrocene (Fc+/Fc) redox couple. A platinum coil was used as counter electrode. The ohmic drop was compensated with a positive feedback system incorporated in the home-built potentiostat. Fluorescence. Solutions were degassed for 15 min with argon prior to use. Data were recorded using an instrument comprised of a 450 W Xe lamp for steady-state measurements and a diode laser for lifetime measurements. The detection system was a single-photon-counting photomultiplier tube in a peltier-cooled housing. All spectra where measured in a perpendicular geometry using 1-cm quartz cuvettes. Steady-state measurements were obtained with 1.8 nm band pass filters and corrected for wavelength dependent intensity variation of the excitation light source. Quantum yields were determined using 9,10-diphenylanthracene in cyclohexane40 (Φf =1.00) as the fluorescence standard with refractive index and differential absorption corrections. In all cases, time-resolved fluorescence decay traces were single exponential and lifetimes were determined using least-squares analysis. All time-resolved measurements were conducted with 7.3 nm band pass filters. Singlet oxygen quantum yields. Singlet oxygen quantum yields, Φ, were determined using instruments and an approach that has been described previously.35,41 For these measurements, the


6

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samples were irradiated at 355 nm (third harmonic of a nanosecond Nd:YAG pulsed laser). The standard used for experiments performed in toluene was phenalenone (Φ=1.00±0.05).42

Crystallographic structure determinations. Crystals suitable for structure determination by single crystal X-ray diffraction were obtained of compounds 10, 11 and 17. The diffraction data were collected using a Nonius KappaCCD employing Mo Kα radiation (λ = 0.71073 Å) on crystals cooled to 122(1) K. The scan parameters were chosen following the recommendations in Ref. [43]. Data reductions were performed with EvalCCD44 for 10 and 11 and DENZO and SCALEPAK45 for 17. All reflections were corrected for background, Lorentz and polarization effects. Absorption correction was performed using the numerical Gaussian integration procedure46 for 17 only within the maXus suite.47 The structures were solved by Direct methods using SIR9748 for 10 and SHELXS9749 for 11 and 17. The structures were refined by full matrix leastsquares on |F|2 values against all reflections with SHELXL97.49 The non-hydrogen atoms were refined with anisotropic displacement parameters, except for the atoms in disorded solvent molecules found in 10 and 11. All hydrogen atoms were introduced using a riding model with idealised geometry. In the crystal structure 10 solvent molecule methylphenylether was cocrystallizated. The solvent molecule was refined as a disordered entity with two molecular positions. The occupancy of the two positions were almost equal, 0.486/0.514(4). Restraints were used for the anisotropic displacement parameters for the carbon atoms in the phenyl group such that the displacements are approximatel isotropic. Also compound 11 was cocrystallized with solvent molecules. The toluene molecule was found in a position over an inversion center, hence four molecules shared the position of which two were unique. The occupancy of the unique molecules were 0.179(5) and 0.321(5). Together with the inversion symmetry this makes 1 molecule in average on this position. The carbon atoms were refined with isotropic displacement


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parameters which within each molecule were restrained to have approximately the same displacement parameter. The detailed crystal data are presented below.

Crystal data for 10. C48H40N2O2, C7H8O, Mw = 784.95 g mol−1, crystal dimensions 0.49×0.30×0.11 mm3, triclinic, space group P-1, a = 10.069(4), b = 13.876(3), c = 16.500(6) Å, α = 109.803(19)◦, β = 94.46(2)◦, γ = 103.692(20)◦, V = 2075.6(12) Å3, Z = 2, ρcalc = 1.256 g cm−3, μ(Mo Kα) = 0.077 mm−1, 1.33 < θ < 25.00◦, of 53270 measured reflections, 7293 were independent (Rint =0.0622) and 5585 observed with I > 2σ(I); R1 = 0.0733, wR2 = 0.1255, GOF = 1.055 for 587 parameters, Δρmax/min = 0.451/−0.246.

