a benefit of lifetime methods 
 is that they are independent of dye concentration ; however , they often 
 require specialized and more costly instrumentation .
intensity - based methods are compatible with common fluorescence 
 detection techniques , even standard digital cameras , but they often require calibration of local dye concentration 
 versus standards . this criterion has been met by ratiometric methods 
 that rely on multicomponent systems combining an oxygen - sensitive 
 phosphor and an oxygen - insensitive standard in an inert , readily processable 
 matrix . when fashioned as nanoparticles or films ,
previously 
 we reported a class of single - component dual - emissive 
 oxygen sensing polymeric materials composed of boron difluoride -diketonates 
 ( bf2bdk ) linked to poly(lactic acid ) ( pla ) , that combine 
 sensor ( phosphorescence , p ) , standard ( fluorescence , f ) , and polymer 
 matrix in one .
specifically , with bf2dbmpla ( dbm = dibenzoylmethane ) and its halogenated , heavy atom congener , bf2dbm(i)pla , we demonstrated that 
 emission wavelengths , relative f / p intensities , and oxygen sensitivities 
 are tunable with heavy atom substitution and polymer molecular weight and that these materials can be exploited for cellular , tissue , and in vivo imaging studies ( e.g. , tumor 
 hypoxia ) . in a recent model study , blending 
 bf2bdk dyes with pla
, 
 it was noted that emission is comprised of  * 
 and intramolecular charge transfer ( ict ) processes . in symmetrical 
 phenyl
 phenyl ( ph  ph ) dyes like dbm ,  * 
 processes dominate , whereas in unsymmetrical ph  np or ph  an 
 systems ( np = naphthyl , an = anthracene ) , ict processes dominate .
computational studies showed that electron density 
 is localized on the larger arene donor ( np or an vs ph ) in the homo 
 but is distributed across the molecule in the lumo .
luminescence data 
 support these claims , with additional ict bands in absorption and 
 emission spectra ( i.e. , for ph  an systems ) and longer radiative 
 lifetimes .
building upon this observation that the larger arene donor 
 dominates the ict emission properties in unsymmetrical dye systems , 
 a set of ph  np dyes with bromide heavy atom substitution were 
 prepared and blended with pla to explore the effects of heavy atom 
 position on luminescence properties .
it 
 was demonstrated that lifetimes , intensities , and oxygen sensitivity 
 can be systematically modulated by placing the heavy atom on the more 
 electron - rich naphthyl versus phenyl ring , with naphthyl ring substitution 
 showing the strongest heavy atom effect ( i.e. , more intense , shorter 
 lived phosphorescence ) .
more recently , this concept was generalized 
 with lewis acid  aryl ketone systems .
here we contribute new findings to the growing boron luminescent 
 materials field by extending investigations to a series of naphthyl - phenyl dye 
covalent attachment of the dye to the polymer is preferred 
 over blends for practical uses , given decreased dye leaching and dye / polymer 
 phase separation at high loadings .
diketonate ligands ( 14 ) , boron initiators ( 58 ) , and associated polymers ( 912 ) were designed with the following fundamental and applied considerations 
 in mind .
first , extended conjugation afforded by the naphthyl ring 
 in np  ph systems ( vs ph  ph in dbm ) can lead to more 
 red - shifted dyes in new color regimes for multiplexing applications . 
 additionally , understanding ways that conjugation length correlates 
 with emission wavelengths for bf2bdk  pla materials 
 is important for generating red - shifted materials for greater tissue 
 penetration of light in biological contexts .
previous studies show 
 that overall conjugation length is not the only determinative factor 
 for emission wavelength , given ict effects .
second , substituents and their placement influence electron density 
 at arene rings and thus material optical properties . though modulation 
 of triplet emission was achieved by heavy atom placement in np 
ph 
 dye / polymer blends , here we test whether this phenomenon extends to 
 dye  polymer conjugates . in the present series
, polymers are 
 attached to dyes via electron - donating alkoxy linkages that can perturb 
 the donor strength of the two arene rings .
thus , polymer and bromide 
 substituent attachments at either phenyl or naphthyl rings are compared .
third , for ratiometric oxygen sensing , strong phosphorescence intensity 
 is desirable . thus , bromide heavy atom substitution is compared to 
 non - brominated systems , and phosphorescence is further modulated via 
 molecular weight tuning , as previously reported .
for example , low molecular weight polymers ( i.e. , higher 
 dye loading ) result in increased intersystem crossing from the singlet 
 to the triplet state , increased relative phosphorescence intensity , 
 and shorter lifetimes .
furthermore , for bf2dbm(i)pla , nearly 
 full range f / p intensity tuning was possible to generate materials 
 for different kinds of sensing through molecular weight tuning alone . 

for instance , materials with strong phosphorescence versus fluorescence 
 serve as turn on sensors , whereas systems with comparable , readily 
 detectable f and p intensities are exploited for ratiometric tumor 
 hypoxia imaging and other biomedical uses . however , the sensitivity 
 of f / p to polymer molecular weight is not known for different dyes , 
 and this is an important parameter for sensor materials design .
fourth , discrete , clearly distinguishable fluorescence and phosphorescence 
 peaks with measurable f / p ratios are important for ratiometric sensing . 
 here
emission wavelengths and can be further modulated 
 by molecular weight , with greater peak separation achievable with 
 higher molecular weights ( i.e. , low dye loading ) .
importantly , systems 
 with poor fluorescence and phosphorescence peak separation are nonetheless 
 useful for gated emission and lifetime sensing modalities , and non - halogenated 
 systems with longer lifetimes result in more sensitive materials for 
 oxygen sensing .
1-[4-(2-hydroxyethoxy)phenyl]-3-(2-naphthyl)propane-1,3-dione , 
 nbmoh ( 1 ) , and bf2nbmoh ( 5 ) were 
 prepared as previously described .
l - lactide , aldrich ) was recrystallized twice 
 from ethyl acetate and stored under nitrogen .
tin(ii ) 2-ethylhexanoate 
 ( sn(oct)2 , spectrum ) , boron trifluoride diethyl etherate 
 ( aldrich , purified , redistilled ) , and all other reagents and solvents 
 were used as received without further purification .
solvents ch2cl2 and thf were dried and purified by over 3  
 molecular sieves activated at 300 c .
all other chemicals were reagent grade from sigma - aldrich and were 
 used without further purification .
h nmr ( 300 mhz ) spectra were recorded 
 on a unity inova 300/51 instrument in cdcl3 .
h nmr were referenced to the signals for the residual protiochloroform 
 at 7.26 ppm , protiodmso at 2.50 ppm , and protioacetone at 2.09 ppm . 

