Fluorescence Excitation and Dispersed Fluorescence Spectra of the First Electronic Excited (S1) State of peri-Hexabenzocoronene (C42H18) Isolated in Solid para-Hydrogen

Large polycyclic aromatic hydrocarbons (PAH) and their cationic, hydrogenated, and protonated derivatives have long been considered as promising candidates for the carriers of the diffuse interstellar bands. peri-Hexabenzocoronene (peri-HBC, C42H18) is a large, compact PAH, and, to the best of our knowledge, the largest centrosymmetric all-benzenoid PAH for which electronic spectroscopy data has been published. In this work, we present the dispersed fluorescence and fluorescence excitation spectra of the first electronic excited (S1) state of peri-HBC isolated in solid para-H2 and provide the first detailed vibronic analysis of observed features. The observed spectra agree with the emission and absorption spectra simulated according to optimized geometries and scaled harmonic vibrational frequencies calculated at the density functional theory (DFT) level using a Franck–Condon Herzberg–Teller approach; the spectral bands are associated solely with vibrational normal modes of approximate e2g symmetry and their combinations with vibrational modes of approximately a1g symmetry. We clearly observed the position of the S1–S0 electronic transition origin of peri-HBC at 22,088 cm–1 (452.7 nm), which was unreported previously. The matrix shift of ∼110 cm–1 to the red relative to the gas-phase value was estimated by comparison of two reported gas-phase bands with our work. Because of the significant deviation from the reported wavelengths of DIB, the weakness of the S1–S0 electronic transitions, and the lack of reported DIB at <400 nm where the intense S4 ← S0 band of peri-HBC is located, peri-HBC is unlikely to contribute to DIB.


INTRODUCTION
Since the discovery of the first diffuse interstellar bands (DIB) by Heger in 1922, 1 well over 500 DIBs have been confirmed. 2,3onetheless, despite their great numbers, the carrier molecules remain a mystery.So far, only the buckminsterfullerene cation C 60 + has been successfully identified as the carrier of five DIB at 957.74, 936.57, 934.85, 942.84, and 963.26 nm, based on laboratory gaseous spectra recorded with a cryogenic ion trap. 4,5arge polycyclic aromatic hydrocarbons (PAH), their cations, and their protonated and hydrogenated derivatives are considered as particularly promising candidates for the DIB carriers; 6−8 laboratory reference spectra of these species suitable for a comparison to astronomical observations, however, are rarely available.
Hexabenzocoronene (HBC, C 42 H 18 ) has two isomers peri-HBC and cata-HBC with D 6h symmetry, as shown in Figure 1.peri-HBC was first synthesized by Clar et al. 9 in 1959.−13 With its compact structure, peri-HBC is regarded to be rather stable under the harsh conditions of the interstellar medium.According to the simulations by Le Page et al., 14 photolytic C 2 H 2 -and H-loss rates of PAH decrease with significantly lower concentrations, is more feasible to obtain suitable spectra, but might suffer from severe and arbitrary shifts in line positions due to interactions of PAH with the matrix host.para-Hydrogen (para-H 2 ) has been frequently employed as a matrix host to record the IR absorption spectra of PAH and their protonated and hydrogenated derivatives; see e.g., refs 27,28 and references cited therein.Consistent with the "softness" of the quantum solid para-H 2 , these spectra show only small matrix shifts for vibrational modes.However, the data for electronic transitions are rare.Over the past years, we have attempted to characterize para-H 2 as a matrix host for electronic spectroscopy by recording dispersed fluorescence and fluorescence excitation spectra of several PAH for which gas-phase spectra at low temperatures are available in the literature. 29,30Our preliminary results indicate consistent redshifts below 100 cm −1 relative to the gas phase for neutral PAH from pyrene (C 16 H 10 ) up to ovalene (C 32 H 14 ).