Crystal data for 11. C48H36N2O2, C7H8, Mw = 764.92 g mol−1, crystal dimensions 0.52×0.18×0.10 mm3, triclinic, space group P-1, a = 8.5037(11), b = 9.8109(13), c = 13.1211(7) Å, α = 92.898(10)◦, β = 97.615(9)◦, γ = 107.974(9)◦, V = 1027.2(2) Å3, Z = 1, ρcalc = 1.237 g cm−3, μ(Mo Kα) = 0.074 mm−1, 1.57 < θ < 25.01◦, of 24859 measured reflections, 3618 were independent (Rint =0.0668) and 2812 observed with I > 2σ(I); R1 = 0.0706, wR2 = 0.1283, GOF = 1.008 for 266 parameters, Δρmax/min = 0.336/−0.251.

Crystal data for 17. C46H34N4O4, Mw = 706.77 g mol−1, crystal dimensions 0.60×0.25×0.16 mm3, orthorhombic, space group Pbca, a = 10.981(2), b = 16.0090(19), c = 19.9870(18) Å, V = 3513.6(9) Å3, Z = 4, ρcalc = 1.336 g cm−3, μ(Mo Kα) = 0.086 mm−1, 2.04 < θ < 32.08◦, of 85028 measured reflections, 6122 were independent (Rint =0.0779) and 4549 observed with I > 2σ(I); R1 = 0.0904, wR2 = 0.1686, GOF = 1.065 for 244 parameters, Δρmax/min = 0.646/−0.244.


8

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Computational details. Geometry optimizations for determining the molecular structures in the present work were performed at the B3LYP/6-31G(d) level using the Gaussian 03 suite of programs.50 Vibrational frequency calculations were carried out on the optimized structures in order to verify that the structures represent minima on the potential energy surface. These geometry optimizations and frequency calculations were performed for the molecules in isolation. The optimized structures were then used as input for response calculations at the CAM-B3LYP/631G(d) level (both in vacou and employing a solvent model) carried out with the DALTON program package.21 Results and discussion All the molecules discussed in the present work can be described with the general structure shown in Chart 1. 

Aryl



Aryl

Aryl





A





D







D

D



A



D

A



A



A

Chart 1: General structure of the sensitizers examined in the present work. The letters A and D refer to electron acceptors and donors, respectively, and illustrate methods by which substituents can be used to influence the electronic character of the chromophore. The π-moiety is either aryl (phenyl, toloyl), alkenyl, acrylonitrile or acrylaldehyde.

The molecules are all centrosymmetric and are built around a core aromatic ring with vinyl substituents in the 1- and 4-positions (phenyl) or the 2- and 6-positions (naphthalene). The enddonor groups are either amine, thio-methyl-ether or methoxy groups. The only end-acceptor


9


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group applied was a substituted imidazole. The acceptor groups on the center aryl ring are bromine , aldehydes, ketones, amides, nitriles, sulfones, fluorine or nitro groups. In continuation of previous works,12,13,15,29,51,52 the success criterion for a good TPA singlet oxygen sensitizer is defined as a large value of the product of the TPA cross section () and the singlet oxygen quantum yield (ΦΔ). Some of the molecular design features for obtaining large TPA cross sections are outlined above, with the presence of a high degree of intramolecular charge transfer (CT) in the molecule as a key feature. However, the extent of CT, both within the sensitizer itself as well as in the sensitizer oxygen complex (intermolecular CT) can have large adverse effects on the efficiency in which singlet oxygen is generated.53-58 In general, CT character in a sensitizer and/or sensitizer-oxygen complex facilitates non-radiative deactivation of the excited state at the expense of energy transfer from the sensitizer to produce singlet oxygen. It is assumed that the extent of intermolecular CT in the sensitizer-oxygen complex will be larger for a sensitizer with a proclivity for intramolecular CT (i.e. intermolecular CT to yield a Sens-O2 state with Sens+··O2- character will more readily occur for a sensitizer with an electron-donating moiety). One particular design principle known to promote higher singlet oxygen quantum yields is the heavy atom effect, which facilitates the efficiency of S1->T1 intersystem (see Figure 1) crossing due to the increasing amount of spin-orbit coupling in the molecule.59,60 In addition, the El-Sayed selection rule59,60 states that intersystem crossing from the S1 to the T1 state becomes allowed if there is a change in the orbital angular momentum to balance the change in spin angular momentum. This is possible when the electronic transition includes excitation from or to an n- or orbital to or from a -orbital. Functional groups where such transitions are likely to occur include carbonyl groups.59,60 Quantification of some of the key photophysical parameters have been carried out in previous work51 whereas the present work is focused on a quantum


10

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mechanical approach to interpret the measured singlet oxygen quantum yields and the TPA cross sections. 1. Changes in the central moiety of the Oligo-phenylene vinylene motif. In the oligophenylene vinylene (OPV) motif the central moiety was systematically changed in order to address the points made above. One-photon absorption properties and fluorescence, fluorescence quantum yields and lifetimes and singlet oxygen quantum yields are presented in Table 1. The E0,0-energies are obtained from the points where the corresponding absorption and emission profiles (normalized intensities) crosses and is thus interpreted as the S1-energies.