polymer molecular weights were 
 determined by gel permeation chromatography ( gpc ) ( thf , 25 c , 
 0.8 ml / min ) using multiangle laser light scattering ( mals ) (  
 = 658 nm , 25 c ) and refractive index ( ri ) (  = 658 nm , 
 25 c ) detection .
polymer laboratories 5 m mixed - c columns 
 ( guard column plus two columns ) along with wyatt technology ( optilab 
 t - rex interferometric refractometer , minidawn treos multiangle static 
 light scattering ( mals ) detector , astra 6.0 software ) and agilent 
 technologies instrumentation ( series 1260 hplc with diode array ( dad ) 
 detector , chemstation ) were used in gpc analysis .
vis spectra 
 were recorded on a hewlett - packard 8452a diode - array spectrophotometer .
steady - state fluorescence emission spectra were recorded on a horiba 
 fluorolog-3 model fl3 - 22 spectrofluorometer ( double - grating excitation 
 and double - grating emission monochromator ) .
time - correlated single - photon 
 counting ( tcspc ) fluorescence lifetime measurements were performed 
 with a nanoled-370 ( ex = 369 nm ) excitation source 
 and a datastation hub as the spc controller .
phosphorescence lifetimes 
 were measured with a 1 ms multichannel scalar ( mcs ) excited with a 
 flash xenon lamp ( ex = 369 nm ; duration < 1 ms )
. 
 lifetime data were analyzed with datastation v2.4 software from horiba 
 jobin yvon .
fluorescence quantum yields ( f ) of initiator 
 and polymer samples in ch2cl2 were calculated 
 versus anthracene as a standard as previously described using the 
 following values : f(anthracene ) = 0.27,nd(etoh ) = 1.360 , nd(ch2cl2 ) = 1.424 .
optically dilute ch2cl2 solutions 
 of the dyes were prepared in 1 cm path length quartz cuvettes with 
 absorbances < 0.1 au .
thin films were prepared on the inner wall 
 of vials by dissolving polymers in ch2cl2 ( 1 
 mg / ml ) and then evaporating the solvent by slowly rotating the vial 
 under a stream of nitrogen .
all compounds 
 were modeled using the gaussian 09 suite 
 of programs using density functional theory .
b3lyp/6 - 31+g(d ) 
 was utilized for ground state geometry optimization with a tomasi 
 polarized continuum for dichloromethane solvent .
the vibrational frequencies 
 for the optimized geometries were all positive , assuring that the 
 geometries are at least a local minimum .
single point energy calculations 
 were used to generate the molecular orbital diagrams utilizing b3lyp/6 - 31g(d ) . 

time - dependent density functional theory , td - b3lyp/6 - 311+g(d ) , was 
 employed for estimates of the absorption spectra , at the respective 
 optimized geometries .
the 
 naphthyl precursor 
 was prepared as previously described with 
 the following exceptions .
6-methoxy-2-acetonapthalene ( 1.5 g , 7.5 
 mmol ) dissolved in 12 m hcl ( 500 ml ) was heated at 90 c for 
 2 h. the hot reaction mixture was filtered through a sintered glass 
 frit ( m ) to remove a greenish - black precipitate .
the filtrate was 
 allowed to cool to room temperature , and then the flask was further 
 cooled in an ice water bath .
the resulting white solid was collected 
 by filtration and was washed with copious quantities of distilled 
 water until the ph of the wash fraction was neutral ( ph  6.5 ) . 

( note : in certain preparations , when the sample was greenish or brownish 
 in color , further purification by recrystallization from hexane / etoac 
 was performed . ) after drying in vacuo , a white powder 
 was obtained : 1.27 g ( 91% ) .
h nmr ( 300 mhz , cdcl3 , ppm ) :  8.41 ( s , 1h , 1-arh ) , 8.00 ( d , 1h , j = 8.7 hz , 8-arh ) , 7.89 ( d , 1h , j = 9.0 , 3-arh ) , 7.72 ( d , 1h , j = 9.0 , 4-arh ) , 7.207.16 ( bm , 2h , 5 , 7-arh ) , 5.42 ( s , 1h , oh ) , 2.71 ( s , 3h , ch3 ) .
6-hydroxy-2-acetonapthalene 
 ( 515 mg , 2.8 mmol ) was dissolved in dmf ( 100 ml ) in the presence of 
 k2co3 ( 1.22 g , 8.8 mmol ) and ki ( 50 mg , 0.3 
 mmol ) and was refluxed at 100 c overnight ( 16 h ) .
dmf was removed 
 via h2o / ch2cl2 extraction ( 1 l of 
 h2o , 100 ml ch2cl2 ) .
the organic 
 layer was washed with h2o ( 2  20 ml ) and brine ( 2 
  20 ml ) and then was dried over na2so4 , filtered , and concentrated via rotary evaporation .
the crude product 
 was purified by column chromatography ( 3:1 hexanes / etoac ) and dried in vacuo to yield a white powder : 460 mg ( 72% ) .
h nmr ( 300 mhz , cdcl3 , ppm ) :  8.40 ( s , 1h , 1-arh ) , 8.02 ( d , 1h , j = 9 , 8-arh ) , 7.88 ( d , 1h , j = 9.0 , 3-arh ) , 
 7.76 ( d , 1h , j = 9 , 4-arh ) , 7.23 
 ( d , 1h , j = 9 , 7-arh ) , 7.18 ( s , 
 1h , 5-arh ) , 4.23 ( t , j = 4.5 , 2h , 
 aroch2ch2oh ) , 4.05 
 ( t , j = 4.5 , 2h , aroch2ch2oh ) , 2.06 ( s , 1h , oh ) .
the ligand precursor was prepared as previously described using 
 6-(2-hydroxy)ethoxy-2-acetonapthalene instead of 1-[4-(2-hydroxyethoxy)phenyl]ethanone . 

