the sunlight - driven 
 generation of the energy carrier h2 from water using earth - abundant 
 materials is considered one of the 
 key processes to generate more sustainable fuels in a postfossil age .
synthetic first - row transition - metal complexes 
 containing co , fe , and ni are under intense development 
 as scalable alternatives to the benchmark h2 evolution 
 catalysts platinum and hydrogenases .
complexes with a catalytic bis(1,5-r-diphospha-3,7-r-diazacyclooctane)nickel(ii ) 
 core , [ ni(p2n2)2 ] , developed by dubois and co - workers , 
 emerged in the past decade as probably the most active synthetic 3d 
 transition - metal electrocatalysts for proton reduction . the catalytic cycle has been studied 
 experimentally and computationally , and the catalyst mimics important features of 
 the hydrogenase active site .
photocatalytic h2 generation requires the efficient 
 coupling of an efficient proton reduction electrocatalyst with a light - harvesting 
 component . in particular , the development and investigation of molecular 
 h2 evolution catalysts made of abundant elements that show 
 stability and activity in water are of major interest .
many reports on homogeneous photocatalytic systems 
 for water reduction using fe or co complexes are available , but there are relatively 
 few using ni - based catalysts .
heterogenization of 
 well - defined molecular catalysts on semiconductor surfaces is an emerging 
 approach for photocatalytic h2 generation .
systems that 
 have been shown to have notable h2 production efficiencies 
 include cobaloximes immobilized on tio2 nanoparticles or cdse / zns quantum dots , and [ fefe]-hydrogenase 
 mimics on cdte or zns quantum dots .
efforts have also been made to integrate 
 molecular catalysts on photoelectrodes , such as an [ fefe ] complex 
 on inp , a cobaloxime catalyst on p - type 
 gap , and fe(dithiolato)(diphosphine ) or cobaloxime catalysts on dye - sensitized nio films .
however , the frequent use of organic 
 solvents , poor light - to - h2 conversion efficiencies , and 
 photoinstabilities are drawbacks in these systems . despite the 
 promising properties of dubois - type catalysts ,
they 
 were previously only applied in homogeneous photocatalytic schemes 
 in the presence of organic solvents or a biological matrix .
surface immobilization of a [ ni(p2n2)2 ] derivative 
 on carbon nanotubes yielded highly active electrodes for h2 generation under strongly acidic conditions . when a [ ni(p2n2)2 ] derivative was immobilized 
 on a p - type silicon photoelectrode , no ( photo)activity for h2 production was reported .
water - soluble 
 [ ni(p2n2)2 ] derivatives were previously not shown 
 to operate in aqueous homogeneous or heterogeneous environments for 
 photocatalytic h2 generation . in this work ,
we report 
 on a novel dubois - type catalyst ( nip ; figure 1 ) , which not only contains 
 a highly electroactive [ ni(p2n2)2 ] core , but 
 also is surrounded by an outer sphere containing four dangling phosphonic 
 acid moieties .
this outer - sphere feature provides the catalyst with 
 good solubility in aqueous solutions and anchors for immobilization 
 onto metal oxide surfaces .
electrochemical 
 studies of nip in an aqueous electrolyte solution demonstrated 
 sustained electrocatalytic activity in the absence of organic solvents . 

subsequently , we studied nip in aqueous photochemical 
 systems with a phosphonated bipyridine - based ru(ii ) photosensitizer 
 ( rup ; figure 1 ) in both a homogeneous 
 system and heterogeneous photocatalytic environments where rup and nip were immobilized on different metal oxide semiconductors .
chemical 
 structures of the doubly charged cationic complexes used 
 in this study : the electrocatalyst nip and dye rup .
the kinetics of charge separation and recombination 
 were studied 
 by time - correlated single photon counting ( tc - spc ) and transient absorbance 
 spectroscopy ( tas ) .
this spectroscopic study reveals that distinct 
 reaction mechanisms are possible in these remarkably versatile photochemical 
 systems .
in particular , we consider whether this coimmobilization 
 strategy on metal oxide surfaces reduces the rapid electron / hole recombination 
 losses typically observed when such photosensitizers and catalysts 
 are covalently attached via molecular linker groups .
the visible - light - driven h2 generation of the 
 different systems was also studied in bulk experiments and revealed 
 that nip can operate with a high photocatalytic turnover 
 frequency for a ni - based molecular proton reduction catalyst in purely 
 aqueous systems .
all synthetic procedures involving 
 air- or moisture - sensitive materials were carried out under n2 by using either a glovebox or schlenk techniques .
chemicals 
 for the synthetic part of this work were purchased from commercial 
 suppliers and used without further purification .
tio2 nanoparticles ( aeroxide tio2 p25 particles ; anatase / rutile ( 8/2 ) mixture , average particle 
 size 21 nm ) were a gift from evonik industries , and zro2 nanoparticles ( 99.9% , 2030 nm ) were obtained from skyspring 
 nanomaterials inc .
nanostructured anatase tio2 and zro2 films were prepared by the doctor blading technique from 
 colloidal pastes as reported previously .
the films were annealed at 450 c for 30 min prior to use , 
 and the anatase phase for tio2 was confirmed by x - ray diffraction 
 studies after heating .
the resulting film thicknesses , determined 
 by profilometry ( tencor instruments ) , were 4 m .
[ ni(p2n2)2](bf4)2 ( nip ) ( figure s1 , supporting information ) and rup(24 ) were prepared 
 according to published procedures .
h nmr spectra were referenced 
 to the solvent residual peaks 
 as an internal reference , and p nmr spectra were referenced to an external standard ( 85% h3po4 in d2o ) .
vis spectra were 
 recorded on a varian cary 50 uv  vis spectrophotometer using 
 quartz glass cuvettes
. high - resolution electrospray ionization mass 
 spectra ( hr - esi - ms ) were recorded on a quattro lc spectrometer , and 
 the theoretical and experimental isotope distributions were compared . 