Table 1:

One-photon optical characterization (absorption, fluorescence and singlet oxygen

quantum yields measurements) of sensitizers with different aromatic cores substituted with diphenylamino-phenylenevinyl.a  abs max [nm] is the absorption maximum, log ε is the log of the extinction coefficient (ε),  em max [nm] is the emission maximum, Φf is the fluorescence quantum yield,  [ns] is the fluorescence lifetime and E0,0 [eV] is the energy of the first excited state. Ph2N Aryl NPh2

Compound

F

2

 [ns]

 em max [nm]

428/4.90 304/4.61

486 517

0.8

1.1

409/4.90 300/4.62

459 481

0.8

1.0

ΦΔ

E0,0 [eV]

TPA [GM]

0.09

N.D.

1970

0.08

N.D.

2030

F

1 F

Φf

 abs max [nm]/log ε

F


11


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Br

0.2

0.4

0.13

N.D.

1410

452 479

0.7

1.0

0.06

N.D.

N.D.

375/4.75 300/4.63

426

0.5

1.6

0.05

N.D.

N.D.

382/4.92 302/4.77

525

0.7

3.3

N.D.

N.D.

N.D.

7

425/2.92 303/4.09

492 524

0.3

0.7

0.46

2.63

1310

8

410/3.02 306/4.05

458 586

0.9

1.1

0.08

2.79

1150

9

472/2.67 304/4.08

531

0.87

1.5

0.13

2.45

2815

10

428/2.90 306/4.05

484 513

0.8

1.1

0.11

2.64

1350

11

480/2.58 394/3.15 347/3.57 303/4.09

601

0.4

2.6

0.15

2.25

2280

12

429/2.89 305/4.07

516

0.7

1.6

0.17

2.57

1970

3

423/4.83 306/4.63

480 507

4

399/4.98 303/4.61

5

Br

F

C8F17

6 C8F17

F


12

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13

416/2.98 302/4.11

475 504

0.8

1.1

0.09

2.70

1460

14

431/2.88 302/4.11

556

0.05

0.2

0.02

2.51

1800

15

419/2.96 303/4.09

481 511

0.1

0.4

0.04

2.68

1040

16

454/2.73 306/4.05

534

0.6

1.5

0.06

2.46

4110

-

0.0

-

0.00

-

-

17

a

Except for 4,5 and 6 all data for

419/2.96 303/4.09

em  abs max [nm]/log ε,  max [nm], Φf, ΦΔ and TPA [GM] are taken from Ref. [51]

As discussed above, the product of the TPA cross section and the singlet oxygen quantum yield truly classifies the efficiency of the two-photon singlet oxygen sensitizer. In order to study this property as a function of intramolecular CT a suitable measure of CT needs to be defined. In previous work describing the TPA properties of 7-16 the 1H-chemical shift of the protons on the central ring in the OPV motif was used as such a measure.51 Indeed a correlation between the CT character of the molecule and the TPA cross section was seen and is reproduced in Figure 2 along the with the curve describing the dependence of · ΦΔ on the intramolecular CT. The OPV analog substituted with a NO2-group (17) is not included, as it does not produce singlet oxygen in a measureable amount.


13


a

b

4500

500

4000 3500

Br

400

3000 300

* 

2500

GM]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2000

200

1500 100

1000

COMe

500 0 7.2

0 7.4

7.6

7.8

8.0

8.2

8.4

8.6

7.2

7.4

7.6

7.8

8.0

8.2

8.4

8.6

Chemical shift [ppm]

Figure 2: (a) TPA cross section as a function of the intramolecular CT for 7-16 (reproduced from Ref. [51]). (b) Dependence between the intramolecular CT and · ΦΔ. The analogs 7 (Br) and 14 (COMe) seem to be outliers. The solid lines are added to guide the eye.

