Gold Nanoclusters: Bridging Gold Complexes and Plasmonic Nanoparticles in Photophysical Properties

nanomaterials

Article Gold Nanoclusters: Bridging Gold Complexes and Plasmonic Nanoparticles in Photophysical Properties

Meng Zhou , Chenjie Zeng, Qi Li, Tatsuya Higaki and Rongchao Jin * Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA * Correspondence: [email protected]



Received: 29 May 2019; Accepted: 25 June 2019; Published: 28 June 2019


Abstract: Recent advances in the determination of crystal structures and studies of optical properties of gold nanoclusters in the size range from tens to hundreds of gold atoms have started to reveal the grand evolution from gold complexes to nanoclusters and further to plasmonic nanoparticles. However, a detailed comparison of their photophysical properties is still lacking. Here, we compared the excited state behaviors of gold complexes, nanolcusters, and plasmonic nanoparticles, as well as small organic molecules by choosing four typical examples including the Au10 complex, Au25 nanocluster (1 nm metal core), 13 diameter Au nanoparticles, and Rhodamine B. To compare their photophysical behaviors, we performed steady-state absorption, photoluminescence, and femtosecond transient absorption spectroscopic measurements. It was found that gold nanoclusters behave somewhat like small molecules, showing both rapid internal conversion (<1 ps) and long-lived excited state lifetime (about 100 ns). Unlike the nanocluster form in which metal–metal transitions dominate, gold complexes showed significant charge transfer between metal atoms and surface ligands. Plasmonic gold nanoparticles, on the other hand, had electrons being heated and cooled (~100 ps time scale) after photo-excitation, and the relaxation was dominated by electron–electron scattering, electron–phonon coupling, and energy dissipation. In both nanoclusters and plasmonic nanoparticles, one can observe coherent oscillations of the metal core, but with different fundamental origins. Overall, this work provides some benchmarking features for organic dye molecules, organometallic complexes, metal nanoclusters, and plasmonic nanoparticles.

Keywords: gold nanomaterials; electron dynamics; phonon dynamics; optical properties

1. Introduction Gold nanomaterials have attracted great interest in both fundamental science and practical applications such as sensing, catalysis, and optoelectronics owing to their unique properties [1–9]. For gold nanoparticles with diameters between 3–100 nm, the strong surface plasmon resonance (SPR) dominates in the absorption spectrum, which is caused by collective excitation of free electrons. In contrast, ultra-small gold nanoparticles consisting of tens to hundreds of gold atoms, often called gold nanoclusters, show multiple absorption bands spanning the UV-visible NIR range because of discrete energy levels [10,11]. The significant progress in ligand-protected (e.g., thiolate) gold nanoclusters has allowed atomically precise control of their size and structure [11–13]. The structure of a thiolate-protected gold nanocluster typically consists of a metal core with Au–Au bonds (~2.8 Å) and surface Au–S staple motifs [11]. The Au(I) complexes, on the other hand, do not have a metal core, albeit gold aurophilic interactions (Au Au distance > 3 Å) often exist owing to the closed-shell ··· d [10] configuration of Au(I) [14,15]. Typically, solutions of gold complexes are colorless because their absorption peaks lie in the ultraviolet (UV) range (shorter than 400 nm). Plasmonic gold nanoparticles (AuNPs), gold nanoclusters, and Au(I) complexes have distinct optical features as a result of their differences in size, structure, and bonding. Therefore, understanding

Nanomaterials 2019, 9, 933; doi:10.3390/nano9070933 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, x FOR PEER REVIEW 2 of 12