elemental analysis was carried out by the microanalytical laboratory 
 in the department of chemistry of the university of cambridge .
bromotrimethylsilane ( 0.25 ml , 1.86 mmol ) 
 was added dropwise to a deep red solution of nip ( 250 mg , 0.143 mmol ) in degassed dichloromethane 
 ( 15 ml ) under a n2 atmosphere at room temperature .
the 
 color changed to deep purple , and the solution was stirred for 2 days 
 at room temperature .
the solvent was removed , followed by addition 
 of deoxygenated meoh ( hplc grade , 15 ml ) to the dark solid residue 
 and stirring under n2 for 1 day at room temperature .
the 
 volume of the solvent was reduced to approximately one - third , and 
 the product was precipitated with et2o ( 25 ml ) . 

after the mixture was stirred for 30 min , the deep purple precipitate 
 was filtered off under n2 , washed with et2o , 
 and dried in vacuo .
the solid was redissolved in meoh , the solution 
 was stirred for several minutes , and the solvent was evaporated under 
 high vacuum to give a purple solid .
h nmr ( 400 mhz , cd3od ,  ) : very broad signals , 8.236.70 
 ( m , 36h , ph ) , 3.604.70 ( m , 16h , nch2p ) , 2.973.22 ( m , 8h , phch2po(oh)2 ) ppm .
p{h } nmr ( 162 mhz , cd3od ,  ) : 25.8 ( po(oh)2 ) , 
 21.0 ( nch2p ) , 13.9 ( nch2p ) ppm .
hr - esi - ms ( meoh , negative ) : m / z calcd for [ m  2h + br ] 1419.1121 , found 1419.1154 ; m / z calcd for [ m  3h ] , 1339.1895 , found 1339.1903 . 

calcd for c60h68br2n4nio12p8hbr2h2o : c , 44.47 ; 
 h , 4.54 ; n , 3.46 ; p , 15.29 ; br , 14.79 . found : c , 44.45 ; h , 4.70 ; n , 
 3.24 ; p , 14.61 ; br , 15.08 .
vis ( meoh ) : max , nm (  ) 510 ( 1100 ) .
electrochemical 
 measurements were 
 performed on an iviumstat or ivium compactstat potentiostat under 
 an argon atmosphere using a three - electrode configuration .
a glassy - carbon 
 disk ( 3 mm diameter ) working electrode , a platinum - wire counter electrode , 
 and a ag / agcl / kcl(sat ) reference electrode were used for cyclic voltammetry 
 ( cv ) .
all potentials were converted to the normal hydrogen electrode 
 ( nhe ) by addition of + 0.197 v. controlled - potential electrolysis ( cpe ) was carried out in a three - necked 
 flask with a three - electrode setup using glassy - carbon rods as working 
 and counter electrodes ( surface area in solution 2 cm ) and a ag / agcl / kcl(sat ) reference electrode .
a solution of nip ( 0.18 mm ) in ascorbic acid ( aa , 0.1 m , ph 4.5 ) was purged 
 with n2 containing ch4 ( 2% ) as the internal 
 gas chromatography ( gc ) standard , and a potential of 0.5 v 
 vs nhe was applied during cpe for 2 h. the faradaic yield was calculated 
 from the amount of h2 accumulated in the headspace , as 
 measured by gc .
cpe of catalyst - free electrolyte at the same potential 
 showed no production of h2 . to prepare tio2 and zro2 films 
 sensitized with rup , an aqueous solution of rup
( 10 l of 4 m ) was spread onto the films ( geometrical 
 surface area 1.5 cm ) , and the solvent was dried in air 
 for 30 min .
for tio2 and zro2 films cofunctionalized 
 with rup and nip , first a solution of nip ( 10 l of 8 m ) in meoh was spread onto the 
 surface of the film , followed by drying for 30 min in air .
then , an 
 aqueous rup solution ( 10 l of a 4 m ) was 
 spread onto the surface of the film and was dried for an additional 
 30 min .
spectroscopic measurements 
 on homogeneous solutions and functionalized tio2 and zro2 films were carried out in water or in aqueous aa solution 
 ( 0.1 m ) carefully adjusted to ph 4.5 with naoh ( 0.1 m ) , unless otherwise 
 stated .
the aqueous solutions were purged with n2 for 15 min prior 
 to the measurements .
the uv  vis and fluorescence spectra of 
 the solutions and films were recorded using a quartz cuvette ( 1 cm 
 path length ) on a perkin 
elmer lambda 35 uv  vis spectrophotometer 
 and a horiba jobin yvon fluorolog luminescence spectrophotometer , 
 respectively .
tc - spc measurements were performed by using a horiba 
 jobin yvon tbx fluorocube system . as the excitation source , a pulsed 
 laser with 467 nm nominal wavelength at a repetition rate of 100 khz 
 was used . the photoluminescence intensity of rup at em 650 nm was measured as a function of time after the excitation 
 pulse , for a fixed data collection period to ensure matched densities 
 of absorbed photons between samples ( 600 s ) .
the instrument response 
 was measured at the full width at half - maximum and showed typically 
 a 200250 ps value .
all tc - spc experiments undertaken on tio2 and zro2 films were measured under aqueous conditions 
 or a 0.1 m aa solution at ph 4.5 in air .
the microsecond to 
 second transient absorption decays were measured using a nd : yag laser 
 ( big sky laser technologies ultra cfr nd : yag laser system , 6 ns pulse 
 width ) .
the second and third harmonics of the laser ( corresponding 
 to 532 and 355 nm , respectively ) were used to excite rup .
the laser intensity was adjusted using neutral density filters 
 as appropriate , with experiments typically employing 350 j 
 cm , and the frequency of the laser pulse was fixed 
 to 1 hz .
a liquid light guide with a diameter of 0.5 cm was used to 
 transmit the laser pulse to the sample .
the probe light source was 
 a 100 w bentham il1 tungsten lamp , and the probing wavelength was 
 selected by using a monochromator ( obb-2001 , photon technology international ) 
 placed prior to the sample .
several high - pass , low - pass and neutral - density 
 filters ( comar optics ) were used to decrease the light arriving to 
 the detector .
the information was passed through an amplifier 
 box ( costronics ) and recorded using a tektronics tds 2012c oscilloscope 
 ( microsecond to millisecond time scale ) and a national instruments 
 ( ni usb-6211 ) daq card ( millisecond to second time scale ) .
all photocatalytic experiments were 
 carried out using a solar light 
 simulator ( newport oriel , 100 mw cm ) equipped 
 with an air mass 1.5 global filter ( am 1.5 g ) .
uv irradiation was filtered 
 using a 420 nm cutoff filter ( uqg optics ) , and ir irradiation was 
 filtered by a water filter ( path length 10 cm ) .
the photoreactor was 
 held at a constant temperature of 25 c in all experiments .
samples 
 were generally prepared in air protected from light by an al foil . 

