there is increasing 
 research interest in the area of solar fuels driven by growing global 
 issues centered around the supply and demand of depleting energy resources . solar fuels provide the potential to convert 
 solar energy into stored energy with applications in transportable 
 fuels and chemical feedstocks . there are several strategies to achieve 
 this including the photocatalytic splitting of water for the production 
 of dihydrogen ( h2 ) and the 
 reduction of carbon dioxide ( co2 ) . traditionally , photochemical h2 production has been achieved 
 through the use of a photosensitizer ( ps ) and a sacrificial electron 
 donor coupled with a catalyst able to reduce protons to h2 ( scheme 1 ) . in this 
 scheme the absorption of light by ps generates an excited state ( ps * ) , 
 which can be reduced by a suitable electron donor ( ed ) to form the 
 reduced photosensitizer , ps . electron transfer 
 to the catalyst then generates the intermediates required for the 
 catalytic reduction of protons . of particular note in this area was 
 the recent work from eisenberg and co - workers , who used a heterogeneous 
 cdse nanocrystal ps to deliver electrons to a nickel - based homogeneous 
 catalyst in water , resulting in continuous h2 production 
 with little drop in activity over 360 h of photocatalysis . in addition to the bimolecular systems described 
 in scheme 1 , photosensitizing 
 units have been successfully linked to catalysts and electron acceptors 
 in an effort to facilitate and direct electron transfer and control 
 charge recombination . research into the utilization 
 of homogeneous photocatalytic systems continues to be investigated 
 widely , and a range of biomimetic catalysts , inspired by the active 
 sites of the hydrogenases , have been studied . these 
 enzymes are naturally occurring catalysts for the interconversion 
 of protons and h2 , with high turnover efficiency at nearly 
 neutral ph in aqueous environments . synthetic 
 efforts have focused on the active sites of the [ fefe ] hydrogenase 
 with complexes such as [ fe2(co)4x(l)(l ) ] ( where x is a dithiolate bridge , and l and l are either carbonyl or phosphine derived ligands ) 
 acting as the catalytic center , with ru- , ir- , or re - derived photosensitizers . the ground state and excited - state properties 
 of [ fefe ] hydrogenase model compounds have been studied in detail 
 using a range of ir spectroscopies at a variety of excitation wavelengths . following excitation , co loss with geminate or diffusion recombination , 
 as well as photoproducts assigned to the breaking and recombination 
 of the fe  fe bond , have been observed . photosensitizing units have been attached directly to the [ fefe ] 
 units , either covalently through a dithiolate bridge integral to the 
 [ fefe ] complex or through ligand substitution reactions at the [ fefe ] 
 center . steady - state and time - resolved 
 spectroscopic studies have probed the photocatalytic mechanisms in 
 linked and bimolecular systems , focusing on electron transfer to the 
 catalyst . transient absorption ( ta ) spectroscopy has monitored electron transfer 
 in bimolecular systems from photogenerated [ ru(bpy)3 ] to [ fe2(co)4x(l)(l ) ] in ch3cn ( x = ( -sch2)2ch2 , l = l = co ; x = ( -sch2)2nch2c6h5 , l = l = co or l = l = p(pyr)3 or l = co l = p(pyr)3 ; x = -cl2bdt , l = l = co ; pyr = n - pyrrolyl ; bdt = 3,6-dichlorobenzene-1,2-dithiolate ) . time - resolved measurements have 
 also been used to study electron transfer in linked systems . ta spectroscopy 
 was used to observe electron transfer in a self - assembled system consisting 
 of an [ fefe ] hydrogenase mimic functionalized with a pendant pyridine 
 group , linked to a zinc porphyrin photosensitizer . similarly , ta spectroscopy has been used to study a zinc 
 porphyrin covalently linked to an [ fefe ] hydrogenase model complex , 
 where rapid electron transfer from the zinc porphyrin to the catalyst 
 was monitored (  = 24  1 ps ) followed by charge recombination 
 (  = 57  1 ps ) in ch2cl2 . time - resolved infrared ( trir ) spectroscopy 
 was used to examine electron transfer in the analogous self - assembled 
 system . again , electron transfer from the photoexcited porphyrin to 
 the catalyst was observed (  = 40  3 ps ) with charge recombination 
 (  = 205  14 ps ) significantly slower than in the covalently 
 linked system (  = 57  1 ps ) . however , turnover numbers ( ton ) in linked systems are generally 
 lower or comparable to the analogous bimolecular , or separated , systems . for example , in a linked dyad containing an 
 iridium - based cyclometalated photosensitizer covalently linked to 
 an [ fefe ] hydrogenase mimic , a maximum ton of 127 was reported , which 
 is similar to the analogous separated system ( ton = 138 ) . we recently reported a detailed mechanistic 
 study into photocatalytic h2 production from an bimolecular 
 system using the photosensitizer [ recl(co)3(bpy ) ] and the 
 catalyst [ nife2 ] ( figure 1 ) , an analogue of the active site of the [ nife ] hydrogenase , which catalyzes h2 production . in 
 this paper we report the synthesis of a linked dyad containing a re 
 diimine based photosensitizer covalently linked to [ nife2 ] , [ re]-[nife2 ] ( figure 1 ) . we investigate the photophysical properties of [ re]-[nife2 ] and monitor excited state formation , electron transfer processes 