In this contribution, we present the dispersed fluorescence and fluorescence excitation spectra of the first electronic excited (S 1 ) state of peri-HBC isolated in solid para-H 2 , thereby extending our series of studied PAH to a PAH containing 42 C atoms, the largest neutral centrosymmetric all-benzenoid PAH for which low-temperature gas-phase spectra are available in the literature.We compare our experimental data to simulated spectra obtained by a Franck−Condon Herzberg−Teller approach on the basis of calculations with the (TD-)-B3PW91/6-311++G(2d,2p) and (TD-)wB97xD/6-311+G-(d,p) methods, and derive the first detailed vibronic assignments associated with the S 1 −S 0 transition of peri-HBC.A comparison to the spectrum reported by Kokkin et al. 22 indicates a matrix red shift of ∼110 cm −1 induced by the para-H 2 environment, consistent with our earlier works. 29,30The use of para-H 2 in the quest for the carriers of the DIB and the relevance of peri-HBC as a potential DIB carrier are briefly discussed.

METHODS
Our para-H 2 matrix isolation/laser-induced fluorescence setup has been described in detail elsewhere; 29−31 therefore, only a brief summary is given here.The sample substrate, a nickelplated copper flat mounted to the second stage of a closed-cycle helium refrigerator (Sumitomo F-50 compressor), is cooled to ∼3 K and serves as a matrix substrate and a reflective surface for spectroscopy.To characterize the sample, infrared spectra were recorded using a Fourier transform infrared spectrometer (FTIR, Bruker IFS66v) equipped with a KBr beam splitter and a Hg−Cd−Te detector cooled to 77 K; typically, 100 scans over the range 500−4500 cm −1 at a resolution of 0.25 cm −1 were acquired.
To record fluorescence spectra, the sample was irradiated with the output of an optical parametric oscillator (OPO, EKSPLA NT340) pumped by a frequency tripled Nd:YAG laser (EKSPLA NT300) operated at a repetition rate of 10 Hz.The OPO output beam was expanded to a diameter of ∼1.5 cm through a telescope to maximize the overlap with the sample.Emitted light was collected with a convex lens (f = 50 mm) and transmitted to the spectrograph via an optical fiber.The spectrograph consisted of a monochromator (Andor Shamrock SR500i, focal length 0.5 m) with a holographic grating (2400 grooves mm −1 , reciprocal linear dispersion 0.83 nm mm −1 ), and an intensified charge-coupled device (iCCD, Andor iStar DH320T-18U-73, 1024 × 225 pixel, pixel size 26 μm × 26 μm).The entrance slit width of the monochromator was set to ∼25 μm for dispersed fluorescence recordings; in this The Journal of Physical Chemistry A configuration, each pixel of the iCCD corresponds to 0.022 nm, that is ∼1 cm −1 at a wavelength around 470 nm.To increase the throughput, the slit width was increased to 1.5−2.0mm for the acquisition of lifetime and fluorescence excitation spectra.Emitted light was typically collected, starting 10 ns after excitation of the sample, for 75 ns.Absolute emission wavelengths were calibrated against a Hg(Ar) lamp; all dispersed fluorescence spectra have been corrected for wavelength-dependent variations in the sensitivity of the grating and the detector as specified by the manufacturer.
Fluorescence excitation spectra were obtained on stepping the OPO wavelength at increments of 0.1 nm and probing the recorded dispersed fluorescence spectra over a specific wavelength range.The line width of the OPO beam is <6 cm −1 in the range up to 400 nm and <4 cm −1 between 400 and 450 nm.Fluorescence excitation spectra displayed here have been corrected for variations in laser power with emission wavelength according to the specifications of the OPO system provided by the manufacturer.
Due to the extremely low vapor pressure of peri-HBC, preparation of premixed mixtures of the sample with para-H 2 was challenging.Sample mixtures were therefore generated by passing a flow of para-H 2 (flow rate ∼15 STP cm 3 s −1 ) over a sample of solid peri-HBC heated to ∼300 °C.para-H 2 was prepared from normal-H 2 (Chiah-Lung, purity 99.9999%) by employing a Fe(III)-oxide catalyst cooled to ∼13 K; with this configuration, the minimum mole fraction of ortho-H 2 attainable was estimated to be ∼100 ppm.