the closed-shell d [10] configuration of Au(I) [14,15]. Typically, solutions of gold complexes are colorless because their absorption peaks lie in the ultraviolet (UV) range (shorter than 400 nm). NanomaterialsPlasmonic2019, 9 gold, 933 nanoparticles (AuNPs), gold nanoclusters, and Au(I) complexes have distinct2 of 13 optical features as a result of their differences in size, structure, and bonding. Therefore, understanding the photodynamics will help to deepen the understanding of their electronic thestructures photodynamics and optical will helpproperties to deepen [2,7,9,12,16– the understanding18]. The ofelectron their electronic dynamics structures of plasmonic and optical gold propertiesnanoparticles [2,7, 9has,12, 16been–18 ].intensively The electron investigated dynamics of [1 plasmonic9,20]. Since gold the nanoparticles 1990s, El-Sayed has beenand intensivelycoworkers investigatedhave probed [the19,20 size]. Sinceand shape the 1990s,dependent El-Sayed electron and dynamics coworkers of havemetallic probed AuNPs the [21,22]. size and Hartland shape dependentand Vallee electron groups dynamics have extensively of metallic AuNPsinvestigat [21ed,22 ].the Hartland phonon and dynamics Vallee groups and electron–phonon have extensively investigatedcoupling of thedifferent phonon sized dynamics AuNPs and [23,24]. electron–phonon In recent years, coupling there of has diff erentalso been sized research AuNPs [on23, 24the]. Inexcited recent state years, dynamics there has of also ligand-protected been research gold on the na excitednoclusters state [1,2,25–27]. dynamics The of ligand-protected electron and phonon gold nanoclustersdynamics of [1the,2, 25nanoclusters–27]. The electron were found and phonon to be dependent dynamics ofon the both nanoclusters size and werestructure found [28–30]. to be dependentStamplecoskie on both and size Kamat and [31] structure found [28 that–30 ].the Stamplecoskie dynamics of andAu(I) Kamat complexes [31] found are different that the dynamicsfrom that ofof Au(I)gold complexesnanoclusters. are diDespitefferent fromsuch thatprogress, of gold a nanoclusters. systematic comparison Despite such of progress, the photo-dynamics a systematic comparisonbetween them of theis still photo-dynamics lacking. between them is still lacking. Here,Here, wewe chosechose RhodamineRhodamine BB (RB), (RB), Au Au1010(SR)(SR)1010 complex,complex, AuAu2525(SR)(SR)1818 nanocluster, andand 1313 nmnm diameterdiameter AuNPsAuNPs protected by citrate (Figure (Figure 11)) as typical examples examples to to compare compare the the photophysics photophysics of ofsmall small molecules, molecules, gold gold complexes, complexes, nanoclusters, nanoclusters, and and nanoparticles. We employedemployed time-resolvedtime-resolved fluorescencefluorescence and and femtosecond femtosecond transient transient absorption absorpti spectroscopyon spectroscopy to probe to their probe excited their state excited behaviors. state Thebehaviors. electron The and electron phonon and dynamics phonon are dynamics discussed are and discussed compared and in compared detail. The in obtaineddetail. The results obtained are ofresults great are importance of great toimportance understand to their understand optical response their optical and furtherresponse promote and further their applications promote their in sensingapplications and optics. in sensing and optics.

Figure 1. (Upper panel) Photograph of dilute solutions of Rhodamine B, sub-nanometer Au10(SR)10 Figure 1. (Upper panel) Photograph of dilute solutions of Rhodamine B, sub-nanometer Au10(SR)10 complex, 1 nm Au25(SR)18 nanocluster, and ~13 nm diameter gold nanoparticles. (Lower panel) complex, 1 nm Au25(SR)18 nanocluster, and ~13 nm diameter gotld nanoparticles. (Lower panel) Molecular structures of Rhodamine B, Au10(SR)10 (where, R: Ph– Bu), and Au25(SR)18 (where, R: Molecular structures of Rhodamine B, Au10(SR)10 (where, R: Ph–tBu), and Au25(SR)18 (where, R: CH2CH2Ph). Color labels: yellow, S atoms; all other colors are for Au. The R groups and citrate stabilizersCH2CH2Ph). on ~13Color nm labels: Au nanoparticles yellow, S atoms; are not all shown. other colors are for Au. The R groups and citrate stabilizers on ~13 nm Au nanoparticles are not shown. 2. Materials and Methods 2. Materials and Methods. Sample preparation: Rhodamine B was purchased from Sigma–Aldrich (St. Louis, MO, USA) and usedSample as received.preparation: Au Rhodamine10(SR)10 (where, B was SR purchased= 4-tert-butylbenzenethiolate), from Sigma–Aldrich (St. Au Louis,25(SR) MO,18 (where, USA) SRand= used2-phenylethanethiol), as received. Au10 and(SR) 1310 (where, nm diameter SR = Au4-tert-butylbenzenethiolate), nanoparticles protected by Au trisodium25(SR)18 (where, citratewere SR = prepared2-phenylethanethiol), according to theand literature 13 nm diameter [32,33]. Au nanoparticles protected by trisodium citrate were prepared according to the literature [32,33].

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Steady state absorption and photoluminescence: UV-vis absorption spectra were measured on a Shimadzu UV-3600plus spectrometer (Kyoto, Japan). Steady-state photoluminescence was measured on a Fluorolog-3 spectrofluorometer from Horiba Jobin Yvon (Piscataway, NJ, USA). Time-resolved luminescence: fluorescence lifetimes were measured with a time-correlated single photon counting technique from Horiba Jobin Yvon (Piscataway, NJ, USA); a pulsed LED source (376 nm, 1.1 ns) was used as the excitation source. Transient absorption spectroscopy: Femtosecond transient absorption spectroscopy was carried out using a commercial Ti:Sapphire laser system (SpectraPhysics, 800 nm, 100 fs, 3.5 mJ, 1 kHz) (Santa Clara, CA, USA). A pump pulse was generated using a commercial optical parametric amplifier (LightConversion, Vilnius, Lithuania). A small portion of the laser fundamental was focused into a sapphire plate to produce a supercontinuum in the visible region, which overlapped in time and space with the pump. Multi-wavelength transient spectra were recorded using dual spectrometers (i.e., signal and reference) (Thorlabs, Newton, NJ, USA) equipped with array detectors whose data rates exceed the repetition rate of the laser (1 kHz). Solution samples in 1 mm path length cuvettes were excited by the tunable output of the OPA (pump). Nanosecond transient absorption measurements were conducted using the same ultrafast pump pulses along with an electronically delayed supercontinuum light source with a sub-nanosecond pulse duration (EOS, Ultrafast Systems, Sarasota, FL, USA).