the reaction vessel was sealed with a rubber septum , and air was replaced 
 by n2 containing 2% ch4 ( internal gc standard ) . 

the irradiated cross section of the solution in the vials was approximately 
 3.3 cm .
h2 evolution was monitored by gc measurements 
 with an agilent 7890a series gc equipped with a 5  molecular 
 sieve column .
the gc oven temperature was kept constant at 45 c , 
 n2 was used as a carrier gas at an approximate flow rate 
 of 3 ml min , and a thermal conductivity detector 
 ( tcd ) was used .
samples ( 1520 l ) for headspace gas 
 analysis were taken from the reaction vessel headspace in 30 min intervals 
 for 2 h in the screening experiments . for long - term measurements
the response factor of the thermal conductivity 
 detector for h2 compared to ch4 was 1.91 . in a typical experiment ,
a freshly prepared aa solution 
 was titrated to the desired ph using naoh ( 0.1 m ) or hbf4 ( 0.1 m ) and diluted to a final aa concentration of 0.1 m ( the final 
 ph was confirmed ) .
stock solutions of rup ( 1.0 or 2.0 
 mm in h2o ) and nip ( 2.0 mm in meoh ) were added 
 in the desired ratio to the aqueous aa solution ( 0.1 m ) to reach a 
 final volume of 2.25 ml , leaving 5.59 ml of gas headspace in the vial . 

control experiments in the absence of meoh were carried out and confirmed 
 a comparable photocatalytic activity .
zro2 or 
 tio2 nanoparticles ( 2.5 mg ) were placed in a vial containing 
 an aa solution and dispersed by sonication for 5 min . to load the 
 particles ,
first nip was added and after stirring for 
 several min rup was added , resulting in a final volume 
 of 2.25 ml , leaving 5.59 ml of gas headspace in the vial .
the adsorption of nip on tio2 and zro2 nanoparticles was quantified by recording 
 the difference uv  vis spectrum of a solution of nip in aa ( 0.1 m , ph 4.5 , 2.25 ml ) before and after exposure to tio2 or zro2 nanoparticles ( 2.5 mg ) .
the solution was 
 stirred with the nanoparticles for 30 min , followed by centrifugation 
 ( 7000 rpm , 5 min ) between the measurements .
adsorption of rup on nanoparticles after coadsorption with nip was estimated 
 by adding rup ( 0.05 mol ) to a suspension of nip - sensitized tio2 or zro2 particles 
 ( 0.02 or 0.1 mol nip loading on 2.5 mg nanoparticles ) . 