All quantum-chemical calculations were performed with the Gaussian 16 program package, Revision B.01. 32 We conducted geometry optimizations and harmonic-frequency calculations for the electronic ground state and selected electronic excited states with two methods of (time-dependent) density functional theory, (TD-)B3PW91/6-311++G(2d,2p) and (TD-)wB97xD/ 6-311+G(d,p); in the latter functional, a dispersion correction to treat electrostatic interactions is implemented. 33Vibrationally resolved electronic absorption and emission spectra were simulated according to the optimized geometries and scaled harmonic vibrational frequencies of the electronic states involved by the Franck−Condon Herzberg−Teller approach.For comparison to our experimental results, we convoluted the computed stick spectra with a Gaussian line shape using a full width at half-maximum (fwhm) of 30 and 25 cm −1 for the simulated emission and absorption spectra, respectively; these parameters were chosen to match the line shapes observed in our experiments.Hamonic vibrational frequencies were scaled by empirical scaling factors of 0.982 (B3PW91) and 0.972 (wB97xD), respectively, to account for systematic deviations due to calculation errors and effects of the matrix environment; we determined these scaling factors in our earlier works 29,30 by comparing the experimental IR spectra of a variety of planar neutral PAH isolated in solid para-H 2 studied in our laboratory to those calculated with these two methods.

Dispersed Fluorescence Spectrum of peri-HBC.
The dispersed fluorescence spectrum of peri-HBC isolated in solid para-H 2 is depicted in Figure 2a.Emission was monitored upon excitation at 372.7 nm (26831 cm −1 ) corresponding to excitation to the S 4 (E 2u ) state.In matrix isolation spectroscopy, vibronic relaxation of excited states by internal conversion is typically fast, and, hence, emission typically occurs from the ground vibrational level of the lowest electronic excited state of the same spin multiplicity.The electronic transition origin, 0 0 0 band, can be located by comparing the dispersed fluorescence and the fluorescence excitation spectra as illustrated in Figure S1 and Table S1; it corresponds to a very weak feature observed at 452.7 nm (22088 cm −1 ).The spectral bands are sparse, with the most intense band located at 471.53 nm (880 cm −1 from the origin) and five notably less intense features at 459.95, 463.32, 465.67, 476.29, and 480.53 nm (347, 505, 614, 1093, and 1278 cm −1 from the origin).

The Journal of Physical Chemistry A
normal modes of e 1u symmetry correspond to mostly in-plane deformations asymmetric with respect to the inversion center.Vibrational wavenumbers for all normal modes of the S 0 and the S 1 states obtained with the wB97xD and B3PW91 methods are listed in Table S2.Rouilléet al. 21 optimized the ground-state geometry of peri-HBC employing various combinations of functionals and basis sets and pointed out that optimizations strictly retaining D 6h symmetry were only successful at the B3LYP/6-311G level.Likewise, our calculations indicate an outof-plane deformation of the molecule in its electronic ground state, and, consequently, degenerate vibrational normal modes of e symmetry are slightly split; the frequencies of e symmetric vibrations listed in Table S2 correspond to the averages.