3. Results and Discussions Steady-state absorption and photoluminescence spectra: From the steady-state spectra, one can clearly observe the differences between small molecules and different-sized gold nanomaterials. The RB dye showed significant absorption peaks at ~550 nm and a shoulder at ~520 nm (Figure2A), which originated from 0–0 and 0–1 vibronic peaks of S1 state, respectively. Other absorption peaks at shorter wavelengths were transitions from the ground state to higher excited states than S1. The fluorescence spectrum of RB exhibited a mirror image to the S0–S1 absorption band, with a Stokes shift of 0.1 eV. The Au10 complex, on the other hand, showed absorption in the UV region only, with a peak at 346 nm and a shoulder at 380 nm (Figure2B). The higher energy transitions ( λ < 300 nm) originated from intraligand (IL) transitions, while the lower energy transitions (346 and 370 nm) should arise from ligand to metal or metal to ligand charge transfer (LM/MLCT) modified by Au(I)–Au(I) interactions [34,35]. The Au10 complex showed no observable photoluminescence (PL); however, some of the other gold complexes were reported to exhibit strong PL [36–38]. Unlike gold complexes, the Au25 nanocluster exhibited multiple absorption bands spanning the entire UV-Vis range (Figure2C). Theoretical calculations revealed that the absorption band at 670 nm was from the sp to sp transition while absorption bands at shorter wavelengths involved both sp to sp and d to sp transitions [10]. The Au25(SR)18 exhibited weak photoluminescence (QY < 1%) related to the surface [39]. However, one can observe that the PL peak (centered at 750 nm) overlapped with the lowest absorption band, which indicates that the emission in the visible region may not be the intrinsic PL of Au25 nanoclusters. As the size of particles further increased, more and more gold atoms contributed to the electronic states, and finally SPR emerged [8] in the UV-vis spectrum, such as the ~13 nm Au nanoparticles (Figure2D). Plasmonic gold nanoparticles had a continuous electronic band (i.e., the conduction band), and only 4 had a very weak photoluminescence (QY = 10− )[40]. Below, we further discuss the excited state behaviors of these four species, from which one can find their molecular and plasmonic behaviors. Nanomaterials 2019, 9, 933x FOR PEER REVIEW 4 4of of 12 13

Figure 2. Steady-state optical spectra of ((AA)) RhodamineRhodamine B;B; ((BB)) complexcomplex AuAu1010(SR)(SR)1010;;( (C) nanocluster Au25(SR)(SR)1818, ,and and (D (D) )13 13 nm nm diameter diameter Au Au nanoparticles. nanoparticles.

Organic dyes (Rhodamine B): Before we discuss the photo-dynamics of different different sized gold nanomaterials, we first first discuss the excited state be behaviorhavior of organic dyes such as Rhodamine B B (RB) for an illustration of molecular behavior. The The RB RB has been widely used as a probe in biological and synthetic polyelectrolytepolyelectrolyte systems system [s41 ].[41]. The The excited excited state behaviorstate behavior of organic of dyesorganic has beendyes intensivelyhas been intensivelyinvestigated investigated ever since the ever birth since of time-resolved the birth of spectroscopy time-resolved [42 –spectroscopy44]. An aqueous [42–44]. solution An ofaqueous RB has solutionstrong luminescence of RB has strong (QY luminescence= 30%) and the (QY PL = lifetime 30%) and is determinedthe PL lifetime to be is 1.7determined ns in water to be (see 1.7 Figure ns in waterS1 in the(see Supplementary Figure S1 in Materials).the Supplementary It is worth Materi notingals). that It theis worth excited noting state lifetimethat the of excited RB is highly state lifetimedependent of onRBsolvent is highly polarity dependent (Figure on S1), solvent which haspolarity been reported(Figure S1), previously which [has45]. been In our reported current work,previously the transient [45]. In our absorption current spectroscopicwork, the transi measurementent absorption on RBspectroscopic was performed measurement with excitations on RB wasat 360 performed nm and 560 with nm, excitations respectively, at 360 to excite nm and the sample560 nm, to respectively, the second and to excite first singlet the sample excited to state. the secondUpon photo-excitation, and first singlet one excited can observe state. excited Upon statephoto-excitation, absorption (ESA) one around can observe 450 nm, excited ground state absorptionbleaching (GSB), (ESA) and around stimulated 450 nm, emission ground (SE) state between bleaching 500 nm (GSB), and 700 and nm (Figurestimulated3A,B). emission With 360 (SE) nm betweenexcitation, 500 one nm can and observe 700 nm an ultrafast(Figure 3A,B). decay (~100With fs)360 in nm GSB excitation, at 520 nm one (Figure can 3observeC, blue dip)an ultrafast as well decayas a rise (~100 in SE fs) at in 630 GSB nm at (Figure520 nm3 C,(Figure black). 3C, On blue the dip) other as hand, well as the a ultrafastrise in SE decay at 630 component nm (Figure was 3C, black).absent withOn the excitation other hand, at 560 the nm ultrafast (Figure 3decayD); thus, component the 100 fswas component absent with was excitation assigned asat internal560 nm (Figureconversion 3D); from thus, S2 theto S1001 state. fs component From the kinetic was assigned traces, ESA, as internal GSB, and conversion SE decay simultaneously from S2 to S1 state. from 1From ps to the 3 ns kinetic and the traces, transient ESA, absorption GSB, and SE (TA) decay lifetime simultaneously (1.7 ns) matched from well 1 ps with to 3 thens PLand lifetime the transient (Table absorptionS1 in SI). The (TA) TA lifetime spectra and(1.7 decayns) matched dynamics well of with RB we the observed PL lifetime here (Table matched S1 wellin SI). with The the TA results spectra of previousand decay studies dynamics [44,45 of]. RB we observed here matched well with the results of previous studies [44,45].