the crude product was purified by silica column chromatography ( 6:1 
 hexanes / etoac ) to give a tan solid : 1.12 g ( 68% ) .
h nmr 
 ( 300 mhz , cdcl3 , ppm ) :  8.40 ( s , 1h , 1-arh ) , 8.00 ( d , 1h , j = 9 , 8-arh ) , 7.86 ( d , 1h , j = 9.0 , 3-arh ) , 
 7.57 ( d , 1h , j = 9.0 , 4-arh ) , 7.26 
 ( d , 1h , j = 9.0 , 7-arh ) , 7.19 ( s , 
 1h , 5-arh ) , 4.74 ( t , 1h , j = 3.5,o  ch * ( ch2)o ) , 4.31 ( t , 2h , j = 4.5 , aroch2ch2or ) , 3.92 ( t , 2h , j = 4.2 , aroch2ch2or ) , 3.55 ( t , 2h , j = 6 , och2c3h6ch ( r)o ) , 2.70 ( s , 3h , ch3 ) , 1.801.65 ( bm , 6h , och2c3h6ch ( r)o ) . hrms 
 ( esi , tof ) m / z calcd for c19h23o4 315.1596 [ m ] ; found 315.1596 .
h nmr ( 300 mhz , cdcl3 ) :  
 17.03 ( s , 1h , oh ) , 8.49 ( s , 1h , 1-arh ) 8.107.90 ( m , 4h , 2,6-arh , 3,5-arh , ) , 7.887.71 
 ( m , 2h , 7,8-arh ) , 7.63 ( d , j = 9 , 1h , 3-arh ) , 7.03 ( d , j = 9 , 2h , 3 , 5-arh ) , 6.93 
 ( s , 1h , cochco ) , 4.19 ( t , j = 4.2 , 
 aroch2ch2oh ) , 4.03 
 ( m , 2h , aroch2ch2oh ) , 
 3.48 ( s , 1h , ch2ch2oh ) . hrms 
 ( esi , tof ) m / z calcd for c21h18o4br 413.0388 [ m + h ] ; found 
 413.0401 .
h nmr ( 300 mhz , cdcl3 ) :  17.02 
 ( s , 1h , oh ) , 8.49 ( s , 1h , 1-arh ) ,
8.067.99 ( bm , 3h , 8-arh , 2,6-arh ) 7.90 ( d , j = 9 , 1h , 3-arh ) 7.81 ( d , j = 9 , 1h , 4-arh ) , 7.577.51 ( bm , 
 3h , 3,4,5-arh ) , 7.25 ( d , j = 9 , 1h , 7-arh ) , 7.20 ( s , 1h , 
 5-arh ) 6.98 ( s , 1h , cochco ) , 4.25 ( t , j = 6 , 2h , aroch2ch2oh ) , 4.06 ( t , j = 6 , 2h , 
 aroch2ch2oh ) , 2.2 ( s , 1h , aroch2ch2oh ) . hrms ( esi , tof ) m / z calcd for c21h19o4 335.1283 [ m ] ; found 335.1289 .
h nmr ( 300 mhz , cdcl3 ) :  16.96 
 ( s , 1h , oh ) , 8.48 ( s , 1h , 1-arh ) , 8.00 ( d , j = 8.7 , 1h , 8-arh ) , 7.947.90 ( m , 3h , 2 , 6-arh , 3-arh ) , 7.81 ( d , j = 8.7 , 1h , 4-arh ) , 7.64 ( d , j = 8.7 , 2h , 3 , 5-arh ) , 7.26 ( d , j = 8.7,1h , 7-arh ) , 7.19 ( s , 1h , 
 5-arh ) , 6.94 ( s , 1h , cochco ) , 4.25 ( t , j = 4.2 , 2h , aroch2ch2oh ) , 4.094.04 ( m , 2h , aroch2ch2oh ) , 2.05 ( s , 1h , ch2ch2oh ) .
hrms ( esi , tof ) m / z calcd for c21h18o4br 413.0388 [ m + h ] ; found 413.0376 . a representative preparation 
 for difluoroboron initiators is provided for complex 6 below .
h nmr ( 300 
 mhz , cdcl3 ) :  8.76 ( s , 1h , 2-arh ) , 8.21 ( d , 2h , 2,6-arh ) , 7.98 ( m , 
 5h , 4,7,9,10-arh , 
 cochco ) , 7.68 ( m , 2h , 5,6-arh ) , 7.08 ( d , 2h , 3,5-arh ) , 4.23 ( t , j = 4.2 , 2h , aroch2ch2oh ) , 4.05 ( t , j = 4.2 , 
 2h , aroch2ch2oh ) . hrms 
 ( esi , tof ) m / z calcd c21h16bo4f2na 405.1099 [ m + na ] ; found 405.1094 .
the ligand 
 nbmoh , 2 ( 250.0 mg , 0.605 mmol ) , was added to a flame - dried 
 two - neck round - bottom flask under nitrogen and was dissolved in thf / ch2cl2 ( 20/20 ml ) to give a pale yellow solution . 