The molecules 2-6 do not fit directly into the series 7-17 but they do give some insight into the photophysical processes taking place in the OPV motif. For example comparing 2 and 3 one would expect that 3 would have a much higher quantum yield than 2 (see for example 7 and 8). However, the quantum yields are not significantly different. Though there is a decade in difference between the non-radiative rate constants, which is the sum of the rate constants for internal conversion (kic) and intersystem crossing (kisc): For 2 the non-radiative rate constant is ~1.3·109 s-1, whereas for 3 the rate constant is 1.3·1010 s-1. It has previously been demonstrated that the energy level of the triplet state can otherwise lower the singlet oxygen quantum yield


14

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despite expectations of a higher quantum yield.51 This will be further elaborated with calculations below. The “configuration” of the “building blocks” i.e. the connectivity was probed on compound 4 and 5, where only a negligible difference in ΦΔ was observed despite differences in fluorescence quantum yields and lifetimes. The non-radiative decay lifetimes amounts to approximately the same value; ~1.4·109 s-1 for 4 and ~1.3·109 s-1 for 5. The bromine (7) and methyl-ketone (14) analogs do not seem to follow the trend of the other molecules in the series 7-16 as observed in Figure 2: The bromine analog has a · ΦΔ value larger than what would be expected from the amount of intramolecular CT present in the molecule, whereas the opposite is seen for the methyl-ketone. However, 7 cannot be directly compared to the rest of the series due to the heavy atom effect, which increases the rate for intersystem crossing because of the mass of the bromine atoms. It is particularly interesting that 14 has a low singlet oxygen quantum yield (0.02) compared to the other molecules. Likewise, the tert-butylketone has a similar low quantum yield (0.04). It can be speculated that the methyl-ketone analog has considerably lower two-photon singlet oxygen production efficiency than expected because of the possible presence of a keto-enol tautomer reaction. Such a reaction can act as an energy sink. Thus a populated Sn state can isomerize to an enol-tautomer with a lower intersystem crossing efficiency than the keto-tautomer. The oxygen lone pair orbitals in the enol tautomer will no longer be in plane. This will most like result in a decreased orbital overlap and eventually cause a lower n->π* transition matrix element. The bromine analog on the other hand has higher twophoton singlet oxygen production efficiency than expected. This can possibly be ascribed to the heavy atom effect. Electronic structure calculations. In Ref. [51] the singlet oxygen quantum yield of 7 and an analog, which includes two carbonyl groups, showed an unexpected order. Additional carbonyl


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groups did not give an expected higher singlet quantum yield. Instead the singlet oxygen quantum yield was lowered. This finding was ascribed to the differences in triplet energy levels of the two compounds. In order to pursue the triplet energy levels as an explanation for the differences in singlet oxygen quantum yields, singlet and triplet excitation energies were calculated for 7-17 along with the corresponding (singlet) oscillator strengths (f) (CAM-B3LYP/6-31G(d) level). This exchange-correlation functional is especially promising for transitions showing a large degree of charge-transfer; see the discussion below. Furthermore, the basis set 6-31G(d) represents a reasonable balance between computational cost and accuracy. The results of these calculations are presented in Table 2 which refers to calculations performed in vacuum The calculations were validated by also calculating the T1-energy for the analog of 7 containing two carbonyl groups51 obtaining a value of 2.00 eV, which is to be compared to the experimentally determined value of 1.66 eV. For 7 a calculated T1-energy is found to be 1.63 eV and the experimentally determined value is 1.53 eV. Even though there is a difference of 0.34 eV and 0.10 eV, respectively, for the calculated and experimentally determined values the order of the T1-leves are reproduced.