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FigureFigure 3. ((AA–,B) Transient absorption spectra of Rhodamine B (dissolved in in water) water) at at different different time delaysdelays with with excitation excitation of of 360 360 nm nm and and 560 560 nm, nm, respectively. respectively. (C (C–,DD)) Transient Transient absorption decays decays at at differentdifferent probe wavelengths.

ToTo sumsum up,up, the the photophysics photophysics of RBof andRB otherand othe typicalr typical organic organic dye molecules dye molecules can be well can explained be well explainedby the Jablonski by the diagramJablonski and diagram the relaxation and the followsrelaxati theon follows Kasha’s the rule Kasha’s [46], that rule is, [46], the excited that is, state the excitedmolecules state first molecules experience first a rapidexperience non-radiative a rapid non- relaxationradiative to therelaxation lowest singletto the lowest excited singlet state and excited then stateemit and photons then toemit relax photons to the groundto relax state.to the ground state. Gold complexes (Au (SR) ): Gold complexes are composed of one or a few gold atoms Gold complexes (Au10(SR)1010): Gold complexes are composed of one or a few gold atoms coordinated by ligands. The Au (SR) complex is composed of two interlocked Au (SR) rings and coordinated by ligands. The Au10(SR)1010 complex is composed of two interlocked Au5(SR)55 rings and everyevery goldgold atomatom is is connected connected to to two two S atoms S atoms [32]. [32]. Upon Upon photoexcitation photoexcitation at λ = at365 λ nm,= 365 broad nm, positive broad positivesignals weresignals observed were observed (Figure4 A)(Figure with no4A) GSB with signal, no GSB so the signal, transient so the signal transient originates signal solely originates from solelyESA. Suchfrom anESA. observation Such an observation has also been has reported also been in otherreported Au(I) in complexesother Au(I) [ 47complexes]. In the initial[47]. In 36 the ps, initialthe ESA1 36 ps, around the ESA1 780 nmaround decays 780 tonm give decays rise to to give the ESA2rise to at the 535 ESA2 nm (Figureat 535 nm4B). (Figure In the 4B). following In the following2.8 ns, ESA 2.8 at allns, wavelengthsESA at all wavelengths decay dramatically decay bydramatically 90%. Decay by associated 90%. Decay spectra associated (DAS) obtained spectra (DAS)by global obtained analysis by and global singular analysis value and decomposition singular value (SVD) decomposition of the TA data exhibited(SVD) of threethe decayingTA data exhibitedcomponents, three 14 decaying ps, 290 ps, components, and >1 ns (Figure 14 ps,4C,D). 290 ps, Considering and >1 ns that (Figure gold–thiolate 4C,D). Considering complexes show that gold–thiolateLM/MLCT characteristics complexes inshow their LM/MLCT steady-state characteri UV-vis spectra,stics in photo-excitation their steady-state at 365 UV-vis nm can spectra, directly photo-excitationgenerate LM/MLCT at 365 excited nm statecan di [35rectly,37,47 generate]. The first LM/MLCT 14 ps process excited can state be assigned [35,37,47]. to theThe stabilization first 14 ps processand equilibrium can be assigned of LM /toMLCT the stabilization state. In a and previous equilibrium study, weof LM/MLCT probed the state. photodynamics In a previous of study, Au10 complexes dissolved in toluene and dichloromethane and found that the decay lifetime was dependent we probed the photodynamics of Au10 complexes dissolved in toluene and dichloromethane and foundon solvent that polaritythe decay [48 lifetime]. Here, wewas repeated dependent the TAon spectrasolvent ofpolarity Au10 dissolved [48]. Here, in dichloromethanewe repeated the andTA the same processes can be observed with similar time constants to those in the previous study. Upon spectra of Au10 dissolved in dichloromethane and the same processes can be observed with similar 1 3 timephotoexcitation, constants to intersystem those in the crossing previous (ISC) study. from LMUpon/MLCT photoexcitation, to LM/MLCT intersystem occurred in lesscrossing than 100(ISC) fs, 3 fromwhich 1LM/MLCT was not resolved to 3LM/MLCT in our TA occurred measurement. in less The thanLM 100/MLCT fs, which state was was not then resolved stabilized in in our tens TA of 3 * measurement.picoseconds to formThe a3LM/MLCTLM/MLCT statestate, was which then decayed stabilized to the in ground tens stateof picoseconds (Figure5). to form a 3LM/MLCT* state, which decayed to the ground state (Figure 5).