boron trifluoride diethyl etherate ( 115 l , 0.907 mmol ) was 
 added via syringe , and the solution turned bright yellow .
the reaction 
 was stirred at room temperature for 48 h. solvents were removed via 
 rotary evaporation , resulting in a yellow solid .
the crude material 
 was purified by recrystallization in 1:1 etoac / acetone to yield a 
 yellow - orange powder : 143 mg ( 53% ) .
h nmr ( 300 mhz , dmso ) :  9.07 ( s , 1h , 
 1-arh ) , 8.438.38 ( bm , 4h , 3,8-arh , 2,6-arh ) , 8.188.11 
 ( bm , 2h , 5,7-arh ) , 7.98 ( s , 1h , cochco ) , 7.81 ( d , j = 9 , 1h , 4-arh ) , 7.23 ( d , j = 9 , 2h , 3,5-arh ) , 4.98 ( t , j = 5.4 , 1h , oh ) , 4.19 ( t , j = 4.8 , 2h , aroch2ch2oh ) , 3.77 ( t , j = 4.5 , 2h , aroch2ch2oh ) . hrms ( esi , tof ) m / z calcd 
 c21h16bo4f2brna 483.0191 
 [ m + na ] ; found 483.0180 . the ph - np complex 
 was prepared as described for 6 .
h nmr ( 300 mhz , dmso ) :  
 9.07 ( s , 1h , 1-arh ) , 8.39 ( d , j = 9 , 2h , 2,6-arh ) , 8.31 ( d , j = 9 , 1h , 8-arh ) , 8.13 ( d , j = 9 , 1h , 3-arh ) , 8.037.98 
 ( m , 2h , 4-arh , cochco ) , 
 7.81 ( d , j = 9 , 1h , 7-arh ) , 7.68 ( t , j = 6 , 2h , 3,5-arh ) , 7.50 ( s , 1h , 5-arh ) , 7.34 ( d , j = 9 , 1h , 4-arh ) , 5.00 ( t , j = 6 , 1h , oh ) , 4.18 ( t , j = 6 , 2h , aroch2ch2oh ) , 3.80 ( t , j = 6 , 2h , aroch2ch2oh ) . hrms ( esi , tof ) m / z calcd c21h18bo4f2 383.1266 [ m ] ; found 383.1252 .
h nmr ( 300 mhz , dmso ) :  
 9.07 ( s , 1h , 1-arh ) , 8.32- 8.28 ( m , 3h , 2,6-arh , 8-arh ) , 8.13 ( d , j = 9.0 , 1h , 3-arh ) , 8.037.99 ( m , 
 2h , 7-arh , cochco ) , 7.90 
 ( d , j = 9 , 2h , 3,5-arh ) , 7.50 ( s , 1h , 5-arh ) , 7.32 ( d , 1h , 4-arh ) , 5.0 ( t , j = 6 , 1h , oh ) , 4.18 ( t , j = 6 , 2h , aroch2ch2oh ) , 3.80 ( t , j = 6 , 2h , aroch2ch2oh ) . hrms ( esi , tof ) m / z calcd 
 c21h16bo4f2brna 483.0191 
 [ m + na ] ; found 483.0193 .
preparative scale reactions 
 were conducted as follows . the boron initiator and d , l - lactide were placed in a kontes flask and sealed under n2 .
the bulb of the flask was entirely submerged in an oil bath 
 at 130 c .
after the d , l - lactide melted , sn(oct)2 in hexanes was added , and the reaction was heated for the 
 designated time ( 13 h ) .
( see table 
 s1 for reagent loadings and reaction times for specific samples . ) 

the polymer was collected 
 by centrifugation , the filtrate was decanted , and the rubbery solid 
 was redissolved in ch2cl2 and reprecipitated 
 in cold meoh ( 20 c ) .
the resulting solid was reprecipitated 
 from ch2cl2/hexanes , collected by centrifugation , 
 the filtrate was decanted , and the residue was dried in vacuo to give the polymers as foams .
number - average 
 molecular weights ( mn ) , polydispersity 
 indices ( pdis ) , and yields for samples of different molecular weights 
 are collected in table s1 .
polymer 9a was obtained as a yellow crystalline solid : 81 mg ( 45% ) .
mn ( gpc / ri ) = 5400 da , pdi = 1.05 ; mn ( nmr ) = 3800 da .
h nmr ( 300 mhz , cdcl3 ) :  8.76 ( s , 1h , 1-arh ) , 8.20 
 ( d , j = 9 , 2h , 2,6-arh ) , 8.107.90 ( bm , 5h , 3,4,5,8-arh , cochco ) , 7.607.55 ( bm , 
 2h , 6,7-arh ) , 7.05 ( d , j = 9 , 2h , 3,5-arh ) , 5.305.16 
 ( bm , 48h , pla ch  ch3 ) , 
 4.55 ( m , 2h , aroch2ch2oh ) , 4.35 ( m , 2h , aroch2ch2oh ) , 1.57 ( bs , 146 h , pla chch3 ) .
polymer 10a was obtained as a yellow crystalline 
 solid : 151 mg ( 44% ) . mn ( gpc / ri ) = 6800 
 da , pdi = 1.03 ; mn ( nmr ) = 6900 da .
h nmr ( 300 mhz , cdcl3 ) :  8.72 ( s , 1h , 1-arh ) , 8.20 
 ( d , j = 9 , 2h , 2,6-arh ) , 8.108.08 ( m , 2h , 3,8-arh ) , 7.897.87 ( m , 2h , 5,7-arh ) , 7.69 ( d , j = 9 , 1h , 4-arh ) , 7.23 ( s , 1h , cochco ) , 7.05 ( d , j = 9 , 2h , 3,5-arh ) , 5.315.11 
 ( bm , 89h , pla chch3 ) , 4.55 ( m , 
 2h , aroch2ch2oh ) , 4.35 
 ( m , 2h , aroch2ch2oh ) , 
 1.57 ( bs , 205h , pla chch3 ) .
polymer 11a was obtained as a yellow - orange foam : 71 mg ( 64% ) .
mn ( gpc / ri ) = 5200 da , pdi = 1.04 ; mn ( nmr ) = 3300 da .
h nmr ( 300 mhz , cdcl3 ) :  8.73 ( s , 1h , 1-arh ) , 8.20 
 ( d , j = 9 , 2h 2,6-arh ) , 8.10 ( d , j = 9 , 1h , 8-arh ) , 7.95 ( d , j = 9 , 1h , 3-arh ) , 7.85 ( d , j = 9 , 1h , 7-arh ) , 7.71 ( t , j = 6 , 1h , 4-arh ) , 7.58 ( t , j = 6 , 2h , 3,5-arh ) , 7.317.25 ( m , 2h , cochco- , 4-arh ) , 7.19 ( s , 1h , 5-arh ) , 5.305.16 ( bm , 45 h , pla chch3 ) , 4.55 ( m , 2h , aroch2ch2oh ) , 4.35 ( m , 2h , aroch2ch2oh ) , 1.57 ( bs , 144 h , pla chch3 ) .
polymer 12a was obtained as a bright yellow - green 
 crystalline foam : 
 101 mg ( 40% ) .
mn ( gpc / ri ) = 5600 da , pdi 
 = 1.07 ; mn ( nmr ) = 3100 da .
h nmr ( 300 mhz , cdcl3 ) :  8.73 ( s , 1h , 1-arh ) , 8.108.03 ( bm , 4h , 8-arh , 2,6-arh , cochco ) , 7.95 ( d , j = 9 , 1h , 3-arh ) , 7.85 ( d , j = 9 , 1h , 7-arh ) , 7.73 ( d , j = 9 , 2h , 3,5-arh ) , 7.33 ( d , j = 9 , 1h , 4-arh ) , 7.19 ( s , 1h , 5-arh ) , 5.215.11 
 ( bm , 72 h , pla chch3 ) , 4.58 ( t , j = 4.8 , 2h , ochh2ch2 ) , 4.35 ( t , j = 4.8 , 2h , aroch2ch2oh ) , 1.57 ( s , 241 h , pla chch3 ) .
bf2n(br)bmoh ( 30 mg , 0.06 mmol ) and d , l - lactide ( 0.467 
 g , 3.25 mmol ) were placed in a kontes flask sealed under n2 .
the bulb of the flask was entirely submerged in an oil bath at 
 130 c .
after the d , l - lactide melted , sn(oct)2 ( 0.24 mg , 0.60 mol ) in hexanes was added , and the 
 flask was resealed under n2 .
aliquots were drawn up into 
 a pipet tip at the specified times ( table s2 ; figure 1a  c ) until 93% conversion . 

