In the format provided by the authors and unedited.

1

Supplementary Information for

Direct observation of sequential oxidations of a titania-bound molecular proxy catalyst generated through illumination of molecular sensitizers

Hsiang-Yun Chena and Shane Ardoa,b,*

aDepartment of Chemistry, University of California Irvine, Irvine, CA 92697 USA; bDepartment of

Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA 92697 USA

Table of Contents:

Synthesis of dyes and preparation of TiO2 paste ........................................................................................ 2

Spectroscopic and electrochemical data ..................................................................................................... 6

Molar absorptivity spectra on TiO2 ............................................................................................................. 7

Stark effect spectra ...................................................................................................................................... 8

Calculation of the ratio of RuII:RC-11 in (RuII + RC-11)/TiO2 thin films.................................................. 9

Molar absorptivity spectra of TiO2(e–) and oxidized molecules anchored to TiO2 thin films .................. 10

Additional transient absorption procedures, data, spectral modeling, and fits to kinetic models ............. 14

Monte Carlo simulations of transient absorption data .............................................................................. 18

Additional steady-state spectra of (RuII + RC-11)/TiO2 under continuous-wave illumination ................ 20

References ................................................................................................................................................. 21

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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCHEM.2892

NATURE CHEMISTRY | www.nature.com/naturechemistry 1

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Synthesis of dyes and preparation of TiO2 paste

All chemicals were reagent grade or better, unless otherwise specified, and were used without

further purification. Methyltriphosphonium bromide, potassium tert-butoxide, n-butyllithium, 3,5-

dibromobenzaldehyde, tetra-n-butylammonium bromide, and lithium perchlorate (battery grade) were

purchased from Sigma-Aldrich. Cyanoacetic acid, 4,4'-dimethyl-2,2'-bipyridine (dmb), 4,4’-dicarboxylic

acid-2,2’-bipyridine (dcb), hexafluorophosphoric acid (ca. 60% w/w aqueous solution) and lithium

chloride were purchased from Alfa Aesar. 4-(N,N-diphenylamino)benzaldehyde and tetra-n-

butylammonium hexafluorophosphate were purchased from TCI America. Palladium acetate was

purchased from Strem. Tetrahydrofuran (THF), dimethylformamide, acetonitrile, and chloroform used for

synthesis were dried by passing through a column of activated alumina, unless otherwise noted. All other

solvents were used as received without further purification.

Ru(dmb)2Cl2.1–3 In a 25 mL round bottom flask, a mixture of RuCl2(DMSO)4 (242 mg, 0.500 mmol), dmb

(184 mg, 1.00 mmol), lithium chloride (72 mg, 1.7 mmol), dimethylformamide (3 mL, used as received)

was sparged with nitrogen. Under a blanket of nitrogen, the mixture was refluxed overnight while being

stirred. After cooling to room temperature, 20 mL of acetone was added and the mixture was stored in a

freezer at -18 °C. A dark green precipitate was collected via centrifugation. The precipitate was rinsed with

water and centrifuged again. The precipitate was further rinsed with diethyl ether. The two supernatants

were combined and extracted with water and dichloromethane. The dichloromethane layer (purple) was

collected and combined with the precipitate and evacuated to dryness. Yield: 183 mg (0.317 mmol, 63%).

UV–Vis absorption (acetonitrile) λmax 557, 374, 296 nm (absorbance ratio 1:1.1:5.6).

[Ru(dmb)2(dcb)](PF6)2 (RuII).2,4 To 6.3 mL methanol:water (40:60, v/v), cis-Ru(dmb)2Cl2 (144 mg, 0.27

mmol), dcb (61.1 mg, 0.25 mmol) and sodium bicarbonate (75 mg, 0.9 mmol) were introduced, mixed,

and freeze–pump–thawed three times. The solution was refluxed overnight under nitrogen with continuous

3

stirring, and then cooled to room temperature where 1 mL of aqueous 1.0 M HPF6 solution was added

dropwise under stirring. A dark red precipitate formed and was filtered, rinsed with aqueous 0.1 M HPF6

solution followed by diethyl ether, and evacuated to dryness. The product was further rinsed with water

and centrifuged to remove excess HPF6. The precipitate was evacuated to dryness. Yield: 0.163 g (0.163

mmol, 65%). 1H NMR (500.22 MHz, NaOH/D2O) δ 8.82 (d, J = 1.9 Hz, 2H), 8.32 (s, 2H), 8.31 (s, 2H),

7.89 (d, J = 5.8 Hz, 2H), 7.61 (dd, J = 5.8, 1.8 Hz, 2H), 7.57 (d, J = 5.8 Hz, 2H), 7.54 (d, J = 5.8 Hz, 2H),

7.18 (d, J = 5.9 Hz, 2H), 7.15 (d, J = 6.0 Hz, 2H), 2.50 (s, 6H), 2.48 (s, 6H). ESI-MS (TOF): m/z 859.09

([M – PF6-]+), calculated 859.12.