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Figure 4. (A) Transient absorption (fs-TA) data of Au10 complex dissolved in dichloromethane. (B) Figure 4. (A) Transient absorption (fs-TA) data of Au complex dissolved in dichloromethane. TransientFigure 4. (absorptionA) Transient spectra absorption as a function(fs-TA) dataof time of Au delay10 10complex from 0.5 dissolved ps to 36 in ps dichloromethane. and that at 2.8 ns.(B) (B) Transient absorption spectra as a function of time delay from 0.5 ps to 36 ps and that at 2.8 ns. ScatteringTransient dueabsorption to the laserspectra pulse as wasa function excluded of aroundtime delay 800 nmfrom for 0.5 (A ps) and to (36B). ps (C and) Decay that associated at 2.8 ns. Scattering due to the laser pulse was excluded around 800 nm for (A,B). (C) Decay associated spectra spectraScattering (DAS) due toobtained the laser from pulse the was gl obalexcluded analysis around of the800 TAnm fordata. (A )( Dand) Kinetic (B). (C )traces Decay (dots) associated and (DAS) obtained from the global analysis of the TA data. (D) Kinetic traces (dots) and corresponding fit correspondingspectra (DAS) fitobtained (solid line) from at selectedthe global wavele analysisngths, of which the TA shows data. the ( Dqu) alityKinetic of the traces fitting. (dots) and (solidcorresponding line) at selected fit (solid wavelengths, line) at selected which wavele showsngths, the quality which ofshows the fitting. the quality of the fitting.

Figure 5. Diagram for the relaxation dynamics in the Au10 complex. The 290 ps process shows solvent Figure 5. Diagram for the relaxation dynamics in the Au10 complex. The 290 ps process shows dependency (dichloromethane in this study versus 350 ps in toluene in a previous study [48]). solventFigure 5.dependency Diagram for(dichloromethane the relaxation dynamicsin this stud iny theversus Au 10350 complex. ps in toluene The 290 in psa previousprocess showsstudy solvent dependency (dichloromethane in this study versus 350 ps in toluene in a previous study Nanoclusters[48]). (Au (SR) as the example): Unlike homoleptic gold complexes, thiolate-protected [48]). 25 18 gold nanoclusters are composed of a well-defined metal core and surface staple motifs (e.g., Nanoclusters (Au25(SR)18 as the example): Unlike homoleptic gold complexes, –S–Au–S–) [11]. The relaxation dynamics of gold nanoclusters of different structures have been thiolate-protectedNanoclusters gold(Au nanoclusters25(SR)18 as arethe composed example): of a well-definedUnlike homoleptic metal core andgold surface complexes, staple investigated by several groups and the relaxation model is more complicated as a result of multiple motifsthiolate-protected (e.g. –S–Au–S–) gold [11].nanoclusters The relaxation are composed dynamics of aof well-defined gold nanocl metalusters coreof different and surface structures staple contributions of both Au and S atoms to the orbitals [27,28,49,50]. Here, Au25(SR)18 was chosen as an havemotifs been (e.g. investigated –S–Au–S–) by[11]. several The regroupslaxation and dynamics the relaxation of gold model nanocl is ustersmore complicated of different asstructures a result have been investigated by several groups and the relaxation model is more complicated as a result

Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 12 Nanomaterials 2019, 9, 933 7 of 13 of multiple contributions of both Au and S atoms to the orbitals [27,28,49,50]. Here, Au25(SR)18 was chosen as an example to illustrate the photophysics. The Au25(SR)18 structure consisted of an example to illustrate the photophysics. The Au25(SR)18 structure consisted of an icosahedral Au13 core icosahedral Au13 core and six Au2(SR)3 dimeric staple motifs for surface protection (Figure 1) [10]. and six Au2(SR)3 dimeric staple motifs for surface protection (Figure1)[ 10]. With excitation at 490 nm, With excitation at 490 nm, one can observe broad ESA overlapped with GSB peaks at 510 nm, 550 one can observe broad ESA overlapped with GSB peaks at 510 nm, 550 nm, and 675 nm (Figure6A), nm, and 675 nm (Figure 6A), which is a typical feature of gold nanoclusters [2,50,51]. During the which is a typical feature of gold nanoclusters [2,50,51]. During the first picosecond, one can observe first picosecond, one can observe a broad ESA band between 500 nm and 620 nm decaying rapidly a broad ESA band between 500 nm and 620 nm decaying rapidly and the time constant was ~600 fs and the time constant was ~600 fs (Figure 6B). The rapid relaxation was not observed under (Figure6B). The rapid relaxation was not observed under excitation of 800 nm (Figure6C), which excitation of 800 nm (Figure 6C), which suggests that it should be internal conversion from higher suggests that it should be internal conversion from higher to lower excited states. With excitation to lower excited states. With excitation at 800 nm, the TA spectrum at ~0.3 ps was equal to the at 800 nm, the TA spectrum at ~0.3 ps was equal to the spectrum with excitation at 490 nm after spectrum with excitation at 490 nm after 2 ps, which indicates that the 800 nm pulse excited Au25 2 ps, which indicates that the 800 nm pulse excited Au25 directly to the lower excited state. The TA directly to the lower excited state. The TA signal did not decay significantly between 2 ps and 3 ns, signal did not decay significantly between 2 ps and 3 ns, indicating a significantly longer excited state indicating a significantly longer excited state lifetime of Au25 than 3 ns. It was also interesting to see lifetime of Au25 than 3 ns. It was also interesting to see that the nanosecond TA and time resolved-PL that the nanosecond TA and time resolved-PL give different lifetimes (Figure S2 in the give different lifetimes (Figure S2 in the Supplementary Materials). Compared with the dynamics Supplementary Materials). Compared with the dynamics of the Au10 complex, the excited state of the Au10 complex, the excited state dynamics of the Au25 nanoclusters behaved more like that of dynamics of the Au25 nanoclusters behaved more like that of small molecules. small molecules.