Calculations at the TD-B3PW91/6-311++G(2d,2p) level predict a vertical excitation wavelength for the lowest excited singlet (S 1 ) state of approximate B 2u symmetry of 450.81 nm (22,182 cm −1 ); S 2 (B 1u ) and S 3 (E 2g ) were predicted at 429.97 nm (23,257 cm −1 ) and 402.43 nm (24,849 cm −1 ), respectively; they all have an oscillator strength of 0.00.Considering that the ground electronic state is of symmetry A 1g , the lowest-energy transition is symmetry-forbidden, consistent with the very weak 0 0 0 band observed at 22088 ± 12 cm −1 in our experiment.The first excited singlet state with a nonzero oscillator strength f is S 4 (E 1u ), predicted at 373.72 nm (26,758 cm −1 ) with f = 1.46.Vibrational modes contributing to the S 1 −S 0 emission and absorption spectra should therefore gain intensities from interactions with the S 4 state which is feasible only for normal modes of e 2g symmetry, in line with the conclusions drawn by Braüchle. 18he observed spectrum is compared to the simulated S 1 → S 0 emission spectrum, calculated at the B3PW91/6-311++G-(2d,2p) level of theory by a Franck−Condon Herzberg−Teller approach, in Figure 2. The computed stick spectrum (red) was convoluted with a Gaussian line shape of full width at halfmaximum (fwhm) 30 cm −1 (black curve).Assignments for peaks in the dispersed fluorescence spectrum derived from a comparison of our experimental data to the simulated spectrum are provided in Figure 2b and listed

The Journal of Physical Chemistry A
vibrational mode ν 53 (e 2g , predicted at 880, 881 cm −1 in solid para-H 2 ) with approximately total symmetric vibrational normal modes of a 1g symmetry.We therefore assigned the observed progression to the S 1 −S 0 transition.Experimental and theoretically predicted peak positions are in good agreement with a mean absolute deviation of 4 ± 3 cm −1 for peaks contributed solely by fundamental normal modes.
Intensities of peaks in vibrationally resolved electronic spectra are determined by the Franck−Condon factors of the contributing vibrational modes, and, thus, related to the changes in molecular geometry induced by the electronic transition and the vibrational motion.The S 1 −S 0 electronic transition of peri-HBC can be described as a π−π* transition and is a superposition of the orbital transitions between HOMO − 1 and lowest unoccupied molecular orbital (LUMO), and highest occupied molecular orbital (HOMO) and LUMO + 1, respectively, which are depicted in Figure S2.As a result, changes in geometry upon transition between S 0 and S 1 impact mostly the C−C bond lengths, as depicted in Figure S3; the C− H bond lengths remain nearly unchanged and changes in bond angles are generally below 0.2 degrees.
In their discussion of the aromaticity of nonplanar PAH, Antic et al. 34 compared various methods to quantify the deviation from absolute planarity, including an approach based on dihedral angles proposed by Dobrowski et al., 35 who defined the ring planarity index T as where θ i is the CCCC dihedral angle and n is the number of C atoms in the ring.The planarity index of the complete PAH equals the sum of the planarity indices of the individual rings.A planarity index of zero, hence, corresponds to perfect planarity and a planarity index of k 90 2/3 × to complete nonplanarity for all-benzenoid PAH consisting of k benzene rings.For peri-HBC optimized at the B3PW91/6-311++G(2d,2p) level of theory, eq 1 gives planarity indices of 22.6 and 25.7 for the S 0 and S 1 states, respectively, indicating that only a small additional outof-plane deformation of the molecule is induced by the electronic transition.Consistently, only vibrational normal modes of e 2g and a 1g symmetry�these correspond to in-plane deformations of the molecule�contribute to the dispersed fluorescence spectrum; representations of ν 53 (880 cm −1 , e 2g ), the most intense vibrational mode, and ν 6 (1301 cm −1 , a 1g ), a main contributor to combination bands, are depicted in Figure 3. CH-stretching modes which alter only the C−H bond lengths do not appear in the spectrum.A comparison to the IR spectrum of peri-HBC in solid para-H 2 is not feasible due to the different selection rules that apply.