quantitative adsorption of 0.05 mol rup and 0.02 
 mol nip would result in an estimated surface coverage 
 of approximately 400 rup and 150 nip molecules 
 per nanoparticle and approximately 45% coverage of the tio2 surface ( see the supporting information ) .
the external quantum ( photon to h2 conversion ) 
 efficiency ( eqe ) was determined by an led light source ( modulight , 
 ivium ) using blue light (  460 nm , 5 mw cm ) .
the light intensity was measured with a newport thermopile detector 
 ( 818p-020 - 12 ) coupled with an optical power meter ( 1916-r ) .
samples 
 of rup ( 0.3 mol ) and nip ( 0.1 mol ) 
 in solution were used .
the efficiency was determined from 
 the amount of h2 produced after 2 h of irradiation using 
 the equation all analytical measurements were 
 repeated at least three times .
the obtained data were treated as follows : 
 for a sample of n observations xi , the unweighted mean value x0 and the standard deviation  were calculated 
 using the equationsa minimum 
  of 10% was assumed in all 
 experiments .
the light sources and gas chromatographs were calibrated 
 regularly to ensure reproducibility throughout all experiments .
the catalyst nip was synthesized through hydrolysis 
 of the octaethyl phosphonate ester analogue nip   by dealkylation 
 of the ethyl ester groups with trimethylsilyl bromide in dichloromethane ( figure s1 , supporting information ) .
nip was obtained in 81% yield and characterized by h and p nmr spectroscopy , high - resolution mass 
 spectrometry , and elemental analysis for c , h , n , p , and br .
nip is soluble in water ( 0.3 mg ml ) and dissolves well in an aqueous ascorbic acid solution ( 0.1 m , 
 ph 4.5 ) with a solubility of more than 5 mg ml .
nip therefore is an example of a dubois - type ni catalyst 
 that is soluble in aqueous solutions and can be employed homogeneously 
 in organic solvent free aqueous catalytic systems . before studying nip in a purely aqueous electrolyte 
 solution , we compared nip with the previously studied 
 complex nip ( figure s1 ) in acetonitrile / water 
 mixtures and in the presence of added triflic acid . on a glassy - carbon 
 working electrode under ar at a scan rate of 100 mv s ,
cv measurements of nip ( 0.30.5 mm ) show two 
 quasi - reversible waves at e1/2 = 0.38 
 and 0.54 v vs nhe in h2o ( 0.1 m na citrate ph 5)/acetonitrile 
 ( 0.1 m tbabf4 ) ( 1/1 ) .
the two waves are assigned to the 
 ni and ni redox couples , respectively 
 ( figure 2a and figures s2 and s3 ( supporting information ) ) .
addition of triflic acid induced a catalytic current attributed 
 to proton reduction below wsph 4.9 
 following the reduction of ni to ni with 
 an onset potential of 0.3 v vs nhe ( figure 2a ) .
the small cathodic shift in potential of the reduction 
 wave upon acidification is presumably due to protonation of the dangling 
 phosphonate moieties .
thus , the [ ni(p2n2)2 ] core 
 and electrocatalytic proton reduction activity are intact in nip and the electrochemical response is indeed comparable 
 to that of nip in the presence 
 of an organic acid ( figures s2s4 , supporting 
 information ) .
( a ) cv of nip ( 0.5 mm , black trace ) in h2o ( 0.1 m na citrate ph 5)/acetonitrile ( 0.1 m tbabf4 ) 
 ( 1/1 ) , followed by titration with increasing amounts of triflic acid 
 ( 0.678 m in h2o ) .
wsph is the concentration of protons in a mixed 
 solvent ( water / organic solvent ) where the ph was measured in water 
 against an aqueous reference .
( b ) cv 
 of nip ( 0.3 mm , solid red trace ) in an aqueous aa solution 
 ( 0.1 m , ph 4.5 ) . wave ( a ) shows the first reduction wave of nip , and wave ( b ) indicates the onset of catalytic proton 
 reduction .
reduction potentials of possible reactive intermediates 
 of rup in a photocatalytic process and the conduction 
 band potential of tio2 ( ecb(tio2 ) are also shown . a glassy - carbon working electrode , 
 a ag / agcl / kcl(sat . ) reference electrode , and a platinum - wire counter 
 electrode
were employed at room temperature with a scan rate of 100 
 mv s in ( a ) and ( b ) .
electrocatalytic proton reduction by nip was also 
 observed in the absence of organic solvent in an aqueous solution 
 buffered with ascorbic acid ( aa ) or na citrate ( 0.1 m , ph 4.5 ) .
a 
 single , irreversible reduction wave ( a ) was observed at ep = 0.35 v vs. nhe , followed by the onset of a 
 catalytic wave ( b ) at approximately 0.48 v vs nhe ( figure 2b and figure s5 ( supporting 
 information ) ) .
therefore , the catalyst operates with a small 
 overpotential requirement of approximately 0.2 v in comparison to 
 the thermodynamic potential for proton reduction of 0.27 v 
 vs nhe at ph 4.5 .
the reduction potentials of intermediates of rup , which are likely to be formed during the quenching process 
 after photoexcitation ( see below ) are also indicated in figure 2b .
the position of these potentials and the conduction 
 band edge of tio2 suggest that enough driving force for 
 photocatalytic proton reduction with nip would be available 
 when using rup and rup - sensitized tio2 as a light harvester , as described in detail in the kinetic 
 and photocatalytic sections .
controlled - potential electrolysis 
 with nip ( 0.18 mm ) 
 in an aqueous ascorbic acid solution ( 0.1 m , ph 4.5 ) on a glassy - carbon - rod 
 working electrode ( surface area 2 cm ) at 0.5 
 v vs nhe for 2 h confirmed the generation of h2 gas with 
 a faradaic yield of 85  4% ( h2 in the headspace quantified 
 by gc ) . rinsing
the electrode with h2o after cpe and immersing 
 it in a fresh electrolyte solution ( in the absence of nip catalyst ) did not result in the formation of h2 when 
 continuing with cpe at the same potential .
thus , h2 production 
 originates from a dissolved catalyst and not from electrodeposited 
 decomposition products on the electrode .
thus , the water - soluble nip displays electroactivity for the reduction of aqueous 
 protons with a high faradaic yield in the absence of organic solvents , 
 a prerequisite for the use of nip in photocatalytic schemes 
 in water .
the suitability of nip in photocatalytic h2 generation was studied with rup , aa ( 0.1 m ) as a sacrificial electron donor , and buffer ( pka 4.17 ) in aqueous solution .
photoexcitation of rup to rup * ( max 455 nm , mlct ) can result in either oxidative 
 or reductive quenching of the photoexcited state ( figures 3 and 4 ) .
first , the kinetics and mechanisms of et between rup and nip in homogeneous aqueous solution ( rup - nip system ) and anchored onto metal oxide surfaces ( rup - zro2-nip and rup - tio2-nip systems ) were studied by tc - spc and tas . 