percent monomer conversion was determined via h nmr spectroscopy 
 by comparing the integration of monomer versus ( monomer + polymer ) 
 peaks .
molecular masses of dye  polymer conjugates were measured 
 by h nmr spectroscopy and gpc with dad , mals , and ri detectors .
a representative boron polymer synthesis 
 via -diketonate ligand 3 and boron complex initiator 7 is illustrated in scheme 1 for bf2bnmpla 11 .
primary alcohols 
 are installed on the ketones to serve as sites for d , l - lactide polymerization . commercially available 4-hydroxyacetophenone , 
 or 6-hydroxy-2-acetonapthone prepared by acid hydrolysis of 6-methoxy-2-acetonapthone , 

is functionalized with och2ch2oh via 
 williamson ether synthesis and protected with 1,2-dihydropyran for 
 claisen condensation in the presence of nah .
following deprotection 
 and purification by recrystallization , the -diketonate ligands 
 ( 14 ) were boronated with bf3oet2 .
reactions were stirred until ligands 
 were completely consumed , according to tlc . in many instances ,
products 
 precipitated from the reaction mixture over time . after purification 
 by recrystallization powders 58 were 
 obtained .
the structure and purity of boron dye products were confirmed 
 by ms and h nmr spectroscopy .
hydroxyl - functionalized boron 
 initiators 58 were used to produce 
 dye  polymer conjugates 912 by solvent - free ring - opening polymerization 
 of d , l - lactide using a tin octoate catalyst at 130 
 c .
samples were purified by precipitation from ch2cl2/hexane to remove the catalyst and multiple times with 
 ch2cl2/cold methanol until h nmr 
 spectroscopy confirmed all monomer was removed .
reagent loadings , 
 reaction times , and molecular weight data are provided in table s1 .
monomer loadings were increased to 
 achieve higher molecular weights . to test the maximal molecular weight 
 attainable with good molecular weight control , initiator 6
after 24 h , a 21 kda polymer was achieved ( pdi 1.14 ; 84% conversion ) . 

extended reaction time did not result in further chain extension ( 71 
 h , 20.3 kda , pdi 1.17 , 96% conversion ) ; chain transfer or thermal 
 depolymerization may occur .
gpc traces of purified polymer products 
 revealed minor high molecular 
 weight shoulders ( figure 1a ) . in previous studies , 

reactions were typically stopped at 70% conversion to avoid 
 transesterification and broader pdis at high monomer conversion , but here data suggest that other factors are at play .
first , here 
 too , kinetics studies ( e.g. , with 6 , figure 1b , c ) reveal relatively linear mn vs % conversion and pseudo - first - order kinetics plots 
 to 70% conversion , but pdis are higher than usual at the onset 
 ( 1.33 for 23% conversion ) and only drop after 25% 
 conversion ( pdi = 1.05 ) .
second , the high molecular weight shoulders 
 are consistently double the molecular weight of the main polymer peak . 

furthermore , gpc analysis of aliquots shows evidence of this feature 
 throughout this polymerization , not just at high conversions ( figure 1a ) .
these findings point to initiator aggregation 
 rather than diminished control as the polymerization progresses , though 
 at very high conversions ( e.g. , 90% ) slightly increased pdis 
 and nonlinear mn vs conversion is observed , 
 consistent with chain transfer and transesterification .
in fact , the 
 naphthyl dyes show lower solubility in common organic solvents compared 
 to previous bf2dbmoh initiators . for all naphthyl complexes , 
 residual solid initiator was evident in the lactide melt at the onset 
 of polymerization reactions ; however , the mixtures clarified and solids 
 disappeared as the reactions progressed .
gpc trace ( a ) and kinetics 
 plots ( b , c ) for lactide polymerization 
 with initiator bf2n(br)bmoh 6 ( 130 c , 6:lactide : sn(oct)2 = 1:50:1/50 ) . aliquots were 
 taken at 10 , 30 , 60 , 80 , 105 , and 190 min as indicated . while this might point to slow initiation relative 
 to propagation , 
 in fact , no induction period is noted and no tailing is evident on 
 the low molecular weight side of the main polymer peak .
attempts to 
 facilitate the reaction with increased catalyst loading resulted in 
 increased polymerization rate but showed no obvious effect on molecular 
 weight control .
difluoroboron bdk dyes are reported to dimerize and 
 form h - aggregates , which can influence the optical properties .
certain naphthyl initiators 
 show higher percentages of dimeric products than others in the eluting 
 polymer fractions ( 8 : 1012% dimer vs 57 : 57% dimer by mass ) .
alternatively , polymer 
 aggregation could occur during gpc analysis , polymer purification , 
 or processing .
that dimeric shoulders increase slightly in gpc traces 
 for purified polymer samples versus analytical samples from kinetics 
 runs lends some support for this hypothesis .
all complexes have high extinction coefficients ( 48  00065  000 
 m cm ) typical for  * 
 transitions in this family of dyes and high - energy features on dominant 
 peaks . for initiators 5 and 6 , the high - energy 
 shoulders at 402 and 395 nm are more distinct .
previously this has 
 been correlated with boron complex dimerization and ascribed to h - aggregates .
this is consistent with decreased solubility 
 for naphthyl complexes and gpc results in thf solution discussed above . 