FigureFigure 6. ((AA)) TransientTransient absorption absorption data data map map of Auof 25Au(SR)25(SR)18 with18 with excitation excitation at 490 at nm;490 (nm;B) kinetic (B) kinetic traces tracesprobed probed at selected at selected wavelengths waveleng withths excitationwith excitation at 490 at nm. 490 (nm.C) Transient (C) Transient absorption absorption data data map map and andspectra spectra probed probed at 1 ps at with1 ps 800with nm 800 excitation; nm excitation; (D) kinetic (D) traceskinetic probe traces at probe 520 nm at 650 520 nm nm with 650 excitationnm with excitationat 800 nm. at 800 nm.

WithWith excitationexcitation at at 800 800 nm, nm, one canone alsocan observealso observe coherent coherent oscillations oscillations in the first in few the picoseconds first few 1 1 picoseconds(Figure6C,D). (Figure Two frequencies 6C,D). Two (40 frequencies cm − and (40 80 cmcm−–1 )and were 80 exhibited cm–1) were at diexhibitedfferent probe at different wavelengths probe wavelengths(Figure6D), which(Figure have 6D), been which reported have been previously reported [ 51previously]. These oscillations[51]. These oscillations observed in observed TA decays, in TAalso decays, observed also in observed other nanoclusters in other nanoclusters [52,53], were [52,53], assigned were to assigned acoustic to vibrations acoustic ofvibrations the metal of corethe metaland explained core and asexplained displacive as displacive excitation [excitation52,54], similar [52,54], to thatsimilar observed to that inobserved semiconductors in semiconductors and small andmolecules small molecules [54,55]. [54,55]. PlasmonicPlasmonic nanoparticles (13(13 nm nm diameter diameter AuNPs): AuNPs): As As the the particle partic sizele size becomes becomes larger, larger, more more gold goldatoms atoms will contribute will contribute to the electronicto the electronic states, and states, eventually and eventually the bandgap the will bandgap disappear, will giving disappear, rise to givinga transition rise to from a transition non-metallic from tonon-metallic metallic [8 ].to Themetallic TA spectra [8]. The of TA 13 spectra nm AuNPs of 13 showed nm AuNPs a significant showed ableaching significant signal bleaching at 520 nm signal and at ESA 520 on nm two and sides ESA (Figure on two7A). sides We found (Figure that 7A). (i) the We GSB found became that sharper (i) the

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GSB became sharper from 0.2 ps to 5 ps and (ii) the higher pump fluence gave rise to a broader bleachingfrom 0.2 ps signal to 5 ps compared and (ii) the to that higher of pumpthe lower fluence pump gave fluence rise to (Figure a broader 7A,B). bleaching The broadening signal compared of GSB into the that initial of the time lower delay pump (full fluence width at (Figure half maximum7A,B). The decreased broadening from of GSB50 nm in to the 30 initialnm in timeFigure delay 7A) is(full ascribed width atto halfthe heating maximum effect decreased [20]. Upon from photo- 50 nmexcitation, to 30 nm inelectrons Figure7 inA) the is ascribedAuNPs will to the be heating heated toeff ecta very [20 ].high Upon temperature photo-excitation, (e.g., 1000 electrons K, depend in theing AuNPs on the will pump be heatedpower). to Heating a very high the nanoparticle temperature will(e.g., result 1000 K,in dependingbroadening on of the the pump SPR power).peak as Heatingwell as the the nanoparticleGSB in the TA will spectra. result inAfter broadening the electrons of the areSPR heated peak as to well a asvery the high GSB intemperature, the TA spectra. the Afterexcited the state electrons energy are heatedwill fi torsta reach veryhigh equilibrium temperature, via electron–electronthe excited state energyscattering will (manifested first reach equilibrium in the 100 via fs electron–electronrise of the kinetic scattering traces in (manifested Figure 7C). in Subsequently,the 100 fs rise ofthe the energy kinetic traceswill inbe Figuretransferre7C). Subsequently,d from the theelectrons energy willto the be transferredlattice through from electron–phononthe electrons to the coupling lattice through (1–5 ps electron–phonon rapid decay in couplingFigure 7C) (1–5 [3,24]. ps rapid Accompanied decay in Figure by 7theC) [rapid3,24]. decay,Accompanied the GSB by will the also rapid be decay, narrowed the GSB during will alsothe electron–phonon be narrowed during coupling the electron–phonon process (Figure coupling 7A,B). Finally,process (Figurethe energy7A,B). will Finally, dissipate the energyinto the will environment dissipate into by thephonon–phonon environment byrelaxation, phonon–phonon which is dependentrelaxation, whichon the issurrounding dependent onmedium the surrounding (100 ps decay medium in Figure (100 ps 7C). decay As inthere Figure is 7noC). bandgap As there isin plasmonicno bandgap Au in nanoparticles, plasmonic Au there nanoparticles, is no electron there–hole is no separation electron–hole or recombination separation or recombination process as in nanoclusters.process as in nanoclusters.Instead, the Instead, relaxation the relaxation dynamics dynamics can be can described be described by bya a well-established two-temperature modelmodel [ 21[21,56].,56]. With With the the high high pump pump fluence, fluence, one can one observe can aobserve prominent a prominent oscillation 1 -1 oscillationwith a frequency with a of frequency 7.5 cm− (4of ps7.5 in cm periods) (4 ps in in the periods) 13 nm AuNPsin the 13 (Figure nm AuNPs7D). Unlike (Figure that 7D). of the Unlike gold thatnanoclusters, of the gold the nanoclusters, oscillations in the the oscillations metallic AuNPs in the originated metallic fromAuNPs the periodicoriginated shift from of thethe SPR periodic band shiftdue toof the the lattice SPR band expansion due to induced the lattice by laser expansio heatingn induced [20]. In addition,by laser heating the oscillations [20]. In inaddition, the metallic the oscillationsNPs can be wellin the modelled metallic byNPs a classical can be well continuum modelled model by (scalinga classical as 1continuum/D, where Dmodelis the (scaling diameter), as 1/butD,this where model D is fails the diameter), in gold nanoclusters. but this model fails in gold nanoclusters.