the three photocatalytic systems are illustrated in figure 3 , and the mechanistic and kinetic details are summarized 
 in figure 4 and described below .
the rup - nip and rup - zro2-nip systems generate h2 through a reductive quenching mechanism 
 of rup in solution or  on the particle  , 
 respectively .
rup - tio2-nip undergoes 
 oxidative quenching and electrons are transferred  through 
 the particle  .
summary of et kinetics for the three photocatalytic systems studied 
 as determined by tc - spc and tas ( defined as t50% times ) .
tio2 was observed 
 to cause oxidative quenching of the 
 immobilized rup * , resulting in the oxidized intermediate rup ( e(rup / rup * ) = 0.95 v vs 
 nhe ) by electron injection into the conduction 
 band of tio2 ( ecb = 0.55 
 v vs nhe at ph 4.5 ) .
these kinetics were 
 measured by tc - spc to take place in approximately 180 ps , with an 
 injection efficiency of > 95% ( figure 5 ) .
the transient absorption spectrum of 
 a photoexcited rup - tio2 film shows a maximum 
 transient absorption peak at 
 700 nm , corresponding to the absorption spectrum of the oxidized rup ( figure s6 , supporting information ) . in the absence of aa ,
the tio2 conduction band electrons recombine with rup within approximately 1 ms . upon reduction of rup by aa ( t50%  50 s ) , corresponding to the regeneration of rup ,
the resulting photoinjected tio2 electrons 
 exhibit a lifetime of 0.5 s. following the codeposition of nip ( molecular ratio rup / nip of 1/2 ) , the 
 decay of these electrons is accelerated to approximately 1 ms , assigned 
 to et to nip ( figure s7 , supporting 
 information ) . the oxidative et mechanism from rup to nip through the semiconductor (  through particle  
 system , 
 figure 3c ) , is possible with tio2 but not with zro2 , due to the energetic mismatch between rup * and the conduction band of zro2 ( ecb = 1.26 v vs nhe at ph 4.5 ) .
zro2 is therefore unable to accept electrons 
 from rup * and can only be used as a matrix to immobilize 
 the compounds in close proximity on the particle (  on particle  
 system , figure 3b ) .
the reductive quenching 
 through intermolecular et from aa ( e = 1.17 v vs 
 nhe ) to rup * ( e(rup*/rup ) = 1.08 v 
 vs nhe ) was measured 
 by tc - spc to take place in approximately 250 ns with an estimated 
 efficiency of 70% in homogeneous rup - nip and heterogenized  on particle  rup - zro2-nip systems ( figure 5 ) .
photoexcitation of an aqueous rup solution ( 4 m ) containing aa ( 0.1 m , ph 4.5 ) resulted 
 in the appearance of a transient absorption peak at 500 nm ( figure 
 s8 , supporting information ) assigned to 
 the formation of the reactive intermediate rup .
this reduced 
 state of the dye , rup , is a strong 
 reducing agent and has a large driving force for the reduction of nip ( e(rup / rup ) = 1.09 v vs nhe , figure 2 ) .
time - resolved luminescence 
 measurements of rup with 
 and without nip in water and in the presence of aa ( 0.1 
 m ) at ph 4.5 : ( a ) anchored onto the surface of a tio2 and 
 zro2 film ( 10 l of 4 m rup , 
 10 l of 8 m nip ) ; ( b ) in a homogeneous 
 solution ( [ rup ] = 4 m , [ nip ] = 8 
 m ) .
the addition of increasing amounts 
 of nip ( from 0 
 to 16 m ) to a solution containing rup ( 4 m ) 
 and aa ( 0.1 m , ph 4.5 ) results in the linear decrease of the lifetime 
 of rup from 700 to 29 
 s , following first - order kinetics with respect to nip concentration for the intermolecular et between rup and nip ( figure 6 ) .
the second - order rate constant of this et is ket = 1.4  10 m s , indicating that the et kinetics are diffusion 
 limited ( figure 6b and table s1 ( supporting information ) ) .
in addition , the presence 
 of nip resulted in the appearance of a long - lived ( t50%  0.1 s ) bleaching signal assigned 
 to the reduction of ni species ( figure s9 ( supporting information ) ) upon reduction by rup .
further experiments are ongoing to 
 monitor the nickel catalytic species involved in the h reduction reaction .
( a ) transient absorption decays of rup ( 4 
 m ) 
 in an aqueous aa solution ( 0.1 m , ph 4.5 ) after addition of different 
 concentrations of nip ( 0 , 2 , 4 , 8 , and 16 m ) . 

the excitation wavelength was ex 532 nm , and the 
 decays were probed at probe 500 nm .
( b ) calculated pseudo - first - order 
 rate constant ( ket[nip ] ) 
 for the et from rup to nip as a function of the concentration of nip catalyst ( table s1 ( supporting information ) ) .
the reductive quenching 
 of rup * also occurs through 
 intermolecular et from aa to rup * in rup - zro2-nip .
the photoluminescence intensity 
 and lifetime of rup * on nanostructured zro2 films decrease upon the addition of an aqueous aa solution ( 0.1 
 m , ph 4.5 ) , and tc - spc measurements reveal that reductive quenching 
 of rup * to form rup occurs in approximately 250 ns with an estimated et efficiency 
 of 70% ( figure 5b ) .
the cofunctionalization 
 of zro2 with nip ( 8 m ) and rup ( 4 m ) in water does not affect the luminescence of rup , indicating that et between the two molecules requires 
 the formation of rup in 
 the presence of a sacrificial electron donor .
a transient absorption 
 peak at 500 nm corresponding to the signal of rup was also observed in the rup - zro2 system in the presence of aa ( 0.1 m at ph 4.5 ) , with the 
 decay of rup accelerating following 
 the codeposition of nip ( figure s10 ( supporting information ) ) .
it can be concluded that the same 
 reductive et mechanism and similar kinetics ( dependent upon component 
 loadings / concentrations ) take place both in homogeneous media and 
 on anchoring of the molecules onto the surface of a material that 
 does not allow for dye electron injection . although the reductive 
 et mechanism is the same for both homogeneous and the  on - particle  