 and h2 production . we compare the results for [ re]-[nife2 ] with an analogous bimolecular system , [ re]/[nife2 ] ( a mixture of [ recl(co)3(ch3-bpy - cooch3 ) ] and [ nife2 ] , figure 1 ) to evaluate the effects of covalently linking 
 the [ re ] and [ nife2 ] units together and to provide valuable 
 insight for the development of similar catalytic systems . structures 
 of [ nife2 ] , [ nife2]oh , [ recl(co)3(ch3-bpy - coor ) ] ( r = h or ch3 ) , and 
 [ re]-[nife2 ] . reagents were used as received from the suppliers ( sigma - aldrich , 
 acros organic , and fluka ) , and all the reactions , manipulations , and 
 transfers were performed under ar using standard schlenk techniques , 
 unless stated otherwise . for trir , luminescence , and h2 production experiments , acetonitrile ( 99.9% , merck ) and net3 ( > 99.5% , sigma - aldrich ) were distilled under an inert 
 atmosphere of ar from cah2 . for electrochemical measurements , trifluoroacetic acid ( tfa ) and cdcl3 were degassed 
 with ar , stored over 4  molecular sieves , and used without further 
 purification . n(ch2ch2oh)3 ( teoa ; 
 98% , alfa aesar ) was dried under reduced pressure ( 160 c , 1 
  10 mbar ) for 24 h before being stored under 
 ar . [ recl(co)3(bpy ) ] , [ nife2 ] ( figure 1 ) , [ recl(co)3(ch3-bpy - cooh ) ] ( ch3-bpy - cooh = 4-methyl-2,2-bipyridine-4-carboxylic 
 acid ) and [ recl(co)3(ch3-bpy - cooch3 ) ] ( figure 1 , ch3-bpy - cooch3 = 
 4-methyl-2,2-bipyridine-4-carboxylate ) were prepared 
 according to adapted literature methods . elemental analyses were performed 
 by the london metropolitan university ( carlo erba ce1108 elemental 
 analyzer ) . nmr spectra were recorded on a brker dpx 300 spectrometer 
 running at 300.13 mhz for h and at 75.42 mhz for c experiments . electrospray ( es ) mass spectrometric data 
 were recorded by the mass spectrometry service at the university of 
 nottingham using a brker daltonics microtof electrospray mass 
 spectrometer . x - band electron paramagnetic resonance ( epr ) spectra 
 were recorded on a brker emx spectrometer . ftir spectra were recorded in 
 solution cells ( harrick scientific products , inc . ) with caf2 windows using a nicolet 6700 ftir spectrometer , typically at 2 cm resolution . path lengths of 0.25 , 0.39 , 0.5 , or 1 
 mm were used , and all solutions were prepared under an inert atmosphere 
 of ar and degassed by three freeze  pump  thaw cycles . naome ( 0.359 g , 6.64 mmol ) was added to a solution of toluene-3,4-dithiol 
 ( tdt ; 0.520 g , 3.32 mmol ) in anhydrous meoh ( 30 cm ) under 
 ar . the pink solution was stirred for 1 h before dropwise addition 
 of a concentrated solution of [ ni(oac)2.4(h2o ) ] ( 0.414 g , 1.66 mmol ) in meoh ( 10 cm ) . the dark brown 
 solution was stirred for 1 h , after which time a concentrated solution 
 of 1,3-dibromopropan-2-ol ( 0.542 g , 2.49 mmol ) in meoh ( 10 cm ) was added . the reaction mixture was stirred for 24 h at 
 room temperature , after which time a light green precipitate formed . the powder was collected by filtration and washed with cold anhydrous 
 meoh ( 2  5 cm ) and anhydrous isohexane ( 3  
 5 cm ) and dried in vacuo . chromatography of the crude 
 material on silica gel ( fluka , f60 ) using ch2cl2 as eluent yielded one mobile colored fraction , which was isolated . evaporation of this solution to dryness yielded a light green powder , 
 [ ni]oh . yield 0.485 g , 69% . elemental analysis : found ( calculated 
 for c17h18o1s4 ni1 ) c , 48.53 ( 48.01 ) ; h , 4.63 ( 4.27)% . ms ( es ) m / z : 425 ( m + h ) , 447 ( m + 
 na ) . h nmr ( 300 mhz , deuterated dimethyl sulfoxide 
 ( dmso - d6 ) , 298 k ) : h 7.386.82 ( 6h , m , ph - h ) , 4.81 ( 1h , 
 s , oh ) , 3.943.92 ( 1h , m , ch ) , 3.833.50 ( 4h , m , sch2 ) , 2.23 ( 6h , s , ph c nmr ( 75 mhz , dmso - d6 , 298 k ) : c 132.68124.50 ( m , ph - c ) , 64.12 ( s , c - o ) , 
 37.27 ( s , sc ) , 20.9220.36 ( m , ir ( solid ) : 3260 ( s ) , 2919 ( s ) , 
 2863 ( s ) , 2153 ( w ) , 2011 ( w ) , 1715 ( s ) , 1606 ( s ) , 1579 ( s ) , 1512 ( m ) , 
 1460 ( vs ) , 1377 ( s ) , 1326 ( m ) , 1280 ( s ) , 1253 ( vs ) , 1196 ( m ) , 1169 
 ( s ) , 1111 ( vs ) , 1044 ( vs ) , 866 ( s ) , 802 ( vs ) , 772 ( m ) , 689 ( m ) , 638 
 ( m ) , 577 ( w ) , 563 ( w ) , 535 ( s ) , 521 ( vs ) cm . a mixture of [ ni]oh 
 ( 0.301 g , 0.73 mmol ) and fe3(co)12 ( 0.339 g , 
 0.67 mmol ) in anhydrous ch2cl2 ( 40 cm ) was heated to reflux for 16 h under ar . the reaction mixture was 
 subsequently cooled to room temperature , and the dark brown / red solution 
 was filtered to remove a dark brown precipitate . the precipitate was 
 washed with ch2cl2 ( 3  5 cm ) , and the washings were combined with the filtrate . concentration 
 of the filtrate under reduced pressure afforded a dark brown solid , 
 which was washed with anhydrous isohexane ( 3  5 cm ) and dried in vacuo . chromatography of the crude material on silica 
 gel ( fluka , f60 ) using ethyl acetate and petroleum ether ( bp 6080 
 c ; 3/2 , v / v ) as eluent yielded two fractions . a fast - moving 
 weak orange band was identified as [ fe2(co)6tdt ] , an analogue of [ fe2(co)6bdt ] ( bdt = benzene-1,2-dithiolate ) previously reported by 
 perra et al . the second slow - moving brown 
 band was isolated , and removal of the solvent under reduced pressure 
 afforded a black powder , the target complex [ nife2]oh . 