N,N-diphenyl-4-vinylaniline.5 In argon, potassium tert-butoxide (0.903 g, 8.03 mmol) dissolved in 3 mL

of THF was added to a THF solution (4 mL) of methyltriphosphonium bromide (2.60 g, 7.30 mmol) in an

ice bath. After stirring at room temperature for 100 min, 4-(N,N-diphenylamino)benzaldehyde (1.995 g,

7.30 mmol) was added and stirred overnight. Crude 1H-NMR showed that only 10% of the precursor

starting materials were reacted. The mixture was evacuated to dryness at 90 °C for 4 h and then transferred

to a nitrogen glove box. THF was added to the mixture and cooled to -30 °C. Under stirring, 1.6 M n-

butyllithium (3.50 mL, 5.6 mmol) was added. After 30 min, the mixture still showed unreacted

methyltriphosphonium bromide via 1H NMR. 1.6 M n-butyllithium (0.90 mL, 1.44 mmol) was added and

the mixture turned dark yellow. A THF solution of methyltriphosphonium bromide (1.80 g, 6.59 mmol,

pre-dried under vacuum at 90 °C for 4 h) and n-butyllithium (4.10 mL, 6.56 mmol) was added, and the

mixture turned dark green. After 9 h, the reaction was transferred out of the glove box, quenched with

water and extracted with dichloromethane three times. The organic layer was collected, dried on

magnesium sulfate, and filtered, and then the solvent was evaporated. The crude product dissolved in

dichloromethane was filtered through a silica plug using petroleum ether as eluent. The light yellow

portion that came out first was collected and again filtered through a silica plug. The dark yellow portion

4

that moved slowly was discarded. The solvent was evaporated. Yield: 0.75 g (2.77 mmol, 38%). 1H NMR

(299.82 MHz, CDCl3) 6.97 – 7.33 (m, 14 H), 6.66 (dd, J = 17.6, 10.9 Hz, 1H), 5.64 (dd, J = 17.6, 1.0 Hz,

1H), 5.15 (dd, J = 10.9, 1.0 Hz, 1H).

3,5-Bis(4-(diphenylamino)styryl)benzaldehyde.6 N,N-diphenyl-4-vinylaniline (0.700 g, 2.58 mmol),

3,5-dibromobenzaldehyde (0.306 g, 1.16 mmol), palladium acetate (15 mg, 0.067 mmol), potassium

carbonate (0.320 g, 2.32 mmol), tetra-n-butylammonium bromide (0.374 g, 1.16 mmol), and

dimethylformamide (5 mL) were mixed in a Schlenk flask and sparged with argon for 30 min under

stirring. Under a blanket of argon, the reaction was heated to 90 °C for 10 h. After cooling to room

temperature, dichloromethane (50 mL) was added, and the mixture was extracted with an aqueous solution

of HCl three times (1.0 M, 100 mL in total). The organic layer was collected and the solvent was

evaporated. This crude product was dissolved in a small amount of dichloromethane and hexanes was

added. The bottom layer of brown solution was discarded, and the top layer of cloudy light yellow solution

was collected and evacuated to induce precipitation. The cloudy light yellow supernatant contained the

product and was collected. The precipitate also contained the product and impurities and so was again

purified following the same procedures until the color of the precipitate changed from light brown to dark

brown. The supernatants were combined and dried under vacuum yielding a yellow solid. Yield: 0.514 g

(0.798 mmol, 62%). 1H NMR (299.82 MHz, acetone-d6): δ 10.09 (s, 1H), 8.11 (s, 1H), 7.99 (s, 2H), 7.58

(d, J = 8.6 Hz, 4H), 7.44 (d, J = 16.4 Hz, 2H), 7.37 – 7.24 (m, 10H), 7.06 – 7.14 (m, 12 H), 7.03 (d, J =

8.7 Hz, 4H).

3-(3,5-Bis((E)-4-(diphenylamino)styryl)phenyl)-2-cyanoacrylic acid (RC-11).6 To a Schlenk flask,

acetonitrile (6.5 mL), chloroform (6.5 mL) and anhydrous magnesium sulfate (1.05 g) were added and the

mixture was sparged with argon for 30 min. After addition of 3,5-bis(4-

(diphenylamino)styryl)benzaldehyde (0.333 g, 0.517 mmol), cyanoacetic acid (0.072 g, 0.941 mmol), and

5

piperidine (0.05 mL, 0.51 mmol), the mixture was heated to 80 °C for 12 h and then cooled to room

temperature. The reaction mixture was filtered and the solvent of the filtrate was evaporated. The crude

product was dissolved in dichloromethane and filtered through a silica plug using hexanes:ethyl acetate

(8:1, v/v) as eluent to flush out the byproducts. The product which remained on the silica plug was flushed

out with dichloromethane, methanol, and then dichloromethane:methanol (1:1, v/v). The solvent was

evaporated and the solid was rinsed with methanol and evacuated to dryness as a dark yellow solid. Yield:

0.300 g (0.422 mmol, 82%). 1H NMR (499.18 MHz, DMSO-d6) δ 7.96 (s, 1H), 7.90 (s, 2H), 7.87 (s, 1H),

7.53 (d, J = 8.6 Hz, 4H), 7.37 – 7.24 (m, 10H), 7.14 (d, J = 16.3 Hz, 2H), 7.10 – 7.01 (m, 12H), 6.97 (d, J

= 8.7 Hz, 4H). ESI-MS (TOF): m/z 711.21 ([M]+), calculated 711.29.