zro2 systems , charge separation is achieved through two 
 different processes : while in homogeneous media charge separation 
 occurs through the diffusion of molecules into the solution , when 
 the dye and catalyst are immobilized in rup - zro2-nip charge separation can take place through the intermolecular 
 et between neighboring molecules .
the 
 spectroscopic studies revealed that all three photocatalytic 
 systems drive the efficient photoreduction of nip .
all three systems were indeed efficient in 
 producing h2 during irradiation with visible light . consequently , 

the systems were studied and optimized by varying the ph value of 
 the aa solution and concentrations of rup and nip ( table 1 and tables s2s4 ( supporting information ) ) .
the photocatalytic 
 activity and longevity of h2 production was dependent on 
 the conditions and type of system employed . the photocatalytic performance 
 of nip generally varied in all three environments , and 
 the rate of photogenerated h2 was constant over at least 
 1 h in all experiments ( figure 7a ) .
the turnover 
 frequencies based on nip ( tofnip ) were calculated 
 from the amount of h2 accumulated in a photoreactor after 
 1 h irradiation ( tables s3 and s4 ) .
all samples were irradiated with 
 visible light ( am 1.5 g filter , 100 mw cm ,  
 > 420 nm , 25 c ) under an n2 ( 2% ch4 ) 
 atmosphere 
 and a standard solvent volume of 2.25 ml , leaving a gas headspace 
 volume of 5.59 ml .
standard screening samples were irradiated for 
 2 h in aa ( 0.1 m , ph 4.5 ) , and the tof was determined after 1 h irradiation .
homogeneous system with nip ( 0.1 mol ) , rup ( 0.3 mol ) , and 
 different 
 ph values .
rup ( 0.05 mol ) ; 
 in heterogenized systems tio2 or zro2 ( 2.5 mg 
 per sample ) nanoparticles were used as dispersions .
tofnip after 15 min visible 
 light irradiation reported due to rapid bleaching of eosin y. no h2 detected in gc 
 measurements ( limit of detection < 0.01% ) .
( a ) visible - light - driven generation of h2 in a homogeneous 
 aqueous system ( 0.1 m aa , ph 4.5 ) comprised of ( a ) nip ( 0.1 mol ) with different amounts of rup and 
 ( b ) rup ( 0.05 mol ) with low ( 0.02 mol , 
 squares ) and high loadings of nip ( 0.1 mol , circles ) . in a homogeneous rup - nip system , 

a tofnip value of 460  60 h was 
 observed when rup ( 0.05 mol ) was used with a low 
 amount of nip ( 0.02 mol ) in aqueous aa ( 0.1 m , 
 2.25 ml ) .
this rup - nip system was photoactive 
 for 2 h with a final tonnip value of 723  171 ( 15 
  3 mol of h2 ) .
an increasing amount of nip ( 0.1 mol ) resulted in a decreased initial tofnip concomitant with an increased system lifetime and a comparable 
 overall tonnip value of 651  30 ( 65  3 mol 
 of h2 ) after 30 h irradiation ( table 1 and figure 7b ) .
this observation might be 
 explained by the less likely double reduction of a single molecular 
 catalyst at high nip concentrations .
addition of rup or aa to a photodegraded system did not result in reactivation , 
 suggesting that decomposition of nip occurred after approximately 
 700 tons .
addition of nip to a deactivated homogeneous 
 system did result in reactivation , but full photoactivity was not 
 restored , indicating that photodegradation affected not only nip but also other system components such as rup .
indeed , photobleaching of rup became evident after 
 1 h irradiation by recording an electronic absorption spectrum of 
 the homogeneous solution after irradiation in a gastight quartz cuvette 
 under standard conditions ( figure s11 ( supporting 
 information ) ) .
several sets of control experiments were 
 carried out , which showed that no or only negligible amounts of h2 were produced in the absence of rup , nip , aa or light .
the presence of additional buffers such as citrate 
 and acetate did not impede the photocatalytic activity of the original 
 system .
replacement of nip by different ni salts such 
 as nicl2 and nibr2 in combination with 4 equiv 
 of a water - soluble phosphine ( [ 2-(dicyclohexylphosphino)ethyl]trimethylammonium 
 chloride ) resulted only in negligible amounts of h2 .
homogeneous 
 systems using commercially available dyes such as [ ru(bipy)3]cl2 ( tofnip = 220  
 20 h ) or the organic , noble - metal - free dye eosin 
 y ( used as the disodium salt ; tofnip = 189  31 h ) also resulted in efficient photocatalytic h2 production in an aqueous aa solution ( 0.1 m , ph 4.5 ) ( table 1 and table s6 ( supporting information ) ) .
the external 
 quantum efficiency ( eqe ) of the homogeneous photocatalytic system 
 with rup ( 0.3 mol ) and nip ( 0.1 mol ) 
 in aa ( 0.1 m , ph 4.5 ) was measured using an led light source (  
 460 nm , 5 mw cm ) . an eqe value of 9.7  1.2% 
 was determined after 2 h irradiation .
we note that this external yield 
 assumes that all photons emitted by the light source were absorbed 
 by rup and therefore represents a lower estimate of the 
 quantum efficiency of the system .
the photon - to - h2 quantum 
 yield observed for our homogeneous rup - nip system is 
 remarkable in comparison to other photocatalytic systems with molecular 
 3d transition - metal catalysts operating homogeneously or immobilized 
 in aqueous solution .
recently reported quantum yields for homogeneous 
 photocatalytic systems ranged from 0.23 to 0.6% for a co - pentapyridine 
 catalyst with [ ru(bpy)3 ] as photosensitizer in an aqueous system at neutral ph to 4.6% for 
 a cobaloxime - based h2 generation system with an al - porphyrin 
 dye in a water / acetone solvent system . in systems with a molecular catalyst immobilized on a solid - state 
 material , eqes of approximately 11.5% for a cobaloxime immobilized 
 on rup - sensitized tio2 and a ni(teoa)3 ( teoa = triethanolamine ) 
 complex on graphitic carbon nitride , respectively , 
 were reported .
eqes for self - assembled photocatalytic systems 
 using ni salts and 2-mercaptoethanol ( 24.5% ) or dihydrolipoic acid ( 36% qe )
ligands 
 are still benchmarks ; however , a defined catalytically active species 
 has not been reported for these systems .
various 
 homogeneous photocatalytic schemes have been reported for the reduction 
 of aqueous protons with molecular catalysts .
examples of efficient 
 photo - h2 generating systems include a molecular dye combined 
 with [ ni(p2n2)2]-type and 
 fe - based molecular catalysts , but in these systems 
 water / organic solvent mixtures were employed .
photocatalytic schemes 
 for h2 generation which operate in pure aqueous systems 
 are highly desirable . very recently , 