 yield 0.293 g , 62% . elemental analysis : found ( calculated for c23h18o7s4ni1fe2 ) c , 39.98 ( 39.12 ) ; h , 3.12 ( 2.57)% . ms ( es ) m / z : 727 ( m + na ) . h nmr ( 400 mhz , cdcl3 , 298 k ) : h 8.177.20 
 ( 6h , m , ph - h ) , 3.893.84 ( 1h , m , ch ) , 3.74 ( 1h , m , sch2 ) , 3.383.31 ( 1h , m , sch2 ) , 2.992.79 ( 2h , m , sch2 ) , 2.582.49 ( 6h , s , ph c nmr ( 100 mhz , cdcl3 , 298 k ) : c 211.92 
 ( s , co ) , 139.73128.85 ( m , ph - c ) , 66.14 ( s , c - o ) , 44.1842.99 
 ( m , sc ) , 21.0620.86 ( m , solution ir ( ch2cl2 ) : co 2037 ( s ) , 1997 ( s ) and 1958 ( s ) cm . 4-dimethylaminopyridine 
 ( dmap , 10 mg , 0.08 mmol ) and n , n-dicyclohexylcarbodiimide ( dcc , 62 mg , 0.3 mmol ) were added 
 to a solution of [ nife2]oh ( 67 mg , 0.1 mmol ) and 
 recl(co)3(ch3-bpy - cooh ) ( 115 mg , 0.22 mmol ) 
 in ch2cl2 ( 40 ml ) at 0 c , and the reaction 
 was stirred for 48 h at room temperature . the resulting suspension 
 was filtered , and the filtrate was evaporated to dryness to yield 
 the crude product , [ re]-[nife2 ] , which was purified by 
 column chromatography on silica gel ( fluka , f60 ) using ethyl acetate 
 and petroleum ether ( bp 6080 c ; 2/1 , v / v ) as eluent . the intensely colored slow - moving red / brown band was isolated , and 
 removal of the solvent under reduced pressure afforded the target 
 complex [ re]-[nife2 ] . yield 55 mg , 48% . elemental analysis : 
 found ( calculated for c38h26clfe2n2nio11res4 ) c , 37.78 ( 37.82 ) ; 
 h , 2.19 ( 2.17 ) ; n , 2.36 ( 2.32)% . ms ( es ) m / z : 1250.7685 [ m + hcoo ] , 1240.7342 [ m + cl ] . the adduct [ m + cooh ] shows an isotope pattern in good agreement with the 
 theoretical value ( figure s1 ) . h nmr ( 300 mhz , cdcl3 , 298 k )  : 2.39 ( m , 9h , ch3 ) , 3.04 ( m , 2h , sch2chlch2s ) , 3.72 ( m , 
 2h , sch2chlch2s ) , 4.11 ( m , 1h , sch2chlch2s ) , 6.719.38 ( m , 12h , ph - h and py - h ) ppm . c nmr ( 75 mhz , 
 cdcl3 , 298 k )  : 21.83 ( m , ch3 ) , 33.96 ( s , sch2chlch2s ) , 49.19 ( s , sch2chlch2s ) , 118.78156.05 ( m , ph - c and py - c ) , 212.21 ( s , co ) ppm . solution ir : ( ch2cl2 ) 
 (co ) 2037 ( s ) , 2023 ( s ) , 1997 ( s ) , 1957 ( s ) , 1920 ( s ) 
 and 1900 ( s ) cm , (c = o ) 1737 ( w ) 
 cm ; ( ch3cn ) (co ) 2036 
 ( s ) , 2023 ( s ) , 1996 ( s ) , 1957 ( s ) , 1919 ( s ) , 1901 ( s ) cm , (c = o ) 1742 ( w ) cm . picosecond and nanosecond trir spectra 
 were obtained using purpose - built equipment based on a pump details of the equipment and methods used for the trir studies have 
 been described previously . the results 
 were repeated on both instruments , but the data shown in this paper 
 were obtained using the latter , which used a seeded dual titanium 
 sapphire chirped pulse amplifier to generate 10 khz , 4080 
 fs , 0.8 mj pulses at 800 nm . the beam was focused and split into 0.4 
 mj beams , which were used to pump a number of different computer - controlled 
 optical paramagnetic amplifiers ( opas ) . these were able to output 
 a range of synchronized wavelength combinations for use as either 
 pump or probe beams with a 240 to 15  000 nm spectral coverage . a high - precision translation stage ( newport ims600lm ) provided a delay 
 line for the pump beam and generates femtosecond to nanosecond pump once through the sample the ir beam was collimated and focused 
 onto a spectrograph where a 128 mercury cadmium telluride array detector 
 was used to detect the mid - ir probe . access to the nanosecond - to - microsecond 
 time delays was provided by a nanosecond laser system , delivering 
 1064 nm pulses ( 10 khz , 1 ns , 0.05 mj ) . pump energies of 1 j 
 were typically used in experiments reported in this paper . the 355 
 nm harmonic was used , delivering > 5 j energy per pulse , 
 synchronized to the probe beam of the laser system described above . samples were recirculated using a peristaltic pump as the sample was 
 rastered in the x and y planes to 
 minimize cumulative photodamage to both the sample and window during 
 repeated photolysis . to analyze the reaction kinetics of transient 
 species , the spectra were fitted using the minimum number of lorentzian 
 bands . in experiments containing [ re]/[nife2 ] , the intermediate 
 growth and decay of the bands associated with [ recl(co)3(ch3-bpy - cooch3 ) ] overlap 
 with either the parent bleaches of [ nife2 ] or the transient 
 peaks of [ nife2 ] . as a result , the kinetics 
 of [ recl(co)3(ch3-bpy - cooch3 ) ] was measured from the regrowth of the parent 
 bands of [ recl(co)3(ch3-bpy - cooch3 ) ] , as multiple band fitting resulted in more tractable data 
 analysis . for the same reason , the lifetime of [ re]-[nife2 ] was measured from the regrowth of the parent 
 bands of the [ re ] moiety in [ re]-[nife2 ] . an edinburgh instruments fls920 combined fluorescence 
 lifetime and steady - state spectrometer was used for all emission measurements . samples were held in a l  1 cm quartz fluorescence cuvette and 
 either degassed using three freeze  pump  thaw cycles 
 on a schlenk line or prepared in a glovebox . excited - state lifetime 
 measurements were performed using the time - correlated photon counting 
 method , with a pulsed diode laser ( epl-405 , edinburgh instruments ) 
 as an excitation source . cyclic voltammograms 
 were performed using a three - electrode system , with a glassy carbon 
 working electrode ( 6.7 mm diameter ) , a pt wire secondary electrode , 
 and a saturated calomel reference electrode . before each measurement all potentials are referenced to the fc / fc ( fc = 
 ferrocene ) couple , used as an internal standard . . 1 mm ) in ch2cl2 were recorded with [ nbu]4[bf4 ] ( 0.4 m ) as a supporting electrolyte . coulometric measurements 
 were performed using an h - cell at 273 k in ch2cl2 containing [ nbu4][bf4 ] ( 0.4 m ) ; the cell consisted 
 of a pt / rh gauze basket working electrode separated by a glass frit 