Figure 7.7. ((AA,B–B) Transient) Transient absorption absorption spectra spectra of 13 of nm 13 AuNPs nm Au atNPs diff erentat different time delays time with delays an excitation with an excitationof 360 nm of under 360 highnm under and low high pump and fluences;low pump (C )fluences; TA decay (C traces) TA decay at selected traces probe at selected wavelengths; probe wavelengths;(D) kinetic traces (D) withkinetic prominent traces with oscillation prominent features. oscillation features.

A distinct feature of metallic gold nanoparticles is that the electron–phonon coupling is dependent on pump fluence [3,19]. When the pump fluence increased from 80 to 1800 uJ/cm2, one can clearly observe that the electron–phonon coupling slowed down significantly (Figure 8A). After plotting the fitted time constants as a function of laser fluence, a linear relationship can be observed (Figure 8B), which agrees well with previous studies [3,56]. Such a power dependence is only

Nanomaterials 2019, 9, 933 9 of 13

A distinct feature of metallic gold nanoparticles is that the electron–phonon coupling is dependent on pump fluence [3,19]. When the pump fluence increased from 80 to 1800 uJ/cm2, one can clearly observeNanomaterials that 2019 the, 9 electron–phonon, x FOR PEER REVIEW coupling slowed down significantly (Figure8A). After plotting9 of the 12 fitted time constants as a function of laser fluence, a linear relationship can be observed (Figure8B), whichobserved agrees in plasmonic well with previous nanoparticles, studies which [3,56]. Suchcan be a powerwell explained dependence by isthe only two-temperature observed in plasmonic model nanoparticles,[21]. Compared which to the can plasmonic be well explained NPs, the by TA the measurements two-temperature of modelRB, Au [2110 ].complex, Compared and to Au the25 nanoclusters under different pump fluences exhibited no differences in the decay dynamics (Figure plasmonic NPs, the TA measurements of RB, Au10 complex, and Au25 nanoclusters under different pumpS3–5 in fluences the Supplementary exhibited no di Materials),fferences in hence, the decay they dynamics were power (Figures independent. S3–S5 in the This Supplementary serves as a Materials),signature of hence, the non-metallic they were power state. independent. In Table 1, Thisthe servesphotophysical as a signature features of theof non-metallicthe four types state. of Inmaterials Table1, theare photophysicalsummarized. In features Scheme of the1, the four relaxation types of materialsprocesses are as summarized.well as the time In Schemeconstants1, the are relaxationillustrated. processes as well as the time constants are illustrated.