the water - insoluble [ ni(p2n2)2](bf4)2 was incorporated 
 into the photosynthetic protein photosystem i ( psi ) for photocatalytic 
 h2 production in aqueous solution , but only a very limited amount of ni catalyst can be loaded 
 onto dilute enzyme systems and the photostability of the hybrid assembly 
 remained an issue ( more than half of the photoactivity ceased after 
 30 min of irradiation ) .
nickel - thiolate and cobalt - dithiolene catalysts have 
 been reported to achieve several thousand turnovers in organic solvent / water 
 mixtures .
cobaloximes have been reported to evolve h2 in 
 a homogeneous system and on rup - sensitized tio2 in water , but the tofco value never exceeded 20 h and the photon - to - h2 efficiency was only 1% .
the higher 
 photocatalytic activity and eqe of nip in comparison 
 to those of cobaloxime catalysts may be related to the similar reduction 
 potentials required for both reduction steps in nip . 

we have previously shown that the photocatalytic activity of a cobaloxime 
 complex is significantly limited by its relatively slow second reduction 
 kinetics , attributed at least in part to the relatively unfavorable 
 energetics for this second reduction .
the use of a co - pentapyridine catalyst and a [ fefe]-hydrogenase mimic in 
 photocatalytic schemes in an aqueous solution resulted in tofs of 
 approximately 20 and 50 h , respectively .
thus , 
 our rup - nip system compares favorably with 
 previously reported systems in organic solvent free aqueous solution .
the immobilization of a molecular catalyst on a photoelectrode 
 is a prerequisite for fuel generation in a photoelectrochemical device , 
 and the attachment on a semiconductor material is particularly desirable . 

therefore , we extended our studies using nip for h2 generation in heterogeneous photocatalytic assemblies with 
 nanoparticle suspensions as a first step towards an electrode assembly .
we first determined the maximum loading capacity of rup(6c ) and nip on the metal oxide 
 particles by spectrophotometry .
approximately 0.05 mol of rup or nip can be immobilized per milligram of 
 tio2 or zro2 when adding an excess of phosphonated 
 catalyst ( see the supporting information ) .
the metal oxide nanoparticles were 
 loaded by the following procedure : nip was added to a 
 suspension of tio2 or zro2 in aqueous aa solution 
 ( 2.5 mg in 2.25 ml ) , and then rup was added .
nip was loaded first due to the 
 optimized geometry of adsorption of phosphonated bipyridine ligands 
 of rup .
the photoactivity 
 of the suspensions was studied under irradiation with visible light 
 ( am1.5 g , 100 mw cm ,  > 420 nm ) . upon 
 investigating 
 the heterogeneous photocatalytic systems , notably different trends 
 in performance
were observed in comparison to the homogeneous system 
 at lower nip loadings ( figure 8) .
visible - light - driven h2 evolution rate with different 
 amounts of nip and rup ( 0.05 mol ) : 
 ( triangle ) rup - nip ; ( square ) rup - tio2-nip ; ( circle ) rup - zro2-nip . in a rup - zro2-nip system , 
 a low amount of nip ( 0.02 mol ) with rup ( 0.05 mol ) on 2.5 mg of zro2 resulted in a tofnip value of 27  3 h. the results 
 obtained in tas measurements suggest that direct interaction of the 
 quenched dye ( rup ) with nip is required to drive the reaction . 