 from a pt / rh gauze secondary electrode . the saturated calomel reference 
 electrode was placed in a bridge tube containing electrolyte solution , 
 the tip was positioned at the center of the working electrode , and 
 the solution was stirred rapidly during electrolysis using a magnetic 
 stirring bar . catalytic cyclic voltammetry experiments were conducted 
 using a sealed single - compartment electrochemical cell purged with 
 ar . additions of tfa ( 0.1 ml , 0.5 m in ch2cl2 ) to a solution of [ re]-[nife2 ] or [ nife2 ] 
 ( 5 ml , 1 mm in ch2cl2 ) with [ nbu4][bf4 ] ( 0.4 m ) were made using a gastight syringe ( hamilton 
 tll , series 1700 , model 1702 and 1705 ) . vis spectro - electrochemical 
 experiments were performed either at 273 or 243 k with an optically 
 transparent electrode inserted into a modified quartz cuvette with 
 a path length of 0.5 mm . a three - electrode configuration consisting 
 of a pt / rh gauze working electrode , a pt wire secondary electrode 
 ( in a fritted poly(tetrafluoroethylene ) sleeve ) , and a saturated calomel 
 electrode ( chemically isolated from the test solution via a bridge 
 tube containing electrolyte solution and terminated in a porous frit ) 
 was used in the cell . vis 
 spectra were recorded on a perkinelmer lambda 16 spectrophotometer 
 at 1 nm resolution . the spectrometer cavity was purged with n2 , and temperature control at the sample was achieved by flowing 
 cooled n2 across the surface of the cell . ftir and epr 
 spectroscopic measurements were run on aliquots of solutions following 
 electrochemical bulk reduction . h2 production experiments were conducted on purpose - built 
 apparatus , the details of which have been reported previously . irradiation was performed using a xe arc lamp 
 ( oriel instruments ,  < 420 nm cutoff filter ) operating at 
 250 w and h2 was detected on a gas chromatograph ( shimadzu 
 2014 ) using a thermal conductivity detector operating at 50 c . the cyclic voltammograms of [ re]-[nife2 ] ( 1 mm ) in ch2cl2 ( figure 2 ) show two reduction processes at e1/2 = 1.23 v and e1/2 = 1.55 
 v versus fc / fc . the process at e1/2 = 1.23 v versus fc / fc is an electrochemically 
 reversible one - electron process and may be assigned to the reduction 
 of the [ nife2 ] unit in [ re]-[nife2 ] by comparison 
 with the reduction process of [ nife2 ] ( e1/2 = 1.31 v vs fc / fc ) . the second process at e1/2 = 1.55 v versus fc / fc occurs at a comparable 
 potential to [ recl(co)3(bpy ) ] and similar functionalized 
 bipyridine compounds , and it is assigned to the reduction of the diimine 
 ligand within the [ re ] unit . coulometric studies 
 show that this reduction process exhibits multielectron behavior , 
 which may result from the instability of the diimine radical under 
 the conditions of the experiment . the 
 cyclic voltammogram of [ re]-[nife2 ] at 100 mv s shows additional reduction processes at potentials more negative 
 than 1.9 v versus fc / fc ( figure s2 ) . the reduction processes of [ re]-[nife2 ] at e1/2 = 1.23 v and e1/2 = 1.55 v versus fc / fc 
 were investigated further with uv  vis , ftir , and epr spectroscopic 
 methods . cyclic voltammogram of [ re]-[nife2 ] ( 1 mm ) in ch2cl2 containing [ nbu4][bf4 ] ( 0.4 
 m ) as a supporting electrolyte showing the two reduction processes 
 at e1/2 = 1.23 v and e1/2 = 1.55 v vs fc / fc . the one - electron reduction process at e1/2 = 1.23 v versus fc / fc of [ re]-[nife2 ] ( 1 mm ) in ch2cl2 was monitored by uv vis 
 spectroelectrochemistry at 243 k. during the reduction the intensity 
 of the peaks at 372 , 485 , and 700 nm decrease with the simultaneous 
 growth of new absorbances at 364 , 420 , 497 , and 620 nm ( figure 3a and table s1 ) . these new bands are assigned to the reduction of the [ nife2 ] center to form [ re]-[nife2 ] through comparison with previously reported spectroelectrochemical 
 studies on [ nife2 ] . the presence 
 of isosbestic points ( figure 3a and table s1 ) are diagnostic 
 of a conversion between [ re]-[nife2 ] and [ re]-[nife2 ] occurring with no long - lived intermediates 
 on the time scale of the experiment . reoxidation of [ re]-[nife2 ] resulted in the quantitative regeneration 
 of the uv  vis spectrum of [ re]-[nife2 ] , and thus , 
 the reduction process is chemically reversible on the time scale of 
 the experiment . the second reduction process at e1/2 = 1.55 v versus fc / fc ( figure 2 ) was investigated 
 by in situ bulk reduction of [ re]-[nife2 ] at 1.65 
 v versus fc / fc ( 243 k , figure 3b ) . this reduction process is associated 
 with the development of bands at 374 , 488 , 520 , and 699 nm ( figure 3b , table s1 ) , and these bands are assigned to reduction processes 
 that are localized on both the bipyridine ligand of the [ re ] unit 
 and the [ nife2 ] moiety to generate [ re]-[nife2 ] . electrochemical oxidation 
 of this solution generated an almost identical profile to [ re]-[nife2 ] but with a decreased spectral intensity at ca . this 
 difference is most likely due to the instability of the reduced bipyridine 
 radical about the [ re ] center observed previously in the coulometric 
 experiments ( see above ) . vis spectra observed during the electrochemical 
 reduction of [ re]-[nife2 ] ( 1 mm ) in ch2cl2 ( containing 0.4 m [ nbu4][bf4 ] as a 
 supporting electrolyte ) at 243 k. ( a ) reduction of [ re]-[nife2 ] ( black ) to [ re]-[nife2 ] ( red ) . ( b ) reduction of [ re]-[nife2 ] ( black ) to [ re]-[nife2 ] ( blue ) . the ftir spectrum 
 of [ re]-[nife2 ] ( 1 mm ) in ch2cl2 shows 
 six distinct peaks ( figure 4 ) in the metal carbonyl region , which can be assigned to the 
 [ re ] ( 2023 , 1920 , and 1900 cm ) and [ nife2 ] ( 2037 , 1997 , and 1957 cm ) units . one electron 
 reduction at 1.23 v versus fc / fc is associated 
 with a decrease in intensity of bands at 2037 , 1997 , and 1957 cm and the growth of new bands at 1972 , 1925 , and 1890 