A larger out-of-plane deformation associated with the S 1 −S 0 transition is predicted at the (TD-)wB97xD/6-311+G(d,p) level of theory, with planarity indices of 29.7 and 38.4 for the S 0 and S 1 state, respectively.Simulated emission spectra calculated according to geometries optimized at the (TD-)B3PW91/6-311++G(2d,2p) and (TD-)wB97xD/6-311+(d,p) methods are compared in Figure S4 and give an impression of the impact of out-of-plane deformation on the electronic spectra of peri-HBC: simulations at the (TD-)wB97xD/6-311+G(d,p) level predict a non-negligible contribution of combination bands of e 2g vibrational normal modes with ν 29 (30 cm −1 , b 2g ), cf.Table 1.This mode, depicted in Figure S5, corresponds to an out-ofplane motion of the C−H moieties in alternating directions from one outer ring to the next, and resembles the out-of-plane deformation induced by the electronic transition.However, we did not observe clear evidence of these types of combination bands.

Fluorescence Excitation Spectrum of peri-HBC.
The fluorescence excitation spectrum of peri-HBC isolated in para-H 2 obtained on probing fluorescence emission in the range 470.5−472.7 nm (21,155−21,254 cm −1 ), i.e., the most intense feature in the dispersed fluorescence spectrum corresponding to 53 1 0 , is depicted in Figure 4a and compared to the simulated absorption spectrum obtained from calculations with the B3PW91/6-311++G(2d,2p) method.The computed stick spectrum (red) was convoluted with a Gaussian line shape of fwhm 25 cm −1 (black curve).Peak positions and relative intensities from experiments and calculations agree satisfactorily, with the most intense peak observed in the experimental spectrum at 435.7 nm (862 cm −1 above the experimental origin) and predicted at 866 cm −1 above the predicted origin; the mean absolute deviation between peak positions in the simulated and the experimental spectrum is 8 ± 6 cm −1 , slightly larger than for the dispersed fluorescence spectrum but appears reasonable considering the uncertainties induced by applying the scaling factor of vibrational wavenumbers of the ground electronic state to those of the excited electronic state.The larger peak intensities and additional features observed in regions >1000 cm −1 above the origin might contain contributions of the S 2 state which was predicted ∼1075 cm −1 above S 1 , within the spectral range depicted in Figure 4.The simulated S 2 ← S 0 absorption spectrum of peri-HBC is depicted in Figure S6; it consists of an intense peak 951 cm −1 above the extremely weak 0 0 0 origin (relative intensity ∼2%) and, among other weak features, two bands with relative intensities >20% at 204 and 2310 cm −1 above the 0 0 0 band.Our simulations predict maximum intensities for the S 1 and S 2 absorption spectra of 22,340.2 and 536,748 dm 3 mol −1 cm −1 , respectively.The combined spectra of the S 1 and S 2 absorption assuming an energy difference between the two states of 775 and 1205 cm −1 , respectively, are illustrated in Figure S7.Both combinations improve the agreement between the simulated and the experimental spectra either by additional peaks or by changes in the relative intensities; nonetheless, locating the definitive position of the S 2 state in solid para-H 2 from our fluorescence excitation spectrum is difficult at this stage.

The Journal of Physical Chemistry A
Most spectral features in the fluorescence excitation spectrum exhibit a splitting of ∼11 cm −1 , likely due to the occupation of different positions in the matrix lattice by the guest molecules, as was previously observed for the 1-C 9 H 10 radical in solid para-H 2 . 30We therefore determined peak positions in the fluorescence excitation spectrum by fitting the spectrum with a set of Gaussians after smoothening the spectrum by applying a window of ∼10 cm −1 .Partial dispersed fluorescence spectra obtained after excitation at the two different components of one peak in the excitation spectra are displayed in Figure S8 and exhibit the same relative shift.
Similar to the dispersed fluorescence spectrum, the fluorescence excitation spectrum is composed exclusively of vibrational modes of e 2g symmetry and their combination bands with vibrations of totally symmetric a 1g .Assignments for the observed features in the excitation spectrum derived from a comparison to the simulated absorption spectrum are listed in Table 2; all vibrations associated with the S 1 state of peri-HBC calculated at the TD-B3PW91/6-311++G(2d,2p) level are provided in Table S2.
