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World energy demand is predicted to double by 2050 and triple by 2100.  In the long term, solar energy is the most practical and abundant alternative energy source.  The amount of solar energy reaching the earth's surface per hour (4.3 x 10²⁰ J) exceeds the current annual world energy consumption (4.1 x 10²⁰ J).  However, solar energy currently provides just 0.1% of the world's electricity supply.  Efficient solar energy conversion requires absorption of solar illumination followed by reduction and oxidation processes which compete with charge recombination.  The light-to-electrical energy conversion mechanism of dye-sensitized solar cells (DSSCs) using nanocrystalline TiO₂ involves electron injection by the photoexcited sensitizer followed by electron transport through the TiO₂ film and the external circuit.  The oxidized sensitizer is reduced by an electron donor in the electrolyte, which is subsequently re-reduced at the counter electrode. In addition, charge separation at the semiconductor-photosensitizer interface leads to long-lived charge-separated states, enabling productive redox processes to compete with electron-hole recombination. 

We have been examining chalcogenorhodamine dyes that form aggregates on nanocrystalline TiO₂. These aggregated organic photosensitizers broaden the absorbance bands to both the shorter-wavelength side (H-aggregates) and the longer-wavelength side (J-aggregates) of the unaggregated dye absorbance maximum.  Our preliminary data indicated incident-photon-to-current efficiency (IPCE) in several DSSCs based on H-aggregated dyes 1-E (E = O, S, Se, Chart 1) was much higher than in DSSCs based on amorphous, non-aggregated dyes 2-E (E = O< S, Se, Chart 1)).  The carboxylic acid functionality in dyes 1-E and 2-E is necessary to bind the dyes to the TiO₂ surface.

In order to examine the series of dyes 3-E, 4-E, and 5-E (Chart 1) with 9-(carboxyphenyl) substituents for their ability to form H- and/or J-aggregates, it was necessary to devise synthetic schemes that would allow their synthesis.  Dyes 5-E can be prepared by the addition of lithium 2-lithiobenzoate (6) to the corresponding chalcogenoxanthone 7-E, followed by acid-induced dehydration.  This approach does not work for the synthesis of dyes 3-E and 4-E since the corresponding 4-lithio and 3-lithiobenzoates (or the corresponding Grignard reagents), respectively, are thermodynamically unstable and quickly isomerize to 6.  We circumvented this problem using a modified Suzuki-coupling procedure.  The chalcogenoxanthones 7-E reversibly form the triflates 8-E under anhydrous conditions.  While the boronic acids hydrolyze the triflates back to the starting xanthones, the triflates 8-E were stable in the presence of arylboroxins 9 and the addition of sodium carbonate and 10 mol-% of PdCl₂(PPh₃)₂ gave the corresponding 9-arylchalcogenorhodamines (including 3-E and 4-E) in excellent yield.

The series of dyes 3-E, 4-E, and 5-E (Chart 1) behaved quite similarly to the 9-thiophene carboxylic acid derivatives 1-E and 2-E.  Dyes 3-E and 4-E formed H-aggregates on nanocrystalline TiO₂ while dyes 5-E (“traditional” rhodamine derivatives) did not form H-aggregates and gave an amorphous coating of dye on the TiO₂.  In this series of dyes, IPCE values for the unaggregated dyes 5-E on TiO₂ were higher than IPCE values for the H-aggregated dyes 3-E and 4-E in work done in collaboration with Prof. David Watson at the University at Buffalo.  However, IPCE values for dyes 5-E on TiO₂ were smaller than those of dyes 1-E on TiO₂ suggesting that dyes 5-E might be less efficient at converting sunlight into electricity

However, dyes 5-E are among the most efficient photosensitizers ever examined for the evolution of hydrogen from water in either a homogeneous system (Figure 1) or in a heterogeneous system (Figure 2).  The following work was done in collaboration with Prof. Rich Eisenberg and Dr. Theresa McCormick at the University of Rochester. In the presence of triethanol amine (TEOA) as a sacrificial electron donor and the cobalt catalyst shown in Figure 1, dye 5-Se gave nearly 5000 turnovers for the production of hydrogen in 24 h compared to 3500 turnovers for dye 5-S, and no hydrogen evolution with dye 5-O.  The production of hydrogen tracks the triplet yield [as measured by the singlet oxygen yield (Q)], which suggests that electron transfer is more competitive from the longer-lived triplet state..  Further modifications of the system have given turnover numbers of >10,000 after 24 h.  Both 5-Se and 5-S are much more efficient than Eosin Y, which was the previous “best” photosensitizer for this system.

On platinized TiO₂ using TEOA as a sacrificial electron donor, dye 5-Se was far more efficient as a photosensitizer on platinized TiO₂ than Eosin Y, which again was the previous world “best” photosensitizer for hydrogen evolution on platinized TiO₂.  As shown in Figure 2, dye 5-Se gave over 900 turnovers in the first 5 hours of reaction while Eosin Y gave approximately 100 turnovers.  Dyes 5-S and 5-Se were far more efficient in either the homogeneous and heterogeneous systems than the other carboxylic acid containing dyes 1-E, 2-E, 3-E, or 4-E.

During the next reporting period, we shall prepare other chalcogenorhodamine derivatives with carboxylic acid functionality on the amino substituents in order to examine J-aggregation.  We shall also examine the oxidized photosensitizer on TiO₂ by transient techniques in order to examine lifetimes and recombination rates.  We shall also examine ways to optimize the evolution of hydrogen both in heterogeneous and homogeneous systems.  In particular, we shall examine the lifetimes of the reduced photosensitizer as a means to prolong the period of hydrogen evolution.

The PRF support has allowed the PI to become involved with solar fuels and solar electricity by providing support for a core group of students.  The graduate students on this project have benefited not only from the financial support, but also from the breadth of science involved in this project: synthesis, catalysis, photophysics, materials science.  He undergraduate student participating in this project is applying to various programs for graduate studies in chemistry. (He originally was interested in chemical engineering!)