introduction of the bromide heavy atom onto the np site of bf2nbmoh resulted in a slight bathochromic shift ( 3 nm ) and increase 
 in extinction coefficient , whereas bromination of the ph site of bf2bnmoh showed a slightly greater red - shift ( 7 nm ) but a decrease 
 in molar absorptivity .
representative uv / vis spectral data for polymers 
 ( 8 kda samples ) are also provided in table 1 .
( see table s3 for optical properties 
 of polymers with other molecular weights . )
spectra are very similar 
 to corresponding boron initiators with slightly decreased extinction 
 coefficients , typical for these boron bdk pla materials .
fluorescence lifetimes excited with 
 a 369 nm light - emitting diode ( led ) monitored at the emission maxima . 

luminescence data 
 for initiators and representative polymers in 
 solution are given in table 1 .
initiators 5 and 6 with 
 alkoxy donors on the phenyl rings show intense blue emission at 452 
 and 448 nm , respectively , while fluorescence for 7 and 8 with or donors on the naphthyl rings are red - shifted 
 to 505 and 521 nm , respectively .
once pla is grown from the initiators , 
 the emission maxima slightly blue - shift ( 47 nm ) , which may 
 be attributed to a solvatochromic effect or enhanced solubility and 
 thus a diminished tendency to dimerize ( table 1 ) .
boron dye initiators 58 in ch2cl2 showing emission colors . typically , increased conjugation results in red - shifted emission .
naphthyl - phenyl systems here are red - shifted 
 as predicted , compared to previously reported phenyl  phenyl 
 ( i.e. , dbm ) derivatives .
for example , bf2dbmpla showed an absorption maximum , abs = 396 nm , and emission maximum , em = 426 nm , which 
 are both more blue - shifted than data for samples 912 red - shifted ( table 1 ; e.g. 9b : abs = 414 nm , em = 
 458 nm ) .
fluorescence wavelengths 
 for samples 512 with the och2ch2or initiator ( r = h ) and polymer ( r = pla ) tails 
 are consistent with this trend .
for example , 5 ( np - ph ) with alkoxy 
 substitution on the phenyl ring emits at 452 nm and displays a small 
 stokes shift ( 2031 cm ) . in comparison , dye 7 ( ph - np ) with substitution on the naphthyl ring has further 
 red - shifted emission ( i.e. , 505 nm ) and shows a larger stokes shift 
 ( 4121 cm ) .
in addition to red - shifting absorbance , 
 bromide heavy atom incorporation 
 in dyes 6 and 8 also affects emission .
it 
 is well - known that halide heavy atoms result in decreased luminescence 
 lifetimes and quantum yields via enhanced intersystem crossing .
results for brominated and parent ( i.e. , h ) 
 compounds are consistent with this trend . for example , the presence 
 of bromide in bf2n(br)bmoh 6 results in a 
 decrease in fluorescence lifetime compared to bf2nbmoh 5 ( i.e. , 5 :  = 1.55 ns ( no heavy atom ) 
 vs 6 :  = 0.53 ns ( heavy atom ) ) and a decrease 
 in fluorescence quantum yield ( 5 : f = 0.4 vs 6 : f = 0.19 ) . a similar 
 trend is noted in the corresponding polymers 9 and 10 . in samples 6 and 10 ,
heavy atom 
 substitution is on the larger , more electron - rich naphthyl donor , 
 but in 8 and 12 , when the bromide is present 
 on the smaller , less electron - rich phenyl donor , the heavy atom effect 
 is much less pronounced .
lifetimes and 
 quantum yields for 8 and 12 are very similar 
 ( table 1 ) .
these results with initiators and 
 polymer conjugates are consistent with prior bf2bdk model 
 studies indicating moderate to strong ict character for unsymmetrically 
 substituted difluoroboron diketonates . a greater 
 disparity in -electron - donating ability of the two arene rings 
 results in stronger ict character dominated by the more electron - rich 
 aryl group .
previously , 
 it was shown that 
 the electronic transitions of bf2bdks depend on their molecular 
 symmetries . according to td - scf calculations , 

stronger ict is observed for complexes with unsymmetrical diarene 
 ligands , where there is a disparity in electron donor ability from 
 one arene ring ( e.g. , anthracene ) compared to another ( e.g. , phenyl ) . 
 on the other hand ,
when the bdk arene rings are comparable ( i.e. , 
 ph - ph in dbm ) , a delocalized  * model is proposed . 

similar trends are noted here for initiator complexes , 58 , which were chosen as the subjects for computational 
 modeling given their relative simplicity compared to the respective 
 polymers .
molecular orbital diagrams for compounds 5 , 7 , and 8 suggest intramolecular charge transfer 
 ( ict ) ( figure 4 ) .
electron density is localized 
 on the stronger naphthyl donor in the homos for 5 , 7 , and 8 , whereas in the lumos it is distributed 
 across the molecular structure .
in contrast , for 6 the 
 electron density is more distributed through the molecular structure 
 on both homo and lumo and the  to  * transition dominates 
 ( figure 4 ) .
the electron - withdrawing bromide 
 substituent may diminish electron density on the naphthyl ring , making 
 donor capacity of br  np and ph  or rings comparable . 

radiative lifetimes , rad increase with increasing 
 ict character , and this is a trend that 
 is observed for samples 5 , 7 , and 8 ( 3.885.33 ns ) compared to 6 ( 2.89 ns ) , 
 with 6 showing the shortest rad .
additionally , 
 both 7 and 8 have larger stokes shifts compared 
 to 5 and 6 .
this is consistent with a geometry 
 change in the excited state that can be expected for compounds with 
 stronger charge transfer character . as in naphthyl
model studies , these systems show no distinct itc features 
 in absorption or emission spectra .
molecular orbital diagrams 
 for compounds 58 showing the highest 
 occupied molecular orbitals ( homo ) and 
 lowest unoccupied molecular orbitals ( lumo ) in ch2cl2 .
thin films were prepared in vials by slow 
 evaporation of ch2cl2 solutions , followed by 
 drying in vacuo .
optical properties for polymers 912 , including different molecular weights , 
 are presented in table 2 .
fluorescence 
 spectra and lifetime measurements were obtained under ambient conditions 
 ( e.g. , air , 21% oxygen ) .
number - average molecular weights 
 detected by dad , ls / ri detectors in thf solvent .
fluorescence lifetimes excited 
 with 
 a 369 nm light - emitting diode ( led ) monitored at the emission maxima . 