16

Page 17 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Calculated singlet and triplet excitation energies for compounds 7-17 at the CAMB3LYP/6-31G(d) level. Excitation energies (and differences) are presented in eV. f denotes onephoton oscillator strengths. Excited state

7

8

9

10

11

12

13

14

15

16

17

Br

H

CN

OMe

CHO

CO2Et

CONEt2

COMe

COtBu

SO2Me

NO2

Singlet states (E0,0÷1-1Au)

0.50

0.42

0.48

0.42

0.74

0.48

0.58

0.45

0.59

0.63

-

1-1Au

3.13

3.21

2.93

3.06

2.99

3.05

3.28

2.96

3.27

3.09

2.66

1-1Ag

3.73

3.87

3.54

3.84

3.56

3.68

3.83

3.55

3.80

3.62

3.21

2-1Au

4.34

4.39

3.68

4.07

3.58

3.82

4.40

3.66

3.94

4.15

3.34

2-1Ag

4.40

4.39

4.30

4.38

3.66

4.35

4.41

3.78

4.01

4.41

3.80

3-1Au

4.40

4.50

4.36

3.38

3.67

4.40

4.42

3.72

4.06

4.42

3.78

3-1Ag

4.57

4.54

4.42

4.52

4.10

4.42

4.54

4.21

4.41

4.50

3.84

f(1-1Au)

2.77

2.88

2.62

2.61

1.64

2.21

2.68

1.81

2.60

2.57

1.22

f(2-1Au)

0.01

0.05

0.17

0.17

0.99

0.46

0.04

0.17

0.05

0.08

1.46

f(3-1Au)

0.08

0.01

0.04

0.04

0.09

0.05

0.01

0.65

0.04

0.05

0.02

Triplet states 1-3Au

1.63

1.64

1.51

1.54

1.73

1.67

1.80

1.67

1.81

1.76

1.61

1-3Ag

2.26

2.30

2.21

2.29

2.26

2.26

2.32

2.25

2.30

2.26

2.19

2-3Au

3.03

3.07

2.89

2.99

2.75

2.94

3.05

2.83

3.01

3.01

2.47

2-3Ag

3.16

3.14

3.18

3.13

3.05

3.14

3.14

3.15

3.15

3.17

2.47

3-3Au

3.16

3.14

3.10

3.13

2.94

3.17

3.14

3.10

3.34

3.17

2.60

3-3Ag

3.35

3.36

3.34

3.36

3.35

3.35

3.36

3.22

3.36

3.36

3.18

S1÷T1)

1.50

1.57

1.42

1.52

1.26

1.38

1.48

1.29

1.46

1.33

1.05

S1÷T2)

0.87

0.91

0.72

0.77

0.73

0.79

0.96

0.71

0.97

0.83

0.47

S1÷T3)

0.10

0.14

0.04

0.07

0.24

0.11

0.34

0.13

0.26

0.08

0.19


17


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Page 18 of 37

The calculated 1-1Au energies are compared to the experimentally determined E0,0-energies, differences between the two energies are from 0.42 to 0.74 eV. These differences are relatively high and even though the results are quite scattered, there is a tendency for molecules with a large amount of intramolecular CT (e.g. 11 and 16) to have a higher energy difference, than for compounds like 8 and 10, where less intramolecular CT is expected. Several studies where the CAM-B3LYP functional is used have shown a consistent underestimation of excitation energies.61 However, there is a correlation between the E0,0 energies and the excitation energies calculated with the CAM-B3LYP functional in the present study. Previously, a good correlation between measured and calculated optical properties for large aromatic systems has been achieved using the CAM-B3LYP functional29 which is the main motivation for using this level of theory. As described above the amount of CT character in the sensitizer-oxygen complex can have an adverse effect of generating singlet oxygen. The extent of CT in the sensitizer-oxygen complex can be evaluated by the Rehm-Weller equation,62 ECT = F(EMOX ÷ EO2red) + C, where ECT (in eV) is the amount of CT, F is Faraday’s constant, EMOX (in V) is the oxidation potential for the sensitizer (see Table 3), EO2red (in V) is the reduction potential of oxygen and C is a constant that depends on the electrostatic interaction energy which is inversely proportional to the static relative permittivity εr of the solvent and on the differences in solvation energies of the separate ions and the ion pairs. It is assumed that there is little variation of C for the ECT-energies between the sensitizer-oxygen complexes for the sensitizers listed in Table 1. In Figure 3 the singlet oxygen quantum yields are plotted as a function of the oxidation potential of the sensitizer.


18

Page 19 of 37

0.5 Br

0.4 0.3 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2 OMe

0.1 0.0 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 Ox

red

F(EM -EO ) [eV] 2

Figure 3: Singlet oxygen quantum yields as a function of the difference in the oxidation potential for the OPV sensitizer and reduction potentials for oxygen.