TiO2 paste.7 To 15.84 g of titanium isopropoxide, 3.08 mL of glacial acetic acid was added at once under

vigorous stirring. The mixture was continuously stirred for 15 min and was poured into 78.5 mL of

deionized water under vigorous stirring. After stirring for 1h, 1.08 mL of concentrated nitric acid was

added at once. The mixture was heated to 80 °C over 40 min and kept at 80 °C for 75 min. The solution

was allowed to cool and was then diluted to 78.4 mL with water. The mixture was heated in a Teflon-

lined hydrothermal reactor at 220 °C for 12 h. The cooled mixture was sonicated for 5 min using an

ultrasonic horn. Half of the mixture was concentrated to 15 mL followed by centrifugation to obtain the

precipitate. The precipitate was washed by dispersing in ethanol and centrifuging three times. The final

precipitate containing ethanol was directly used to prepare the paste. Stock solutions (A and B) of ethyl

celluloses (Fluka, 46670 and 46680, respectively) in ethanol (10% by weight) were prepared. 84% of the

TiO2 precipitate was mixed with the stock solutions A (4.5 g) and B (3.5 g), terpineol (6.49 g), and ethanol

(8.0 mL) and sonicated for 30 min. The mixture was further concentrated to 9 g using a rotary evaporator

to yield the paste. The paste was further diluted with terpineol for the preparation of the TiO2 thin films

with a smaller roughness factor.

6

Spectroscopic and electrochemical data

Supplementary Figure 1. (a) Molar absorptivity spectra (solid) and photoluminescence (PL) spectra

(dashed) of RuII in acetonitrile (blue, PL λex = 500 nm) and RC-11 in dichloromethane (red, PL λex = 410

nm). Results from single-mode Franck–Condon line-shape analyses (green) are shown for the PL spectra.

(b) Cyclic voltammograms measured at a scan rate of 50 mV/s for Ru/TiO2 and RC-11/TiO2 immersed

in acetonitrile containing 100 mM LiClO4.

Cyclic voltammograms of Ru/TiO2 and RC-11/TiO2 are shown in Supplementary Figure 1. The

oxidation process of RuII/TiO2 was quasi-reversible with Eo' = 1.20 V vs SCE, in good agreement with

the value of Eo' obtained using spectroelectrochemistry. For RC-11/TiO2, the two oxidation events for

RC-11 could not be resolved and a quasi-reversible wave with Eo' = 0.93 V vs SCE was observed. The

peak current of RC-11/TiO2 was about ten times larger than that of Ru/TiO2 due to RC-11 being present

at about two times higher coverage, requiring twice of the number of electrons transferred per molecule,

and exhibiting a larger apparent diffusion coefficient for self-exchange electron transfer.

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Supplementary Table 1. Spectroscopic and electrochemical data for Ru and RC-11.

Ru RC-11 [RC-11]+

Homogeneous: Solution phase a

Eo' (V) for oxidation by CV b 1.21 c 0.82 d 0.97 d

Absorption max (nm) / (104 M-1 cm-1) 287 / 6.3

476 / 1.6

299 / 4.4

376 / 4.7

Emission max (nm) 678 460

E0-0 (eV) e 1.9 2.7

Heterogeneous: Anchored to mesoporous TiO2 thin film

Eo' (V) for oxidation by SEC / CV b 1.22 / 1.20 c 0.78 / 0.93 c 0.89 / 0.93 c

Ideality factor, a 1.44 1.00 1.68

Absorption max (nm) 480

a RuII was dissolved in acetonitrile; RC-11 was dissolved in dichloromethane. b Potential vs a saturated calomel electrode (SCE, KCl saturated) measured using an SCE reference

electrode (KCl saturated), Pt mesh counter electrode, and Pt disk working electrode (for homogeneous

cyclic voltammetry (CV) measurements) and TiO2/FTO working electrode (for heterogeneous

spectroelectrochemistry (SEC) and CV measurements). c In argon-sparged 100 mM LiClO4 acetonitrile.

d In argon-sparged 100 mM tetra-n-butylammonium hexafluorophosphate dichloromethane, because of

the low solubility of LiClO4 in dichloromethane. e Calculated based on a single-mode Franck–Condon line-shape analysis with best-fit values for ℏω,

Huang–Rhys factor (Sm), and full-width-at-half-maximum vibrational mode peak (ṽ1/2) of 1240 cm-1,

0.83, and 1810 cm-1 for Ru and 1800 cm-1, 0.45, and 2620 cm-1 for RC-11.8–10

Molar absorptivity spectra on TiO2

Supplementary Figure 2. Molar absorptivity spectra of RuII/TiO2 (blue) and RC-11/TiO2 (red) immersed

in acetonitrile containing 100 mM LiClO4.