 cm ( figure 4 , green line ) , which are associated with a one - electron reduced 
 [ nife2 ] unit . the down shift of ca . 67 cm for the co bands in [ re]-[nife2 ] relative 
 to those in [ re]-[nife2 ] is in good agreement with shifts 
 reported for other [ nife2 ] centers and is indicative of 
 an increased electron density on the fe centers . a small reduction in the intensity of a band associated with the 
 [ re ] unit ( 2025 cm ) can also be observed in the 
 spectrum along with the growth of a band at 2020 cm . it appears that during bulk electrolysis partial reaction at the 
 [ re ] center occurs . x - band epr spectra of electrochemically generated 
 [ re]-[nife2 ] were recorded as fluid 
 ( 293 k ) and frozen ( 77 k ) solutions ( figure 5 ) . the simulated hamiltonian parameters ( giso = 2.043 ; g11 = 2.014 ; g22 = 2.018 ; and g33 = 2.098 ) are in good agreement with the those reported 
 for [ nife2 ] ( giso = 2.042 ; g11 = 2.099 ; g22 = 2.015 ; and g33 = 2.015 ) . ftir spectra of [ re]-[nife2 ] before and after bulk reduction to [ re]-[nife2 ] in ch2cl2 ( containing 
 0.4 m [ nbu4][bf4 ] as a supporting electrolyte ) 
 at 293 k. ( a ) ftir spectrum of [ re]-[nife2 ] . ( b ) ftir difference 
 spectrum recorded following the reduction of [ re]-[nife2 ] to [ re]-[nife2 ] . the electrocatalytic activity 
 of the first reduction process of [ re]-[nife2 ] ( e1/2 = 1.23 v vs fc / fc ) was 
 investigated using the same procedure that has been employed previously 
 for [ nife2 ] and related compounds . the cyclic voltammogram of [ re]-[nife2 ] shows an increased 
 cathodic peak current and a reduction in the current of the anodic 
 wave ( figure s3 ) , as aliquots of tfa are 
 added to a solution of [ re]-[nife2 ] ( 1 mm ) in ch2cl2 ( containing 0.4 m [ nbu4][bf4 ] as a supporting electrolyte ) . this reduction in the current of 
 the anodic wave is consistent with the removal of [ re]-[nife2 ] from the diffusion layer of the working electrode 
 by a method other than that of heterogeneous electron transfer to 
 the electrode and is diagnostic of an electrocatalytic process . the electrochemical and spectrochemical 
 results are consistent with a first reduction process that is localized 
 at the [ nife2 ] center and a second process that is localized 
 at the [ re ] center in [ re]-[nife2 ] . thus , the photoreduction 
 of the [ re ] unit could provide a route to the photochemical production 
 of [ re]-[nife2 ] , a desired intermediate 
 required for catalytic h2 formation . x - band epr spectra in 
 ch2cl2 ( black ) and the simulated spectra ( red ) 
 for [ re]-[nife2 ] ( a ) as a fluid solution 
 at 293 k ( simulation parameters : giso = 
 2.043 ; wiso = 20 g ) and ( b ) as a frozen 
 solution at 77 k ( simulation parameters : g11 = 2.014 ; g22 = 2.018 ; g33 = 2.098 ; w11 = 11 g ; w22 = 12 g ; w33 = 
 14 g ) . the excited - state properties of the linked dyad [ re]-[nife2 ] and of the analogous uncoupled [ re]/[nife2 ] system 
 were examined by luminescence spectroscopy . luminescence spectra of 
 solutions containing [ re]-[nife2 ] , [ recl(co)3(ch3-bpy - cooch3 ) ] , [ nife2 ] , and 
 [ re]/[nife2 ] ( each at 17 m ) in ch3cn 
 are shown in figure 6 ( ex = 406 nm ) . luminescence spectra for [ re]/[nife2 ] , [ recl(co)3(ch3-bpy - cooch3 ) ] , [ re]-[nife2 ] , and [ nife2 ] ( each at 17 m ) 
 in ch3cn ( ex = 406 nm ) . the observed emission for each solution ( except 
 [ nife2 ] ) was normalized to the absorbance at 406 nm in 
 the uv  vis spectrum . for solutions of [ re]/[nife2 ] and [ re]-[nife2 ] the spectral bands of [ nife2 ] were subtracted before normalization . [ recl(co)3(ch3-bpy - cooch3 ) ] exhibits a broad emission ( max = 678 nm ,  = 25  0.5 ns , figure 6 , green line ) , and the addition 
 of an equimolar amount of [ nife2 ] to the solution resulted 
 in no observable drop in the emission intensity or lifetime of the mlct excited state ( [ re]/[nife2 ] , figure 6 , black line ) . 17 m , [ nife2 ] does 
 not quench the mlct excited state of [ recl(co)3(ch3-bpy - cooch3 ) ] to a detectable extent . in 
 contrast , the emission from [ re]-[nife2 ] is ca . 60% lower 
 in intensity ( max = 650 nm ,  = 24  
 0.5 ns , figure 6 , red 
 line ) when compared to [ re]/[nife2 ] . the drop in emission 
 intensity could suggest quenching of the [ re]-based excited state 
 in [ re]-[nife2 ] by the covalently bound [ nife2 ] moiety . however , the lifetimes are not significantly different , 
 and the emission maximum for [ re]-[nife2 ] is at a higher 
 energy than for [ recl(co)3(ch3-bpy - cooch3 ) ] . we estimated the quenching rate constant ( kq ) for the reductive quenching of the mlct excited states of [ re]-[nife2 ] and [ recl(co)3(ch3-bpy - cooch3 ) ] by net3 , through construction of stern the second - order quenching rate constants 
 are kq = 4.3  0.2  10 m s ( figure s4 , black dots ) and kq = 1.4  0.1  10 m s ( figure s4 , black 
 squares ) for the mlct excited state of [ recl(co)3(ch3-bpy - cooch3 ) ] and [ re]-[nife2 ] , respectively . we used picosecond- and nanosecond - trir spectroscopies to probe 
 the mechanisms of the decay of the excited states in [ re]-[nife2 ] and [ re]/[nife2 ] in ch3cn , where the 
 reaction kinetics can be elucidated and used to monitor electron transfer 
 processes . the ftir spectrum of [ re]-[nife2 ] in ch3cn 
 possesses six (co ) bands associated with the [ re ] ( 2023 , 1919 , 
 and 1901 cm ) and [ nife2 ] ( 2036 , 1996 , 
 and 1957 cm ) units of the molecule ( figure 7a ) . the picosecond - trir spectrum 
 of [ re]-[nife2 ] ( 0.3 mm ) obtained 50 ps after flash photolysis 
 (  = 400 nm ) shows that all parent bands are bleached and that 
 new bands are observed at 2065 , 2009 , 1985 , and 1945 cm ( figure 7b ) . the 
 transient species in this spectrum can be divided into two components , 