The bromine and methoxy analogs are outliers in this series. If these two points are excluded from the analysis, Figure 3 reveals that there seems to be an optimum value for the amount of CT in the sensitizer-oxygen complex. The justification for excluding the methoxy-analog is that the methoxy group is an electron donating group whereas all the other analogs contain electron accepting groups. Furthermore, the bromine analog can be excluded since the mechanism for forming singlet oxygen is a spin-orbit coupling mechanism, which is different from the remaining sensitizers. Thus, by excluding these two compounds we find that there exists an optimum amount of intramolecular CT needed for achieving the highest value of· ΦΔ.


19


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Page 20 of 37

Figure 4: Crystal structures of 11 (CHO) (left), 17 (NO2) (middle) and 10 (OMe) determined by X-ray diffraction.

The difference between the methoxy analog and the other compounds is clearly confirmed by crystal structures of 10, 11 and 17. In the methoxy analogue, 10, the configuration around the N atom suggests that the lone pair of the N atom cannot be conjugated with the distilbene moiety. This is different from both the nitro, 11, and aldehyde, 17, analogues where the configuration around the N atom enables the lone pairs to be in conjugation with the distilbene moiety.

Table 3: Cyclic voltammetry data (in V) for the OPV’s (oxidations carried out in CH2Cl2 and reductions in DMF). All potentials are referenced against the Fc/Fc+·-couple. Compound

Reduction [V]

Oxidation [V]

1 (F4)

-2.318

-1.943

0.472

7 (Br)

-2.122

-1.964

0.472

8 (H)

-2.577

-2.333

0.369

9 (CN)

-2.211

-1.715

0.498

10 (OMe)

-2.499

-2.247

0.285

11 (CHO)

-2.073

-1.598

0.447

12 (CO2Et)

-2.340

-1.942

0.406

13 (CONEt2)

-2.466

-2.164

0.402

14 (COMe)

-2.241

-1.863

0.565


20

Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15 (COtBu)

0.327

16 (SO2Me)

-2.216

-1.765

0.524

17 (NO2)

-1.304

-1.096

0.532

TPA transition probabilities were calculated in different solvents using the response formalism coupled to the polarizable continuum model (PCM) as described in Ref. [63]. Comprehensive descriptions of the relation between calculated and experimentally determined TPA cross sections have been given in the literature.64 In short, the experimental TPA cross section, as discussed above, can be interpreted through the equation,

   

2 2 2 2    g 2    , 15

where  is the fine structure constant, and    is a sum of transition moments available through response calculations (the sum-over-states expression in Eq. 1). The calculated transition moments and excitation energies at the CAM-B3LYP/6-31G(d) level of theory in various solvents for 7-17 are presented in Table 4 for the 1Ag state.


21


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Page 22 of 37

Table 4: Excitation energies and transition moments calculated at the CAM-B3LYP/6-31G(d) level for the 1Ag state in various solvents (cyclohexane, toluene, THF, PhCN and DMSO). To be consistent with the notation above, the excitation energies are described as twice the energy needed to populate the state in a two-photon process.

   [10-31 cm4/eV2]

2·ω[eV] vac

C6H12

PhCH3

THF

PhCN

DMSO

Vac

C6H12

PhCH3

THF

PhCN

DMSO

7 (Br)

3.73

3.61

3.60

3.60

3.61

3.59

3.01

3.89

3.97

4.00

3.94

4.09

8 (H)

3.87

3.76

3.74

3.76

3.77

3.71

1.83

2.18

2.21

2.23

2.21

2.27

9 (CN)

3.54

3.42

3.40

3.42

3.43

3.41

5.73

7.93

8.18

8.04

7.77

8.14

10 (OMe)

3.84

3.73

3.71

3.72

3.72

3.70

1.93

2.40

2.45

2.61

2.66

2.71

11 (CHO)

3.56

3.45

3.44

3.43

3.43

3.42

2.55

4.52

4.70

4.80

4.71

4.95

12 (CO2Et)

3.67

3.56

3.54

3.53

3.53

3.51

3.30

4.43

4.57

4.72

4.70

4.92

13 (CONEt2)

3.83

3.72

3.71

3.70

3.70

3.68

1.76

2.22

2.31

2.46

2.45

2.56

14 (COMe)

3.55

3.45

3.44

3.43

3.44

3.41

3.74

5.10

5.26

5.28

5.19

5.44

15 (COtBu)