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Molar absorptivity spectra of molecules anchored to mesoporous TiO2 thin films (Supplementary

Figure 2) were obtained using the following equation:

𝜀(𝜆) =Abs(𝜆)

1000∙Γgeom (1)

where Abs(𝜆), 𝜀(𝜆), and Γgeom are an absorption spectrum of the molecules anchored to the TiO2 thin

film (after subtracting the background), the molar absorptivity spectrum (M-1 cm-1), and the amount of

molecules per projected geometric area (mol/cm2), respectively.11 The total amount of molecules in each

film (N) was determined by desorbing molecules from the film using a solution of ~0.01 mL of 55%

aqueous tetra-n-butylammonium hydroxide in ~3 mL of acetonitrile for RuII and ~3 mL of ethanol:THF

(1:1, w/w) for RC-11. Each solution containing the desorbed molecules was dried under vacuum and re-

dissolved in 3 mL of acetonitrile with ~0.05 mL of 10% aqueous HCl for RuII and neat dichloromethane

for RC-11. The total amount of molecules desorbed from each film (N') was determined using the Beer–

Lambert law and the solution absorption spectrum, the appropriate molar absorptivity spectrum

(Supplementary Figure 1), and the solution volume. The total amount of molecules initially in each film

(N) was calculated by dividing N' by the desorption yield taken as the ratio of the film absorption spectrum

before and after the desorption process. The total amount of molecules per projected geometric area in

each film, i.e. the projected surface coverage, (Γgeom) was calculated by dividing N by the projected area

of the film.

Stark effect spectra

Local electric fields perturb the absorption spectra of molecules, a phenomenon known as an

electro-absorption Stark effect. Stark effects have been observed at molecules anchored to TiO2 thin

films.2,12–15 Therefore, in order to fully account for observed features in the transient absorption spectra,

9

spectral shifts due to electro-absorption must be taken into consideration. The Stark effect spectra for

RuII/TiO2 and RC-11/TiO2 are shown in Supplementary Figure 3.

Supplementary Figure 3. Absorption spectra of RuII/TiO2 (blue) and RC-11/TiO2 (red) immersed in neat

acetonitrile before (solid) and after (dashed) addition of 100 mM LiClO4. Panel b displays the difference

Stark effect spectra for each, as solid minus dashed.

Calculation of the ratio of RuII:RC-11 in (RuII + RC-11)/TiO2 thin films

For TiO2 thin films that contained both RuII and RC-11 the amount of each molecule was estimated

by spectral modeling, i.e. fitting the absorption spectra (Supplementary Fig. 4) to a linear combination of

molar absorptivity spectra for each molecule anchored separately to a TiO2 film from 420 nm to 700 nm

(Supplementary Figure 2). Data at wavelengths < 420 nm were not included in the fits because the

absorbance values due to RC-11/TiO2 (Supplementary Fig. 2) were often large and imprecise. The

concentrations of molecules used in the dyeing solutions and the best-fit relative surface coverages on

TiO2 thin films are listed in Supplementary Table 2.

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Supplementary Table 2. Dye concentrations in 1.0 mL ethanol:THF (1:1, w/w) dyeing solutions and

calculated relative surface coverages on TiO2 thin films.

Concentration in dyeing

solution (mM)

Fraction of dyes anchored to

TiO2 thin films (from fit)

RuII RC-11 RuII RC-11

1.9 0.1 0.99 0.01

1.8 0.2 0.97 0.03

1.6 0.4 0.87 0.13

1.0 1.0 0.72 0.28

0.1 1.9 0.03 0.97

Supplementary Figure 4. (a) Absorption spectrum of (RuII + RC-11)/TiO2 immersed in neat acetonitrile

and the least-squares best-fit spectrum using a linear combination of the absorption spectra of Ru/TiO2

and RC-11/TiO2. (b) The residuals of the best-fit spectrum from panel a. The films were dyed in a 1.0 mL

ethanol:THF (1:1, w/w) solution consisting of 1.6 mM RuII and 0.4 mM RC-11.

Molar absorptivity spectra of TiO2(e–) and oxidized molecules anchored to TiO2 thin films

The Δ𝜀(𝜆) spectrum of electrons in a TiO2 thin film (TiO2(e–)) was obtained by dividing the

absorption spectral change by the injected charge density. The potential of the TiO2/FTO working

electrode was stepped in increments of -100 mV from the open-circuit potential and held at each potential

until the absorbance differences between two subsequent absorption spectra (measured 15 seconds apart)

were < 0.0005. Although there was no net change in the number of spectrally resolvable electrons at small

11

biases, a background current was still observed. Therefore, for each potential step this background current

was subtracted from the observed current during each step. The molar absorptivity of the TiO2(e–) was

then obtained by dividing the absorption spectral changes from -0.8 V to -0.9 V vs SCE by the background-

subtracted total charge passed (mol) and the surface area (cm2), as shown in Supplementary Figure 5.