excitation source : xenon flash lamp ; rtp lifetime fit to triple - exponential 
 decay . for polymers 911 the emission 
 wavelengths in pla matrices are more red - shifted than in ch2cl2 solution ( for mn  
 78 kda , 9b : ch2cl2 : 458 
 nm , film : 526 nm ; 10a : ch2cl2 : 
 448 nm , pla : 491 nm ; 11b : ch2cl2 : 501 nm , film : 533 nm ) . this trend is consistent with previous reports . dye 
polymer 12a ( mn = 5.6 kda ) is an anomaly ; there is little 
 difference between emission in solution ( 514 nm ) versus film ( 518 
 nm ) , and the emission only blue - shifts further with increasing molecular 
 weight .
previously and here for samples 911 , emission wavelengths for polymers in solution and in films 
 were only comparable for high polymer molecular weights ( i.e. , low 
 dye loading ) , where dyes are diluted and monomeric dye structures 
 are favored .
previous studies showed that bf2bdk 
 luminescence properties 
 are dependent upon the molecular weight of polylactide or dye loading in dye / polymer blends .
shorter dye  dye distances corresponding to low dye  pla 
 molecular weight resulted in lower energy green emission for bf2dbmpla , whereas longer polymer chains ( small dye loadings ) 
 with diminished dye  dye interactions correlate with blue - shifted 
 emission .
polymers 912 also follow this trend , namely , emission wavelength blue - shifts 
 with increasing polymer molecular weight .
for example , polymers 9a  c display emission wavelengths of 526 , 
 496 , and 461 nm , respectively .
furthermore , the sensitivity to dye 
 loading ( i.e. , mn ) varies for different 
 dyes .
for instance , for 9a ( 5.4 kda ) and 9c ( 16.1 kda ) the emission wavelength shifts 65 nm , while from 12a ( 5.6 kda ) to 12c ( 16.6 kda ) it shifts only 
 23 nm ( figures 5 , 6 and 
 table 2 ) . comparison of the emission spectra of 
 dye  polymer conjugates 912 of different molecular weights .
images showing emission color changes for polymers 912 of different molecular weights under air 
 with uv lamp excitation .
fluorescence lifetimes were recorded for bf2bdkpla 
 films 
 under ambient conditions ( tables 2 and table s7 ) . in all cases , lifetimes fit to triple - exponential 
 decay , which may be ascribed to different fluorophore associations 
 or polymer microenvironments in the solid state . as the polymer molecular weight increases , dye 
dye 
 interactions decrease , and the pre - exponential weighted lifetimes , pw0 , steadily decrease . for 
 the entire molecular weight range examined in this study , dye aggregation 
 may be present in polymer films , because only for 12c do lifetimes approach the value measured for monomeric structures 
 in solution .
as expected , the heavy atom has a significant influence 
 on the fluorescence lifetime . for nonhalogenated polymers 9a and 11a lifetimes
are 13 ns , whereas lifetimes 
 for brominated analogues 10a and 12a decrease 
 to 3.7 and 5.0 ns , respectively ( table 2 ) .
phosphorescence measurements 
 for polymer films were performed under 
 a nitrogen atmosphere , and spectra were recorded with a 2 ms delay . 

previous studies showed that bf2bdk dyes exhibit room temperature 
 phosphorescence ( rtp ) in rigid media such as poly(lactic acid ) ( tg  60 c ) . here ,
the phosphorescence maxima vary little ( 538568 nm ) for 
 dyes in this series and for different molecular weights ( i.e. , dye 
 loadings ) of a given dye . for example , the rtp for bf2nbmpla 
 polymers 9a  c is 555 nm , 
 and for its brominated derivatives bf2n(br)bmpla 10a  d the phosphorescence maxima slightly 
 red - shifted to 565 nm . for the bf2bnmpla polymers 11a  c ,
the rtp is 545 nm , and 
 the br derivatives bf2b(br)mpla , 12a  c show slightly blue - shifted rtp at 538 
 nm ( figure 7 ) . normalized delayed emission spectra ( i.e. , 
 phosphorescence + delayed 
 fluorescence ) for polymer films 912 at room temperature under nitrogen ( ex = 369 nm ) . 

the polymers without a bromide heavy atom usually have longer triplet 
 state lifetimes than their heavy atom derivatives .
heavy atoms increase 
 intersystem crossing ( isc ) , decrease quantum yields , and also shorten 
 singlet and triplet state lifetimes .
for example , the phosphorescence 
 lifetime p for bf2nbmpla , 9b ( 7.7 kda ) , is 103 ms , and p for the corresponding 
 brominated derivative , bf2n(br)bmpla , 10a ( 6.8 
 kda ) , is 11 ms . when the heavy atom is present on the weaker 
 phenyl donor instead of the naphthyl ring , the effect is not so pronounced .
12a ( 5.6 kda ) show comparable phosphorescence lifetimes , p of 20 and 26 ms , respectively .
this provides 
 further evidence that it is the stronger arene donor that dominates 
 emission properties in these boron -diketonate systems .
polymer molecular weight has a significant influence on the fluorescence 
 wavelength . because the phosphorescence energy remains relatively 
 constant throughout the series , the singlet 
previously for bf2dbm(i)pla we showed that phosphorescence 
 intensity increases with heavy atom substitution and for lower molecular 
 weights .
nearly full range fluorescence / phosphorescence ( f / p ) intensity 
 tuning was possible ; namely , for high molecular weight polymers , fluorescence 
 was strong and phosphorescence was weak and for low molecular weight 
 polymers , the opposite was true . in this 
 study , we explore how fluorescence and phosphorescence peak separations 
 and f / p intensity ratios vary for the naphthyl dye series .
this is 
 important for ratiometric oxygen sensing , where the oxygen invariant 
 fluorescence signal serves as the standard and oxygen sensitive phosphorescence 
 serves as the sensor .
a comparison of total emission spectra 
 under air ( fluorescence ) 
 with emission under nitrogen ( fluorescence and phosphorescence ) reveals 
 a red shoulder for 9 and slight peak broadening for 11 and 12 under nitrogen ( figure 8) .
this contrasts with bf2n(br)bmpla 10 , with a distinct fluorescence peak at f 490 
 and a more intense phosphorescence peak at 565 nm ( figure 9 ) .
total emission spectra also reveal a much stronger 
 heavy atom effect when the bromide is present on the major donor ( i.e. , 
 naphthalene ring ) . for 11 and 12 ,
phosphorescence 
 and fluorescence emission maxima are comparable and emission bands 
 effectively overlap for the entire molecular weight range investigated . 