3.80

3.69

3.68

3.67

3.67

3.65

1.81

2.43

2.51

2.65

2.66

2.77

16 (SO2Me)

3.62

3.50

3.48

3.48

3.49

3.47

3.69

5.16

5.34

5.42

5.26

5.53

17 (NO2)

3.21

3.08

3.06

3.03

3.03

3.01

5.30

7.91

8.23

7.82

7.42

8.02

It is particularly noteworthy that there is hardly any change in excitation energies when going from one solvent to another. This finding is confirmed experimentally in Ref. [65], where UVVIS spectra of 7 (Br), 9 (CN) and 10 (OMe) were recorded in toluene, cyclohexane, THF, acetonitrile and benzonitrile. For example, the experimentally determined energy difference between the S0 and S1 states for 9 (CN) are 2.45 eV (toluene), 2.52 eV (cyclohexane), 2.42 eV (THF) and 2.34 eV (benzonitrile) as found from the crossing of normalized absorption and fluorescence spectra. Absorption maxima are observed at 2.63 eV (toluene), 2.70 eV


22

Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(cyclohexane), 2.68 eV (THF) and 2.63 eV (benzonitrile) showing an even less pronounced solvent effect. Only a few studies of the influence of the solvent on the TPA properties23,65-67 have been carried out, in which solvent effects are discussed. Usually, solvent effects are identified in correlations between the molecular property in question and the dielectric constant of the solvents or various reaction field parameters. However, no apparent correlation is observed between the solvent type (polarity) and TPA transition moments against either the optical (εo), static dielectric constants or reaction field parameters. An observation previously made by inspection of solvent effects in calculated transition moments is the apparent grouping of the values in two categories; one category for solvents with high static dielectric constants and one for solvents with low static dielectric constants (see Ref. 68 for further discussion). In Figure 5, the term  2   in Eq. (1) calculated for 11 (CHO) is plotted as a function of the optical dielectric constant for each of the solvents considered in the present work. As described above, the energy ω is half the energy of the excited state in a two-photon process.


23


1.44 DMSO

-30

4

cm ]

1.42

2

  [10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

THF

1.40 1.38

Toluene Benzoenitrile

1.36

Cyclohexane

1.34 1.8

1.9

2.0

2.1

2.2

0

Figure 5: Dependence of  2   (calculated for 11 (CHO)) on the optical dielectric constant.

When trying to elucidate solvent effects in experimental data, it is likely that different band shape functions complicate the comparison of TPA cross sections in different solvents. It is outlined in the following how to deconvolve  2   from experimentally determined TPA spectra and these values are then compared to calculated ab initio results.

Well determined TPA cross sections exist for 7 (Br) and 9 (CN) over a broad spectral range as shown in Figure 6 (the values for the TPA cross sections as a function of wavelength were taken from Ref. 28 and 51).


24

Page 25 of 37

a

b

3000 2500 2000

 [GM]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1500 1000 500 0

1.9

2.0

2.1

2.2

1.3

1.5

1.7

1.8

2.0

 [eV]

Figure 6: TPA spectra in frequency space of 7 (Br) (a) and 9 (CN) (b). Band shape functions consisting of a sum of Gaussian functions (two for 7 (Br) and three for 9 (CN)) fitted to the measurements are shown as solid lines. Dotted lines indicate the regions in which Gaussian functions were fitted.

Band shape functions consisting of a sum of Gaussian functions (two for 7 (Br) and three for 9 (CN)) were fitted to the experimental cross section (Figure 6). These band shape functions were subsequently normalized.69,70 The same analysis was also made for the carbonyl analogue in Ref. 51 (Ac) and the data from this analysis is also presented below. In Table 5, the values of the band shape functions are presented at absorption maximum along with the values for  2   derived by Eq. (1).


25


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Page 26 of 37

Table 5: Experimentally and theoretically derived TPA cross sections. The solvent used in the PCM calculations is toluene. Experiment g(2  ) [s-1]

 /eV  ( )

Theory

[cm4·eV-2]

 2  

[cm4]



[eV]

 ( )

[cm4·eV-2]

 2  

[cm4]

7 (Br) 3.47·10-14

2.05

1.28·10-30

5.38·10-30

1.80

0.40·10-30

1.29·10-30

9 (CN) 4.47·10-14

1.47

4.18·10-30

8.99·10-30

1.70

0.82·10-30

2.36·10-30

Ac1

1.47

2.09·10-30

4.50·10-30

1.77

0.10·10-30

0.31·10-30

1

5.52·10-14

Calculations do not include a PCM model. Data from Ref. 51 were used to derive the parameters

presented.