et between rup and nip  on the particle  
 as observed by the spectroscopic studies above becomes difficult under 
 such dilute conditions due to the spatial separation of the compounds 
 on the zro2 surface .
when the amount of nip added to zro2 is increased to 0.1 mol , a significant 
 enhancement in tofnip to 92  26 h was observed ( figure 8 and table s4 ( supporting information ) ) .
we note that , 
 at low concentrations of compounds carrying phosphonic 
 acid groups , attachment on zro2 or tio2 is not 
 quantitative , presumably due to competitive binding of aa to metal 
 oxides .
spectrophotometric studies indicate 
 that nip is almost quantitatively ( > 80% ) adsorbed 
 on 
 zro2 , whereas only approximately 10% of rup adsorbs on nip - modified zro2 ( table s7 ( supporting information ) ) . when the zro2 nanoparticles were loaded with nip ( 0.02 mol ) 
 and rup ( 0.05 mol ) , separated by centrifugation , 
 and redispersed in fresh aa ( 0.1 m , ph 4.5 ) , the amount of h2 produced was 0.16 mol after 1 h irradiation ( in comparison 
 to 0.54 mol of h2 before centrifugation ) .
thus , 
 the photodriven h2 production in bulk experiments can best 
 be described as a mixture of et between surface - immobilized catalysts 
 through an  on particle  mechanism and from solubilized rup to surface - bound nip .
a rup - tio2-nip system displays 
 a tofnip value of 51  7 h at 
 a low nip loading of 0.02 mol on 2.5 mg of tio2 ( table 1 ) .
the rate of h2 production reached a maximum with a nip loading of 
 0.1 mol , whereupon a tofnip value of 72  5 
 h and an overall tonnip value of 278 
  19 h ( after 30 h ) was obtained ( table s5 
 ( supporting information ) ) . under 
 these conditions , at least 80% of nip and more 
 than 20% of rup are attached on tio2 , as measured 
 by spectrophotometry ( figures s12 and s13 ( supporting 
 information ) ) . once nip or rup
is 
 bound to the tio2 surface , it can not easily be removed 
 from the solid - state material .
redispersion of loaded particles in 
 a fresh aa solution did not result in the detection of significant 
 amounts of rup or nip in solution .
loading 
 the tio2 nanoparticles with nip ( 0.02 mol ) 
 and rup ( 0.05 mol ) , centrifugation , and resuspension 
 in fresh aa ( 0.1 m , ph 4.5 ) resulted in a tofnip value 
 of 24 h during irradiation .
thus , 50% of the photocatalytic 
 activity remained , thereby establishing the importance of the role 
 of the conduction band and the  through particle 
this experiment and the significantly 
 higher rate of photocatalytic h2 production for rup - tio2-nip in comparison to rup - zro2-nip at very low loading of the particles 
 with nip ( table 1 ) support a preferential 
  through particle 
et mechanism for tio2 : 
 a mechanism which does not require the direct electronic communication 
 of rup and nip as needed on zro2 .
these results show that photocatalytic h2 production 
 can be achieved with a dubois - type catalyst attached on a solid - state 
 semiconductor .
however , to this point limitations by the loading capacity 
 of materials used and by competitive binding of electron donor or 
 electrolyte are still being faced when investigating such hybrid materials . 

work is in progress to overcome these limitations by investigating 
 the binding modes of the molecular components and photocatalytic activity 
 of rup - nip systems on thin films and electrode 
 materials .
in summary , we describe 
 a novel [ ni(p2n2)2]-type 
 h2 evolution catalyst ( nip ) , which is soluble 
 in water and can be immobilized on metal oxide surfaces .
nip is electrocatalytically active in organic solvent free aqueous solution 
 and evolves h2 with an onset potential of only 0.48 
 v vs nhe under mild conditions ( ph 4.5 ) .
photocatalytic and spectroscopic 
 studies were performed with nip in three different systems 
 in a purely aqueous solution containing aa .
a homogeneous rup - nip system operates through reductive quenching of rup * in solution .
the heterogeneous rup - zro2-nip system shows the same et mechanism , and 
 zro2 acts merely as a matrix to retain the attached molecules 
 closely together , hence facilitating et  on the particle  . 
 in rup - tio2-nip
mechanism , where rup * is oxidatively quenched upon injection of an electron into the conduction 
 band of tio2 , which can subsequently be harvested by nip .
a high tof value of 460  60 h for light - driven 
 h2 evolution with a molecular 3d transition metal catalyst 
 in pure aqueous solution was obtained , with tons of approximately 
 700 for nip .
advanced spectroscopic methods ( tc - spc and 
 tas ) confirmed that directed et from rup to nip occurs efficiently in all systems on the nano- to microsecond time 
 scale .
losses due to charge recombination are minimized , as et occurs 
 efficiently within the lifetimes of the excited species .
the highly 
 efficient et from dye to proton reduction catalyst is also reflected 
 in the high photon to h2 quantum yield of the homogeneous 
 system of almost 10% in the presence of the sacrificial electron donor 
 aa .
work is in progress to assemble a photoelectrode with nip for use in a photoelectrochemical water splitting cell .
 the generation of renewable h2 through an efficient 
 photochemical route requires photoinduced electron transfer ( et ) from 
 a light harvester to an efficient electrocatalyst in water . here , 
 we report on a molecular h2 evolution catalyst ( nip ) with a dubois - type [ ni(p2rn2r)2]2 + core ( p2rn2r = bis(1,5-r-diphospha-3,7-r-diazacyclooctane ) , 
 which contains an outer coordination sphere with phosphonic acid groups . 
 
 the latter functionality allows for good solubility in water and immobilization 
 on metal oxide semiconductors . 
 electrochemical studies confirm that nip is a highly active electrocatalyst in aqueous electrolyte 
 solution ( overpotential of approximately 200 mv at ph 4.5 with a faradaic 
 yield of 85  4% ) . 
 photocatalytic experiments and investigations 
 on the et kinetics were carried out in combination with a phosphonated 
 ru(ii ) tris(bipyridine ) dye ( rup ) in homogeneous and 
 heterogeneous environments . 
 time - resolved luminescence and transient 
 absorption spectroscopy studies confirmed that directed et from rup to nip occurs efficiently in all systems 
 on the nano- to microsecond time scale , through three distinct routes : 
 reductive quenching of rup in solution or on the surface 
 of zro2 (  on particle  system ) or oxidative 
 quenching of rup when the compounds were immobilized 
 on tio2 (  through particle 
  system ) . 
 our 
 studies show that nip can be used in a purely aqueous 
 solution and on a semiconductor surface with a high degree of versatility . 
 a high tof of 460  60 h1 with a ton of 723 
  171 for photocatalytic h2 generation with a molecular 
 ni catalyst in water and a photon - to - h2 quantum yield of 
 approximately 10% were achieved for the homogeneous system . 
Introduction
Experimental Section
Results and Discussion
Conclusions