as expected , for 11 and 12 , the overall 
 intensity at the emission band maximum increases under nitrogen ( fluorescence 
 + phosphorescence ) versus air ( fluorescence ; phosphorescence is quenched ) . 

though not useful for ratiometric oxygen sensing , materials with small 
 singlet triplet energy gaps are of interest for high efficiency oled 
 materials . for polymer 9 , in contrast , distinct fluorescence 
 and phosphorescence bands are observed for the high molecular weight 
 polymer 9c with maximal peak separation .
however , at 
 this lower dye loading the phosphorescence is weak relative to fluorescence , 
 corresponding to a smaller dynamic range for oxygen sensing .
total emission 
 spectra for polymers 912 under air 
 and under a nitrogen atmosphere .
fluorescence and phosphorescence 
 peaks are well separated , and the phosphorescence intensity is strong 
 for all samples 10a  d ( mn = 726 kda ) ( figure 9 ) . 

even the sample with the lowest dye loading has strong phosphorescence 
 intensity and could be useful for sensing , minimizing the valuable 
 dye reagent and possible toxicity in a biological environment . what 
 is more , the fluorescence and phosphorescence peaks are well aligned 
 with the blue and green channels of rgb ccd cameras , allowing for 
 ratiometric sensing with a simple hand - held imaging modality .
( a ) total emission 
 spectra showing relative fluorescence and phosphorescence 
 intensities for films of bf2n(br)bmpla 10a  d of different molecular weights under n2 .
( b ) images showing emission ( excitation by uv lamp ) : top , 
 under ambient conditions ; bottom , under n2 .
in summary , naphthyl - substituted 
 bf2bdks with and without 
 bromide heavy atoms were prepared for use as initiators to generate 
 polymers with different molecular weights .
compared to previous dbm 
 systems , the naphthyl materials displayed red - shifted absorbance , 
 fluorescence , and phosphorescence .
even when the general structure and the conjugation 
 length of the dye are the same , the polymer attachment site 
whether 
 on the naphthyl or phenyl ring  plays an important role in electron 
 density delocalization and optical properties .
computational studies 
 and homo and lumo analysis for the dye initiators 5 ( och2ch2oh attached to phenyl ring ) and 7 ( och2ch2oh attached to the naphthyl 
 ring ) revealed that both have ict character . nevertheless , the experimentally 
 stokes shift for 5 is small and for 7 it 
 is large in solution . in the solid state ,
fluorescence is sensitive 
 to the position of 
 polymer attachment whereas phosphorescence remains relatively constant . 

consequently , fluorescence and phosphorescence peaks are well separated 
 for polymers with blue - shifted fluorescence ( e.g. , 9 ) 
 but are poorly separated for those with the most red - shifted fluorescence 
 ( 11 ) . aside from altering the dye molecular structure , 
 it is also possible to adjust fluorescence energies and emission lifetimes 
 via polymer molecular weight . increasing the molecular weight decreases 
 dye  dye interactions and results in blue - shifted fluorescence , 
 decreased fluorescence lifetimes , increased phosphorescence lifetimes , 
 and a larger singlet to triplet energy gap . achieving fluorescence 
 and phosphorescence peak separation in these ways is important for 
 ratiometric sensing .
heavy atom substitution and placement also 
 play an important role 
 in optical properties .
halide substitution shifts the homo from ict 
 to  * character ( e.g. , 5 vs 6 ) , as supported by computational studies and radiative lifetimes . 

fluorescence lifetimes and quantum yields decrease significantly 
 upon bromide substitution on the major donor , namely the naphthyl 
 ring ( 6 ) , whereas the heavy atom effect is considerably 
 weaker for bromide substitution on the minor arene donor ( e.g. , phenyl 
 ring ) ( 8) .
dyes without heavy atoms such as bf2nbmpla have longer phosphorescence lifetimes and thus are more sensitive 
 to oxygen .
furthermore , it is possible 
 to fine - tune the lifetime and thus oxygen sensitivity with polymer 
 molecular weight ( i.e. , dye loading ) .
dyes such as bf2bnmpla 
 and bf2b(br)nmpla with very small singlet  triplet 
 energy gaps , indistinguishable fluorescence and phosphorescence signals , 
 could be of interest for oled materials .
finally , bf2n(br)bmpla , 
 with red - shifted emission , good fluorescence and phosphorescence peak 
 separation , strong phosphorescence versus fluorescence intensity , 
 and emission wavelengths that are well aligned with the blue and green 
 channels of a ccd camera make this material an excellent candidate 
 for ratiometric oxygen imaging .
 luminescent materials are important 
 for imaging and sensing . 
 aromatic 
 difluoroboron -diketonate complexes ( bf2bdks ) are 
 classic fluorescent molecules that have been explored as photochemical 
 reagents , two - photon dyes , and oxygen sensors 
 . a series of bf2bdks with naphthyl and phenyl groups was synthesized , and 
 photophysical properties were investigated in both methylene chloride 
 and poly(lactic acid ) ( pla ) . 
 polymer molecular weight and dye attachment 
 site along with bromide heavy atom placement were varied to tune optical 
 properties of dye  pla materials . 
 systems without heavy atoms 
 have long phosphorescence lifetimes , which is useful for lifetime - based 
 oxygen sensing . 
 bromine substitution on the naphthyl ring resulted 
 in intense , clearly distinguishable fluorescence and phosphorescence 
 peaks important for ratiometric oxygen sensing and imaging . 
Introduction
Experimental Section
Results and Discussion
Conclusions