Figure 8. (A) Kinetic traces probe at 520 nm with different pump fluences; (B) electron–phonon time Figure 8. (A) Kinetic traces probe at 520 nm with different pump fluences; (B) electron–phonon time constants as a function of pump fluence. constants as a function of pump fluence. Table 1. Characteristic features of molecules, complexes, nanoclusters, and plasmonic nanoparticles (PL: photoluminescence;Table 1. Characteristic ESA: features excited of state molecules, absorption; complexes, GSB: ground-state nanoclusters, bleach; and plasmonic SE: stimulated nanoparticles emission; IC:(PL: internal photoluminescence; conversion; ISC: ESA: intersystem excited state crossing). absorption; GSB: ground-state bleach; SE: stimulated emission; IC: internal conversion; ISC: intersystem crossing). Steady-State Structure Eg PL Transient Absorption Structure Eg Steady-stateabs. abs. PL Transient absorption ESA + GSB + SE, Dye molecules MultipleMultiple bands bands ESA + GSB + SE, Dye molecules IC, ISC, long lifetime e.g., No core >1–2 eV (e.g., π–π π π Yes No core >1–2 eV (e.g. – Yes (ns),IC, ISC, long lifetime (ns), e.g., RhodamineRhodamine B B transition) transition) power independencepower independence Multiple bands ESA (predominant),ESA (predominant), Multiple bands Complexes,Complexes, ISC, long lifetime μ NoNo core core > >22 eVeV (charge(charge transfer, transfer, Yes Yes ISC, long lifetime ( s–ns), e.g.,e.g., Au Au10(SR)(SR)10 (µs–ns), 10 10 CT) CT) power independencepower independence ESA + GSB, acoustic Core + MultipleMultiple bands bands ESA + GSB, acoustic (metal vibrations, vibrations, Nanoclusters, Core + surface Nanoclusters, surface ~2.5~2.5 eV eV to zero to (metalcore-based core-based+ Yes IC, ISC, varying e.g., Au25(SR)18 (staple motifs) Yes IC, ISC, varying lifetime e.g., Au25(SR)18 (staple zero metal+ metalligand ↔ ligand lifetime (ns–ps), ↔ (ns–ps), motifs) CT)CT) power independence GSB, acousticpower independence Plasmonic NPs, Single-band vibrations, Plasmonice.g., 13 NPs, nm Core + surface Zero SPR Negligible GSB, acoustic vibrations, Core + Single-band SPR short lifetime (ps), e.g.,AuNPs 13 nm Zero (nanospheres) Negligible short lifetime (ps), surface (nanospheres) power dependence AuNPs power dependence

Nanomaterials 2019, 9, x FOR PEER REVIEW 9 of 12 observed in plasmonic nanoparticles, which can be well explained by the two-temperature model [21]. Compared to the plasmonic NPs, the TA measurements of RB, Au10 complex, and Au25 nanoclusters under different pump fluences exhibited no differences in the decay dynamics (Figure S3–5 in the Supplementary Materials), hence, they were power independent. This serves as a signature of the non-metallic state. In Table 1, the photophysical features of the four types of materials are summarized. In Scheme 1, the relaxation processes as well as the time constants are illustrated.

Figure 8. (A) Kinetic traces probe at 520 nm with different pump fluences; (B) electron–phonon time constants as a function of pump fluence.

Table 1. Characteristic features of molecules, complexes, nanoclusters, and plasmonic nanoparticles (PL: photoluminescence; ESA: excited state absorption; GSB: ground-state bleach; SE: stimulated emission; IC: internal conversion; ISC: intersystem crossing).

Structure Eg Steady-state abs. PL Transient absorption Multiple bands ESA + GSB + SE, Dye molecules No core >1–2 eV (e.g. π–π Yes IC, ISC, long lifetime (ns), e.g., Rhodamine B transition) power independence Multiple bands ESA (predominant), Complexes, No core >2 eV (charge transfer, Yes ISC, long lifetime (μs–ns), e.g., Au10(SR)10 CT) power independence ESA + GSB, acoustic Core + Multiple bands vibrations, Nanoclusters, surface ~2.5 eV to (metal core-based Yes IC, ISC, varying lifetime e.g., Au25(SR)18 (staple zero + metal ↔ ligand (ns–ps), motifs) CT) power independence Plasmonic NPs, GSB, acoustic vibrations, Core + Single-band SPR e.g., 13 nm Zero Negligible short lifetime (ps), Nanomaterials 2019, 9, 933 surface (nanospheres) 10 of 13 AuNPs power dependence

Scheme 1. Relaxation pathways and time constants of four types of materials. G stands for ground state, e–e scattering stands for electron–electron scattering, e–ph coupling stands for electron–phonon coupling.

4. Conclusions In summary, we have compared the photophysical properties of small organic molecules, gold complexes (no explicit core), nanoclusters (with a core and surface Au–S staples), and metallic-state nanoparticles by choosing four examples (Rhodamine B, Au10(SR)10, Au25(SR)18, and 13 nm diameter AuNPs). Femtosecond transient absorption spectroscopy was used to determine their relaxation time constants and photodynamics. Overall, gold nanoclusters behave similarly to small molecules, for example, showing a rapid internal conversion (<1 ps) and a long-lived excited state lifetime (~100 ns). In Au(I) complexes, LM/MLCT charge transfer states dominate the relaxation dynamics and no sub-picosecond relaxation was observed. On the other hand, the electron dynamics of plasmonic gold nanoparticles can be well explained by a two-temperature model, with no electron–hole separation or recombination being observed. Overall, the revealed features in photodynamics of the four different materials provide some benchmarking features and are expected to be of great importance for understanding their electronic structures and broadening their applications in various fields in future work.

Supplementary Materials: The Supplementary Materials are available online at http://www.mdpi.com/2079-4991/ 9/7/933/s1. Author Contributions: R.J. designed the study; M.Z., C.Z., Q.L. and T.H. performed the experiments and analyzed the data. All authors discussed the results and commented on the manuscript. Funding: This research was funded by the National Science Foundation (DMR-1808675). Conflicts of Interest: The authors declare no conflict of interest.

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