The calculated excitation energies, ω, in Table 5 differ by 0.25 eV (12 %) and 0.30 eV (20 %) from the experimental values. The deviation in the  ( ) values are somewhat larger, however, the relative order is predicted correctly and given the theoretical level in terms of solvent model, the deviation is still impressively small. Many theoretical rationalizations of two-photon properties employs a fixed value for the band shape function (e.g. 2.77·10-14 s-1), which is not completely off when comparing with the values in Table 5. However, there is a significant difference between the two numbers presented and more measurements should be conducted to supplement these values. 2. Changes in the end-groups of the OPV motif. In the OPV motif the central moiety was systematically changed in order to evaluate the effect of the electron-donating effect of the endgroups on the singlet oxygen quantum yields. The singlet oxygen quantum yields along with photophysical parameters are presented in Table 6.


26

Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 6:

One-photon optical characterization (absorption, fluorescence and singlet oxygen

quantum yields measurements) of sensitizers with different end-groups on the vinyl-moieties of a 1,4-dibromo-2,5-divinylbenzene core. Br end-group end-group Br

End-group

Φf

 [ns]

0.3

0.7

0.46

0.5

0.6

N. D.

450 477

0.2

0.4

0.26

356/4.69

404 428

0.1

0.3

0.16

362/4.76

412 437

0.2

0.3

0.23

373/4.68

427 453

0.2

0.3

0.23

 abs max [nm]/log ε

 em max [nm]

425/4.80

492

303/4.60

524

N

397/4.50

475

N

304/4.13

504

383/4.67 343/4.50 330/4.43 293/4.58

20

21

7

Ph2N

18

19

22

N

MeO


ΦΔ

27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

23

416/4.72

Me2N

Page 28 of 37

490 519

0.2

0.4

0.39

MeO

24

0.23

MeO MeO

25

O

398/4.60

N

486

0.2

0.5

The most striking result presented in Table 6 is that the strongest electron donor (diphenylamino, 7) also results in the highest singlet oxygen quantum yield. In particular, when comparing the singlet oxygen quantum yields, ΦΔ, for 23 (0.39), 19 (0.26) and 7 (0.46) it becomes apparent that the electron-donating properties of the amino-functional group are beneficial for achieving a high quantum yield. The reason for this is that the lone-pairs on the N atoms are, to a larger extent, delocalized in the pendent fused phenyl ring of the carbazole as opposed to the delocalization into the less conformational restrained phenyl groups of the diphenylamino group. Finally, no delocalization of the N lone-pairs other than into the distyrylbenzene skeleton is possible for 23. It also appears that more energy is lost to vibrational relaxation in for example 20, 21 and 22 in comparison to 23 and 7.

Conclusion A series of phenylene vinylenes have been characterized with quantum mechanical calculations, electrochemical measurements and their ability to produce singlet oxygen in a two-photon absorption scheme. It has been shown that even though charge transfer will result in higher two-


28

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photon absorption cross sections it is at the expense of the singlet oxygen quantum yield. Thus, it was demonstrated that in order to tune a molecule for optimum two-photon singlet oxygen sensitization specific charge transfer properties are essential. Quantum mechanical calculations at the DFT level provided insight into the band-shape function for two-photon processes and it was demonstrated that there is a reasonable agreement between experimental and calculated numbers. However, the maximum value for the band-shape function varied by a factor of two among the three molecules investigated. This value is essential for calculation of two-photon absorption cross sections, thus computational studies that do not take this value into account (i.e. uses the same value for a series of molecules) are not very accurate.

Acknowledgement. The authors would like to than Mikkel Jørgensen, Frederik C. Krebs and Peter R. Ogilby for valuable discussions.

Supporting Information Available: Synthetic procedures and characterization of compounds not described previously in Ref. [51]. Crystallographic cif files of compound 10, 11 and 17. This material is available free of charge via the Internet at http://pubs.acs.org.


29


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269x208mm (300 x 300 DPI)


Comparison between Theoretically and Experimentally Determined

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