The change in molar absorptivity spectra (Δ𝜀(𝜆)) on oxidizing molecules bound to TiO2 were

calculated using Supplementary Equation 1, and therefore required values for Γgeom and the difference

absorption spectrum for complete oxidation of the molecules (Abs(𝜆)). Γgeom was calculated using

Supplementary Equation 1 and the absorption spectrum before redox titrations (Abs(𝜆) ) and the

corresponding 𝜀(𝜆) spectrum that was previously determined (Supplementary Figure 2). The procedures

used to determine ΔAbs(𝜆) varied depending on the redox event. For [RC-11]+/0/TiO2 (Supplementary

Figure 5), ΔAbs(𝜆) was determined by subtracting the absorption spectrum before oxidation of [RC-11]0

from the spectrum at an early stage of oxidation (0.76 V vs SCE), where features due to [RC-11]+ were

first observed yet [RC-11]2+ features were absent. The resulting difference spectrum was scaled to match

the ΔAbs at 519 nm, an [RC-11]2+/+/TiO2 isosbestic point that was constant at potentials ≥ 0.86 V vs SCE,

in order to obtain ΔAbs(𝜆) at maximum oxidation for [RC-11]+/0/TiO2. For [RC-11]2+/0/TiO2

(Supplementary Figure 5), ΔAbs(𝜆) was determined by subtracting the absorption spectrum before

oxidation of [RC-11]0 from the spectrum at maximum conversion to [RC-11]2+ (1.51 V vs SCE). For

RuIII/II/TiO2 (Supplementary Figure 5) a similar procedure was used, where ΔAbs(𝜆) was determined by

subtracting the absorption spectrum before oxidation of RuII from the spectrum at maximum conversion

to RuIII (1.83 V vs SCE).

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Supplementary Figure 5. Calculated molar absorptivity difference spectra for (a) RuIII/II/TiO2 (blue),

using a RuII/TiO2 sample, and TiO2(e–) (purple), using a TiO2 only sample, and (b) [RC-11]+/0/TiO2 and

(c) [RC-11]2+/0/TiO2. In panels b and c, molar absorptivity difference spectra (blue) were obtained using

RC-11/TiO2, while scaled absorption difference spectra (dotted-green) were obtained using co-anchored

(RuII + RC-11)/TiO2. The red spectra in panels b and c are approximate molar absorptivity difference

spectra for [RC-11]+/0/TiO2 and [RC-11]2+/0/TiO2, respectively, as proposed to exist in co-anchored films.

The spectra of co-anchored (RuII + RC-11)/TiO2 thin films during steady-state irradiation

measurements were modeled using Δ𝜀(𝜆) for TiO2(e–), RuIII/II/TiO2, [RC-11]+/0/TiO2 and [RC-

11]2+/0/TiO2. However, the experimentally measured RC-11/TiO2 spectra (Supplementary Figure 5)

required slight modification because the absorption peaks due to RC-11 species in co-anchored films

differed slightly from those measured individually. These modified Δ𝜀(𝜆) spectra could not be determined

directly using co-anchored films (i.e. using the data in Supplementary Fig. 6) because oxidation of RC-

11 first required oxidation of the neighboring RuII dyes and therefore the extent of RC-11 oxidation could

not be controlled and RC-11 spectral features were masked by spectral features due to RuIII. Therefore,

the modified Δ𝜀(𝜆) spectra (Supplementary Figure 5) were generated by linearly shifting the Δ𝜀(�̅�)

spectra of [RC-11]+/0/TiO2 and [RC-11]2+/0/TiO2, i.e. the blue spectra but as a function of wavenumber,

so that their peaks aligned with the corresponding peaks observed in the co-anchored (RuII + RC-11)/TiO2

thin film. For both [RC-11]+/0 and [RC-11]2+/0, the peak at ~480 nm was red-shifted by ~160 cm-1, which

is < 4 nm; for [RC-11]+/0 the peak at ~970 nm was blue-shifted by ~460 cm-1 and for [RC-11]2+/0 the peak

at ~680 nm was blue-shifted by ~630 cm-1 (Supplementary Figure 5). The rationale for this procedure is

13

described in more detail below and is valid when nearby environmental effects (i.e. the identity of nearest-

neighbors) affect the absorption peak energies but not the molar absorptivities, which is an assumption

with literature precedent.16,17

Supplementary Figure 5 shows the ΔAbs(𝜆) of a co-anchored film at 0.8 V vs SCE (green), where

features due to [RC-11]+ were first observed yet [RC-11]2+ features were absent. This spectrum has clear

features at ~930 nm due to [RC-11]+ but the spectrum is not that of [RC-11]+/0/TiO2 only, because the

peak at ~480 nm is a mixed absorption change due to [RC-11]+/0 and RuIII/II. The majority contribution at