 those associated with the [ re ] and those with the [ nife2 ] units . the parent bands associated with the [ nife2 ] 
 center rapidly reform (  = 70 ps , figure s5 ) as the bands at 2009 and 1985 cm decay , 
 which is similar to processes observed in the control experiment following 
 the photolysis of [ nife2 ] , in which a short - lived transient 
 is formed (  = 90 ps ) with bands shifted to lower wavenumber . this species is tentatively assigned to the 
 homolysis of the fe  fe bond in [ nife2 ] , which has 
 also been observed after photolysis of synthetic analogues of the 
 active sites of the [ fefe ] hydrogenases . the nanosecond - trir spectrum of [ re]-[nife2 ] recorded 
 2 ns after photolysis (  = 355 nm ) only shows depletion of the 
 parent bands associated with the [ re ] moiety ( 2023 , 1919 , and 1901 
 cm ) and transient bands at 2065 , 1995 , and 1960 
 cm ( figure 7c , blue line ) . the species formed is characteristic of the 
 production of a re - diimine mlct excited state where the 
 (co ) bands shift up in energy due to the oxidation of the re 
 center ( [ re]*-[nife2 ] ) . such 
 behavior has been reported for numerous [ recl(co)3(diimine ) ] mlct excited states , and the shift in the (co ) bands 
 to higher wavenumber has been well - documented . following photolysis , the mlct bands decay 
 (  = 20  1 ns , figure 7d , blue dots ) , and the parent bands reform ( figure 7d , black dots ) . this 
 lifetime (  = 20  1 ns ) is of the same magnitude as the mlct emission lifetime at 650 nm (  = 24  0.5 
 ns ) , measured using luminescence spectroscopy ( see above ) and suggests 
 that the excited state [ re]*-[nife2 ] is not quenched by 
 electron transfer to the [ nife2 ] unit . ftir , ps - trir , and ns - trir 
 spectra of [ re]-[nife2 ] in ch3cn . ( b ) ps - trir difference spectrum taken 50 ps after flash 
 photolysis at 400 nm . ( c ) ns - trir difference spectrum taken 2 ns after 
 flash photolysis at 355 nm . ( d ) trir kinetic traces for the decay 
 of the [ re ] * center ( blue dots ) and regrowth of the ground - state bands , 
 [ re ] ( black dots ) . figure 8 shows the nanosecond - trir spectra obtained 
 following excitation (  = 355 nm ) of [ re]-[nife2 ] 
 ( 0.3 mm ) in a solution of ch3cn containing net3 ( 2 m ) . the spectrum obtained 2 ns after photolysis ( figure 8b ) is identical to the spectra 
 obtained above in the absence of net3 ( figure 7c ) and shows the formation 
 of a mlct excited state at the [ re ] center . however , in 
 this experiment the mlct bands decay ( figure 8e , blue dots ,  = 12 
  0.5 ns ) as 60% of the parent [ re ] moiety bands recover ( figure 8e , black dots ) to 
 form a new species with bands at 2004 , 1896 , and 1897 cm ( figure 8c ) , which 
 accounts for the remaining 40% of the parent bleach . through comparison 
 with similar experiments on re - diimine complexes , this new species can be assigned as [ re]-[nife2 ] formed through reductive quenching of [ re]*-[nife2 ] by net3 . on a longer time scale ( 10010  000 
 ns ) [ re]-[nife2 ] decays (  = 3100 
  100 ns , figure 8e , red dots ) , and the remaining bands associated with the [ re ] center 
 recover . concomitantly the (co ) bands of the [ nife2 ] unit are bleached ( figure 8e , black circles ) , and bands associated with the formation 
 of [ re]-[nife2 ] grow in at 1970 , 1926 , 
 and 1889 cm ( figure 8d ) . thus , electron transfer involving [ re]-[nife2 ] occurs to form [ re]-[nife2 ] via a reductive quenching pathway from the mlct of the photosensitizing unit . the growth of [ re]-[nife2 ] ( 10010  000 ns ) is biexponential 
 ( figure 8e , green dots ) ; 
 a possible consequence of its formation from [ re]-[nife2 ] and from the decomposition of the oxidized electron 
 donor ( net3 ) . this process 
 has been observed previously for similar re - diimine complexes following 
 reductive quenching by net3 or teoa . we have also observed similar behavior in photolysis experiments 
 involving [ recl(co)3(bpy ) ] and [ nife2 ] in a 
 solution of ch3cn containing net3 . in this example decomposition of net3 resulted in the reduction of [ nife2 ] to [ nife2 ] . once formed , [ re]-[nife2 ] remains stable on the time scale of this experiment ( up to 70 s ) . ftir and 
 nanosecond - trir spectra of [ re]-[nife2 ] in a solution of 
 ch3cn containing net3 ( 2 m ) . nanosecond - trir difference spectra taken ( b ) 2 ns , 
 ( c ) 50 ns , and ( d ) 50 s after flash photolysis at 355 nm . ( e ) 
 trir kinetic traces for the decay of the mlct excited 
 state , [ re]*-[nife2 ] ( blue ) ; the growth and decay of [ re]-[nife2 ] ( red dots ) , the growth of [ re]-[nife2 ] ( green ) , the regeneration of the [ re ] 
 center ( black  ) , and the loss of the [ nife2 ] center 
 ( black  ) . a previous picosecond - trir 
 investigation of [ nife2 ] in ch3cn has shown 
 that a short - lived transient (  = 90 ps ) is formed following 
 photolysis with characteristic bands shifted to lower wavenumber , 
 namely , [ nife2 ] * ( see above ) . the presence of net3 did not affect the lifetime or band positions of [ nife2 ] * , and no direct interaction between [ nife2 ] * , [ recl(co)3(bpy ) ] , or the mlct excited state of [ recl(co)3(bpy ) ] was observed on these time scales . therefore , to simplify the analysis of the spectra in this 
 study , solutions containing [ nife2 ] at time delays of less 
 than 2 ns after excitation are not reported . figure s6a shows the ground - state ftir spectrum of [ re]/[nife2 ] with three bands associated with [ recl(co)3(ch3-bpy - cooch3 ) ] ( 2023 , 1919 , and 1901 cm ) and three with [ nife2 ] ( 2035 , 1995 , and 1955 cm ) . the nanosecond - trir spectrum recorded 2 ns after 
 the photolysis of a solution of [ re]/[nife2 ] ( both at 1 
 mm ) in ch3cn is shown in figure s6b . depletion of the parent [ recl(co)3(ch3-bpy - cooch3 ) ] bands are observed with the formation of 