~480 nm is from [RC-11]+/0, because the net ΔAbs is positive and the magnitude of the molar absorptivity

of [RC-11]+/0 and RuIII/II are very similar in this region. Supplementary Figure 5 shows the ΔAbs(𝜆) of a

co-anchored film at 1.0 V vs SCE (green), where features due to [RC-11]2+ were first observed yet RuIII

features were small. This spectrum has clear features at ~680 nm due to [RC-11]2+ but the spectrum is not

that of [RC-11]2+/0/TiO2 only, because again the peak at ~490 nm is due to a mixed absorption change due

to [RC-11]+/0 and RuIII/II. In addition, the peak at ~930 nm is due to absorption from complete conversion

of [RC-11]0 to [RC-11]+ with only a small fraction of [RC-11]2+, which is clear when the spectral

component due to [RC-11]+ is removed (Supplementary Figure 5)

14

Supplementary Figure 6. (a) Steady-state absorption spectra of (RuII + RC-11 (87:13))/TiO2 under

various applied bias potentials. Inset: Absorbance at 677 nm and 483 nm (indicated by the arrows in the

main figure) vs applied bias potential (dots) and the non-linear least-squares best fits (lines) to a modified

version of the Nernst equation (Equation 1) with E10' = 1.08 V and E2

0' = 1.19 V vs SCE, and a1 = 0.92

and a2 = 1.43, respectively. (b) Absorption spectra over time after applying a potential step of 1.15 V vs

SCE.

Additional transient absorption procedures, data, spectral modeling, and fits to kinetic models

To help increase the signal-to-noise ratio of the data, smoothing was performed. For data displayed

on a linear time scale, each data point was averaged over ± 100 ns (± 50 pts). For data displayed on a

logarithmic time scale, each data point was averaged over ± 25 ns (at < 3 μs), ± 100 ns (at 3 – 10 μs), ±

200 ns (at 10 – 90 μs), ± 2.5 μs (at < 1 ms, 1 MΩ termination), and ± 10 μs (at > 1 ms, 1 MΩ termination),

and then the data was decimated evenly on the logarithmic time scale.

Transient absorption data and spectra (Supplementary Figure 7) of RuII/TiO2 and RC-11/TiO2 thin

films returned cleanly to baseline in less than 20 ms, shorter than the time between subsequent pump laser

pulses (100 ms). The transient absorption data at 480 nm for RuII/TiO2 was solely attributed to RuIII,

because 480 nm is an isosbestic point for the RuII Stark effect (Supplementary Figure 3). RC-11/TiO2

(Supplementary Figure 7) exhibited induced absorption features peaked at ~520 nm and a shoulder at long

wavelengths due to the formation of [RC-11]+ together with featureless weak absorption features that

increased toward the near-infrared spectral region due to TiO2(e–), consistent with the observations in Ref.

15

6 that RC-11 can sensitize TiO2. A similar result has been reported for other bis(triarylamine) dyes

anchored to TiO2 thin films.18

Starting at the peak signal after the laser pulse (50 ns), the transient-absorption data were fit well

by a stretched-exponential function,

ΔAbs(𝜆) = ΔAbs(𝜆)o exp[-((t – to)/τ)𝛽] (2)

where ΔAbs(𝜆)o, to, τ, and 𝛽 are the exponential scale factor, the time (sec) at the half-height of the initial

rise in signal,19 a characteristic time constant (sec), and a stretch parameter which ranges from 0 < 𝛽 < 1

and is inversely related to the width of the underlying Lévy distribution of lifetimes, respectively.11 Non-

linear least-squares best fits to the data at 480 nm were obtained when τ = 620 ± 40 μs and 𝛽 = 0.66 ± 0.04

(Supplementary Figure 7), which were then used as non-adjustable parameters in order to fit the data at

720 nm, with the only adjustable parameter being ΔAbs(𝜆)o. These data were also fit using a single-

exponential function (𝛽 = 1), and while the best fits (τ = 470 μs) were inferior to those using a stretched-

exponential function, the exponential first-order kinetic model was straightforward to implement in the

Monte Carlo simulations described below as a process that obeyed a Poisson distribution. For RC-11/TiO2,

the same processes were performed and non-linear least-squares best fits were obtained when τ = 320 ±

20 μs and 𝛽 = 0.49 ± 0.02 (Supplementary Figure 7). These data were also fit to a biexponential function

(τa = 30 ± 5 μs and τb = 690 ± 90 μs) for implementation in the Monte Carlo simulations. The same

stretched-exponential fitting process was performed on the data in Figure 4. For Figure 4a, non-linear

least-squares best fits resulted in τ and 𝛽 values that were the same as those obtained for the data in

Supplementary Figure 7. These values were then used as non-adjustable parameters for fits to the data in

Figure 4e, where the only adjustable parameter was ΔAbs(𝜆)o. The same process was performed with the

data in Figure 4d and Figure 4h, where non-linear least-squares best fits were obtained when τ = 72 ± 2

μs and 𝛽 = 0.45 ± 0.02.