 a species at 2067 , 1997 , and 1959 cm characteristic 
 of the production of a re - diimine mlct excited state ( [ recl(co)3(ch3-bpy - cooch3 ) ] * ) . [ recl(co)3(ch3-bpy - cooch3 ) ] * decays back 
 to the ground state (  = 22  1 ns ) as the parent bleaches 
 for [ recl(co)3(ch3-bpy - cooch3 ) ] recover and the rate of decay is similar to the mlct 
 emission (  = 25  0.5 ns ) measured above using luminescence 
 spectroscopy . the co bands corresponding to [ nife2 ] were not observed , which suggests that electron transfer 
 from [ recl(co)3(ch3-bpy - cooch3 ) ] * 
 to [ nife2 ] does not occur to an extent that can be monitored 
 in these experiments . figure s7 shows 
 the nanosecond - trir spectra recorded following photolysis (  
 = 355 nm ) of [ recl(co)3(ch3-bpy - cooch3 ) ] ( 0.5 mm ) in a solution of ch3cn containing net3 ( 1 m ) . bands characteristic of [ recl(co)3(ch3-bpy - cooch3 ) ] * are observed to form 4 ns after 
 excitation ( figure s7b ) . [ recl(co)3(ch3-bpy - cooch3 ) ] * (  = 
 10  2 ns ) is reductively quenched by net3 to form 
 [ recl(co)3(ch3-bpy - cooch3 ) ] ( 43% ) with bands at 2000 , 1891 , and 1894 cm , while the remaining 57% decays back to the ground state . the rise 
 in [ recl(co)3(ch3-bpy - cooch3 ) ] is observed as two processes over two different 
 time scales ( figure s7d , red dots ) . the 
 first phase ( 070 ns ) , which occurs on a similar time scale 
 to the decay of the mlct excited stated , is associated 
 with the reductive quenching process described above involving [ recl(co)3(ch3-bpy - cooch3 ) ] and net3 . the second phase ( 701000 ns ) can be explained by considering 
 the decomposition of the oxidized electron donor net3 + , which 
 may result in the reduction of a second equivalent of [ recl(co)3(ch3-bpy - cooch3 ) ] as was observed 
 previously for the reduction of [ re]-[nife2 ] . [ re]/[nife2 ] ( 1 mm ) was investigated in a solution of ch3cn 
 containing net3 ( 2 m ) . [ recl(co)3(ch3-bpy - cooch3 ) ] * is observable 2 ns after photolysis 
 as a transient in the spectrum ( figure s8b ) . this mlct excited state decays (  = 10  
 2 ns , figure s8e , blue dots ) to the ground 
 state ( 57% ) and through reductive quenching by net3 ( 43% ) 
 to form [ recl(co)3(ch3-bpy - cooch3 ) ] ( figure s8c ) . over 
 greater time scales ( 501500 ns ) , the co bands associated with 
 [ recl(co)3(ch3-bpy - cooch3 ) ] are lost (  = 250  10 ns , figure s8e , red dots ) as this species reduces 
 [ nife2 ] to [ nife2 ] to generate 
 a spectrum with bands at 1969 , 1921 , and 1896 cm ( figure s8d ) . [ nife2 ] remains stable for the time scale of the experiment 
 ( 70 s ) . the most noticeable difference in the excited - state 
 electron transfer mechanisms in [ re]-[nife2 ] and [ re]/[nife2 ] is the lifetime of the reduced photosensitizer . the monoexponential 
 decay of [ recl(co)3(ch3-bpy - cooch3 ) ] (  = 250  10 ns , figure s8e , red dots ) is significantly shorter 
 than that measured for [ re]-[nife2 ] 
 (  = 3100  100 ns , figure 8e , red dots ) under similar conditions . this faster 
 rate for [ re]/[nife2 ] was investigated further by the determination 
 of the rate constants of the electron transfer processes in both [ re]-[nife2 ] and [ re]/[nife2 ] . nanosecond - trir experiments 
 have shown that the reduction of the [ nife2 ] center in 
 [ re]-[nife2 ] can be achieved via the photochemical formation 
 of [ re]-[nife2 ] and subsequent electron 
 transfer to generate [ re]-[nife2 ] . we 
 performed concentration - dependent studies to determine the mechanism 
 of this electron transfer process in [ re]-[nife2 ] and [ re]/[nife2 ] . for [ re]/[nife2 ] only intermolecular electron 
 transfer is possible ( eq 1 ) with the observed rate constant expected to exhibit pseudo - first - order 
 kinetics and to be dependent on [ nife2 ] concentration . however , in the case of [ re]-[nife2 ] , both intra- and intermolecular 
 electron transfer processes are feasible ( eqs 2 and 3 ) . therefore , 
 concentration - dependent analysis of the observed rate constant for 
 electron transfer should be able to distinguish between the two mechanisms . the observed rate constant can be calculated from the reciprocal of 
 the lifetime of either [ re]-[nife2 ] 
 or [ recl(co)3(ch3-bpy - cooch3)].123 to 
 quantify the rate of electron transfer in [ re]/[nife2 ] 
 the concentrations of all components were kept constant ( 0.5 mm [ recl(co)3(ch3-bpy - cooch3 ) ] ; 1 or 2 m net3 ) except for the concentration of [ nife2 ] , which 
 was varied between 0.34 and 1.00 mm . the lifetime dependence of [ recl(co)3(ch3-bpy - cooch3 ) ] on the concentration of [ nife2 ] is shown in table s2 with the pseudo - first - order rate plot 
 in figure 9a . since 
 [ nife2 ] is in excess , its concentration remains almost 
 constant , and so the observed pseudo - first - order rate constant can 
 be used to calculate the second - order rate constant for electron transfer , kinter = 3.8  0.5  10 m s. this is in a similar 
 range to the value reported for electron transfer between [ recl(co)3(bpy ) ] and [ nife2 ] , kinter = 6.7  10 m s. in a 
 similar experiment , the concentration of [ re]-[nife2 ] was 
 varied between 0 and 0.3 mm ( figure 9b and table s2 ) . this concentration 
 range was selected to reduce the overall absorbance at the excitation 
 wavelength ( 355 nm ) . a similar trend to that for [ re]/[nife2 ] is observed : as the concentration of [ re]-[nife2 ] is 
 increased the observed electron transfer rate constant also increases . however , it is not directly proportional to the concentration of [ re]-[nife2 ] and does not pass close to the origin ( figure 9b ) . thus , assuming that these two processes are the only significant 
 processes occurring following photolysis , then the gradient of the 
 linear fit provides an estimate of the intermolecular electron transfer 
 rate constant , kinter = 5.9  0.7 
  10 m s. 