16

Supplementary Figure 7. Transient absorption data as a function of time at the indicated probe

wavelengths (a, c) and spectra at the indicated delay times (b, d) for RuII/TiO2 (incident (absorbed) pump

fluence = 0.66 (0.37) mJ/cm2 per pulse) and RC-11/TiO2 (incident (absorbed) pump fluence = 3.7 (~0.09)

mJ/cm2 per pulse). The red lines in panels a and c are two non-linear least-squares best fits of the 480 nm

data to a stretched-exponential function, Eq. S2, and then scaled to the 720 nm data.

In order to verify that the ΔAbs(𝜆) observed using co-anchored (RuII + RC-11)/TiO2 thin films

(Figure 3) was due to direct excitation of RuII and not RC-11, two control experiments were performed

using RC-11/TiO2 thin films. The first control experiment demonstrated that absorption of the pump laser

light by RC-11 in the co-anchored film did not result in electron injection from the excited-state of RC-

11, i.e. RC-11*, into TiO2. Immediately after exciting maximum-coverage RC-11/TiO2 with a pump

fluence similar to that used for the experiments with co-anchored films (0.71 mJ/cm2 per pulse), ΔAbs ≈

0 was observed when probed at 720 nm, and only ΔAbs ≈ 0.0003 was immediately apparent when the

pump fluence was increased three-fold (Supplementary Figure 8). The small intensity of these absorption

17

features was likely due to the weak absorbance of RC-11 at the pump wavelength and negligible excited-

state electron injection from RC-11* into TiO2. Therefore, it is reasonable to assume that essentially no

[RC-11]+ was formed from direct laser excitation of RC-11, a hypothesis further supported by data

obtained for co-anchored (RuII + RC-11 (~99:1))/TiO2 thin films (Figure 3a) where a relatively large

ΔAbs ≈ 0.0011 was observed, yet the mole fraction of RC-11 was only ~0.01. The second control

experiment demonstrated that [RC-11]+ generated during the laser pulse (i.e. from oxidation by a nearest-

neighbor RuIII formed after sub-nanosecond electron injection from Ru* into TiO2) did not directly inject

electrons into TiO2 by absorbing light from the long-time tail of the same pump laser pulse. Exciting RC-

11/TiO2 containing significant [RC-11]+ generated by applying a bias of 0.69 V vs SCE (Supplementary

Figure 8) and with a pump fluence similar to that used for the co-anchored film, resulted in ΔAbs ≈ 0 when

probed at 720 nm (Supplementary Figure 8), where [RC-11]2+ absorbs strongly. This suggests that [RC-

11]+ does not inject a measureable amount of electrons into TiO2, even though [RC-11]+ has a large molar

absorptivity at the pump wavelength (532 nm). Therefore, it is reasonable to assume that essentially no

[RC-11]2+ was formed from direct excitation of [RC-11]+, a hypothesis further supported by data obtained

for co-anchored (RuII + RC-11 (~99:1))/TiO2 (Figure 3a) where no peak at ~680 nm was observed initially.

Supplementary Figure 8. Transient absorption data as a function of time at the indicated probe

wavelength(s) for (a) RC-11/TiO2 (incident pump fluence (mJ/cm2 per pulse) = 0.53 (blue), 0.71 (green),

2.0 (red), and 3.7 (cyan)) and (b) [RC-11]+/TiO2 (incident pump fluence = 0.53 mJ/cm2 per pulse). Inset:

Equilibrium absorption spectra measured at the indicated applied bias potentials.

18

Supplementary Figure 9 shows the spectral modeling components used in the best fits reported in

Figure 3a and Figure 3c. To assess whether inclusion of contributions due to the Stark effect were justified,

the same analyses were performed in the absence of components due to the Stark effect. A superior fit to

specifically the low-energy region of the spectrum was obtained when the RuII Stark effect was included.

Supplementary Figure 9. Spectral modeling components used in the best fit (red) of the maximum

transient absorption difference spectrum shown in green in Figure 3a and Figure 3c (green) with (a, c) or

without (b, d) spectral contributions from the RuII/TiO2 Stark effect.