 the positive intercept of this line at 0 m [ re]-[nife2 ] 
 ( figure 9b ) suggests 
 that intramolecular electron transfer also occurs , the first - order 
 rate constant of which can be estimated at kintra = 1.5  0.1  10 s. it must be noted that this simplification of the electron transfer 
 mechanism does not include any potential effects resulting from the 
 decomposition of the electron donor , net3 . ( a ) 
 [ re]/[nife2 ] in a solution of ch3cn containing 
 net3 , linear fitting ( solid line ) gives kinter = 3.8  0.5  10 m s ( gradient ) . ( b ) [ re]-[nife2 ] in 
 a solution of net3 and ch3cn , linear fitting 
 ( solid line ) gives kinter = 5.9  
 0.7  10 m s ( gradient ) and kintra = 1.5  0.1 
  10 s ( x = 0 intercept ) . dashed 
 lines are extensions of the linear fits . to assess the stability of [ re]-[nife2 ] and [ re]/[nife2 ] during continued photolysis , the ftir spectrum was monitored 
 following irradiation using white light from a light - emitting diode 
 ( led ;  < 420 nm cutoff filter ) . 5 s 
 after 1 min of photolysis of [ re]-[nife2 ] , and the bands 
 originating from [ re]-[nife2 ] ( 1970 , 
 1926 , and 1889 cm ) are observed with an associated 
 loss in the bands from the [ nife2 ] unit . in addition , bands 
 at 1938 and 1909 cm develop , and by comparison 
 with our previous studies on [ nife2 ] in ch3cn , these can be assigned as originating from the 
 formation of a second isomer of [ re]-[nife2 ] , where the two of the terminal carbonyl bands of the [ nife2 ] unit bridge across the two fe atoms . depletion of the parent bands 
 of the [ re ] center is also evident and a broad positive feature is 
 observed at 2013 cm , which is in a similar region 
 to related products reported following chemical reduction of [ recl(co)3(bpy ) ] . these include [ recl(co)3(bpy ) ] ( 1998 , 1880 , and 1866 cm , in ch3cn ) , 
 [ re(ch3cn)(co)3(bpy ) ] ( 2017 
 and 1902(br ) cm , in ch3cn ) , and [ re(co)3(bpy)]2 ( 1990 , 1952 , 1886 , and 1863 cm in tetrahydrofuran ) ; however , the precise 
 origin of the band in our experiment could not be determined . the 
 apparent loss of bands associated with the [ re ] center upon photolysis , 
 without the clear formation of a new species , suggests that decomposition 
 of the [ re ] diimine center has occurred to yield products that are 
 not clearly observable in this region of the ir spectrum . a 
 similar spectral profile was observed for [ re]/[nife2 ] 
 after 1 min of photolysis . almost complete depletion of the bands 
 originating from both [ recl(co)3(ch3-bpy - cooch3 ) ] and [ nife2 ] can been seen with the formation 
 of both isomers of [ nife2 ] ( figure s9 ) . this apparent decomposition of the 
 photosensitizer in [ re]/[nife2 ] was not observed when [ recl(co)3(bpy ) ] was used in place of [ recl(co)3(ch3-bpy - cooch3 ) ] . in 
 this example no apparent loss in the bands of [ recl(co)3(bpy ) ] could be observed after 30 s of photolysis under similar conditions . thus , it appears that functionalizing the bipyridine ligand in [ re]-[nife2 ] and [ recl(co)3(ch3-bpy - cooch3 ) ] reduces the stability of the photosensitizer when compared 
 to [ recl(co)3(bpy ) ] , on continuous photolysis in reducing 
 conditions . ftir spectra of [ re]-[nife2 ] before and after 
 steady - state photolysis in a solution of ch3cn containing 
 net3 ( 1 m ) . photolysis 
 was performed using an led white light source and a  < 420 
 nm cutoff filter . we investigated the use of [ re]-[nife2 ] and [ re]/[nife2 ] for photoinduced h2 production 
 using both net3 and teoa as sacrificial electron donors . [ recl(co)3(bpy ) ] has been used previously as a photosensitizer 
 for photocatalytic h2 production from [ nife2 ] with a ton of 55 under optimal conditions . using similar conditions , [ re]-[nife2 ] ( 0.1 
 mm ) was investigated in a solution of ch3cn containing 
 teoa ( 1 m ) and [ hteoa][bf4 ] ( 0.1 m ) as a proton source . only a trace amount of h2 could be detected from this system , 
 with a ton of ca . 1 based on [ re]-[nife2 ] as the catalytic 
 moiety . replacing teoa with net3 had a negative effect 
 on the total amount of h2 evolved . in a solution containing 
 [ re]/[nife2 ] ( both at 0.1 mm ) , teoa ( 1 m ) , and [ hteoa][bf4 ] ( 0.1 m ) in ch3cn , again only a trace amount of 
 h2 was evolved . thus , from ftir and h2 production 
 studies it appears that when either [ re]-[nife2 ] or [ re]/[nife2 ] are used for h2 production , rapid decomposition 
 of the esterified photosensitizer occurs , resulting in an almost complete 
 loss in catalytic activity toward h2 production . we have investigated a linked dyad , [ re]-[nife2 ] ( figure 1 ) , designed for photoinduced 
 electron transfer and proton reduction . following flash photolysis , picosecond - trir and nanosecond - trir spectroscopies have monitored 
 the reduction of the [ nife2 ] catalytic moiety in [ re]-[nife2 ] and probed the mechanisms of electron transfer . excitation 
 of the [ re ] center results in the population of the mlct 
 excited state , [ re]*-[nife2 ] . reductive quenching of [ re]*-[nife2 ] by net3 forms [ re]-[nife2 ] , which subsequently undergoes electron transfer to the [ nife2 ] center to form [ re]-[nife2 ] . the mechanisms of this electron transfer step were probed by concentration - dependent 
 studies and suggest that both intra- and intermolecular electron transfer 
 processes are involved ( kinter = 5.9  
 0.7  10 m s , kintra = 1.5  0.1  10 s ) . as expected , in the analogous bimolecular 
 system ( [ re]/[nife2 ] ) , only intermolecular electron transfer 
 could be observed ( kinter = 3.8  
 0.5  10 m s ) . one of the key advantages for directly linking the photosensitizer 
 and catalyst is to increase the rate of electron transfer . however , 
 in this study the overall observed rate of electron transfer is significantly 
 slower in [ re]-[nife2 ] than in [ re]/[nife2 ] . 
 during continuous photolysis , rapid decomposition of the photosensitizing 
 unit in both [ re]-[nife2 ] and [ re]/[nife2 ] this may , in part , help to explain 
 the poor activity of this dyad and the bimolecular system toward h2 evolution in acidic media . in the ester - linked dyad probed 
 in this study , linking the catalytic and photosensitizing moieties 
 appears to be detrimental to both the electron transfer rate and the 
 chemical stability of the system on continuous photolysis . however , 
 careful consideration in the development of future systems can overcome 
 these problems so that the benefits of linked dyads can be exploited .
the synthesis , photophysics , and 
 photochemistry of a linked dyad ( [ re]-[nife2 ] ) containing 
 an analogue ( [ nife2 ] ) of the active site of [ nife ] hydrogenase , 
 covalently bound to a re - diimine photosensitizer ( [ re ] ) , are described . 
 following excitation , the mechanisms of electron transfer involving 
 the [ re ] and [ nife2 ] centers and the resulting decomposition 
 were investigated . 
 excitation of the [ re ] center results in the population 
 of a diimine - based metal - to - ligand charge transfer excited state . 
 reductive quenching by net3 produces the radically reduced 
 form of [ re ] , [ re] ( kq = 1.4  0.1  107 m1 s1 ) . 
 once formed , [ re] reduces the 
 [ nife2 ] center to [ nife2] , and this reduction was followed using time - resolved infrared spectroscopy . 
 the concentration dependence of the electron transfer rate constants 
 suggests that both inter- and intramolecular electron transfer pathways 
 are involved , and the rate constants for these processes have been 
 estimated ( kinter = 5.9  0.7  
 108 m1 s1 , kintra = 1.5  0.1  105 s1 ) . for the analogous bimolecular system , only 
 intermolecular electron transfer could be observed ( kinter = 3.8  0.5  109 m1 s1 ) . 
 fourier transform infrared spectroscopic 
 studies confirms that decomposition of the dyad occurs upon prolonged 
 photolysis , and this appears to be a major factor for the low activity 
 of the system toward h2 production in acidic conditions .