Monte Carlo simulations of transient absorption data

Monte Carlo simulations supported the experimental transient absorption data (Figure 3) and the

general observation that significantly more [RC-11]2+ was generated when a low coverage of RC-11 was

used. Spherical particles (14.7 nm in diameter determined from an average of 83 particles using a plan-

view scanning electron micrograph) were modeled with molecule locations at the vertices of an order-5

19

regular triangular tessellation of each face of an icosahedron, resulting in 252 vertices. The tessellation

vertices were chosen to represent the locations of molecules because the resulting number of vertices was

within 5% of the calculated average number of molecules per TiO2 particle observed experimentally. Each

particle, and thus each set of 252 vertices, was randomly rotated around each of the two rotational axes in

spherical polar coordinates to randomize the exact locations of the vertices. Optical excitation and ultrafast

excited-state electron injection from Ru* into TiO2 were simulated to occur in one time step (i.e. a delta-

function laser pulse) including absorption of linearly-polarized light that followed the Beer–Lambert law

and occurred via a radially oriented transition dipole moment in RuII. Simulations were performed on a

set consisting of a single column of particles (e.g. beads on a string), which is a valid approximation

assuming a film porosity of (1 – π/6 = 0.48), and incorporated a time step of 2 ns and a self-exchange "hole"

hopping process between dyes (i.e. RuaIII + Rub

II RuaII + Rub

III, where a and b are the dye labels) that

followed a Poisson distribution. The experimentally quantified number of RC-11 molecules were

randomly distributed at tessellation vertices across all particles and were flagged as oxidized only when a

RuIII dye was located as a nearest neighbor. (There were 5 or 6 nearest neighbors depending on the vertex.)

The simulations used experimentally determined values for the molar absorption coefficient of RuII/TiO2

(3136 M-1 cm-1) at the excitation wavelength (532 nm), the quantum yield for excited-state electron

injection from Ru* into TiO2 calculated based on the initial bleach signal observed using Ru/TiO2 (0.34),

the molar ratio of catalysts to dyes (1:99 and 28:72), the film thickness (3.9 μm and 3.2 μm) and the

absorbance at the excitation wavelength (0.256 and 0.193) both corrected for the film being positioned at

a 45° angle, and the incident laser pump fluence (0.79 and 0.87 mJ/cm2 per pulse).20 The models also

included first-order kinetic processes for TiO2(e–) recombination to RuIII (τ = 470 ns) and [RC-11]+ (τa =

30 μs and τb = 690 μs) obtained from the best fits above, which each followed a Poisson distribution. The

models did not take into consideration physical occlusion of molecules at particle necking regions,

20

interparticle electron transfer, or TiO2(e–) recombination to [RC-11]2+. The average signals from the

simulations (± one standard deviation) are consistent with the experimental observations and the best-fit

simulation data over the first 60 µs (τhop = 800 ns, to the nearest hundred nanoseconds) are shown in Figure

3b.

Additional steady-state spectra of (RuII + RC-11)/TiO2 under continuous-wave illumination

Supplementary Figure 10 shows the spectral modeling components of the 40 mW/cm2 (0.9 Suns)

data in Figure 5, and the spectral modeling results when the difference absorption spectrum of RuII/TiO2

obtained under continuous-wave illumination was used. Inferior best fits were obtained using the

difference absorption spectrum of RuII/TiO2 obtained from spectroelectrochemistry (Figure 2b), as

evidenced by the poor fit of the shoulder region at ~550 nm.

Supplementary Figure 10. (a) Spectral modeling components used in the best fit (red) of the steady-state

difference spectrum at 40 mW/cm2 (0.9 Suns) in Figure 5 (blue) for a (RuII + RC-11 (97:3))/TiO2 thin

film. (c) Steady-state difference spectrum for RuIII/II/TiO2 under 532 nm continuous-wave laser

illumination at an absorbed irradiance of ~16 mW/cm2, which was used for the spectral fitting in panel a.

(d) The same data and analysis as in panel a but using the spectrum from Figure 2b in place of the

absorption difference spectrum for RuIII/II/TiO2. Panels b and e show the residuals of the best-fit spectra

from panels a and d, respectively.

The spectral modeling results shown in Supplementary Figure 10 also suggests that the RuII Stark

effect is opposite of that which is typically observed after electrons are injected into TiO2.2,14,15 This

observation implies that the RuII dyes are affected by increased positive charge density during the

21

experiment and thus the TiO2(e–) must be screened by more charge than they contribute. One hypothesis

is that the RC-11 molecules lie flat on the TiO2 surface and so when [RC-11]2+ is formed, a net positive

charge results at the TiO2 interface; another is that transiently the ion distribution is such that an

overcompensation of charge exists.21 This new observation, whose complete understanding is beyond the

scope of this work, will be the focus of future studies.

Supplementary Figure 11. Steady-state difference spectra (dark blue) for a (RuII + RC-11 (97:3))/TiO2

thin film under the indicated irradiances of 532 nm continuous-wave laser illumination (a, c) and least-

squares best-fit spectral modeling results with (red, panels a and c) and without (light blue, panel a)

spectral contributions from [RC-11]2+. Spectra were smoothed by averaging over ± 1 nm. Panels b and d

show the color-coded residuals of the best-fit spectra from panels a and c, respectively, and the [RC-

11]2+/0 spectrum (dash-dotted orange) as a scaled version of the red difference spectrum in Figure 2d.

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