No position for the forbidden 0 0 0 band of S 1 ← S 0 of gaseous peri-HBC has been previously reported in the literature.To assess the influence of para-H 2 as a matrix host on the electronic spectrum of peri-HBC, we therefore compare the reported spectra directly.Kokkin et al. 22 recorded the 2C2PI spectrum of jet-cooled peri-HBC over the range 408−453 nm and located the two most intense peaks at 433.52 nm (23,067 cm −1 ) and 426.41 nm (23,452 cm −1 ), respectively, with an intensity ratio of approximately 1:3.The peak at 23067 cm −1 likely corresponds to the absorption band 53 0 1 , located 862 cm −1 above the 0 0 0 band, i.e., at 22,950 cm −1 , implying a matrix shift of 117 cm −1 .The high-resolution peak at 23,452 cm −1 of gaseous peri-HBC likely corresponds to the absorption band 48 0 1 , but the observed band in para-H 2 located 1263 cm −1 above the 0 0 0 band, i.e., at 23351 cm −1 , includes bands 49 0 1 and 48 0 1 . If we use the difference between the peak of 48 0 1 (1286 cm −1 ) and the convoluted peak position of 49 0 1 and 48 0 1 (1279 cm −1 ) to correct for the difference, we obtain a matrix shift of 94 cm −1 .We hence estimate that the matrix shift of the transition origin is ∼106 ± 11 cm −1 from the average of these two values.
Rouilléet al. 21 report three peaks in the absorption spectrum of peri-HBC in solid Ar at 424.00 (23,585 cm −1 ), 430.75 (23,215 cm −1 ), and 438.25 nm (22,818 cm −1 ); the latter, most intense feature, was also observed in solid Ne at 434.40 nm (23,020  The Journal of Physical Chemistry A cm −1 ).Under the assumption that the most intense feature corresponds to 53 0 1 , matrix shifts of +47 and +249 cm −1 relative to the gas-phase value can be deduced for solid Ne and solid Ar, respectively.Our work thus suggests an impact of the para-H 2 environment on the S 1 −S 0 transition of peri-HBC smaller than for solid Ar but larger than for solid Ne as a matrix host, in line with our earlier study of the smaller, bowl-shaped PAH sumanene (C 21 H 12 ). 29The relative intensities of the two absorption features discussed here are inversed between the 2C2PI study of Kokkin et al. 22 and the matrix isolation studies; the relative intensities in our fluorescence excitation spectrum of peri-HBC are consistent with those in the absorption spectra in solid Ne and Ar presented by Rouilléet al. 21 The variations in efficiencies of ionization upon excitation to various vibronic levels in 2C2PI study might be the reason for this discrepancy in intensity.
The most intense band in the S 1 ← S 0 fluorescence excitation spectrum of peri-HBC in para-H 2 is located at 22,950 cm −1 or 435.7 nm, corresponding to the peak at 23,067 cm −1 or 433.52 nm in the gas phase.The DIB density in the 400−450 nm region is low; in their survey of DIB toward HD204827, Hobbs et al. 36 identified three DIB in this range, located at 425.901, 436.386, and 442.819 nm with fwhm of 0.105, 0.046, and 2.25 nm, respectively.A contribution of peri-HBC to the DIB hence remains unlikely considering the significant deviation in wavelength and the extremely low oscillator strength of the forbidden S 1 −S 0 transition; the first symmetry-allowed transition for peri-HBC, the S 4 −S 0 transition predicted at ∼373 nm, lies outside the wavelength range where DIB are commonly observed.

CONCLUSIONS
We presented the first full vibronic analysis of the dispersed fluorescence and fluorescence excitation spectra associated with the symmetry-forbidden S 1 −S 0 transition of peri-HBC.We recorded the spectra by para-H 2 matrix isolation spectroscopy and analyzed them based on simulated spectra obtained from Franck−Condon Herzberg−Teller calculations according to optimized geometries and scaled harmonic vibrational wavenumbers calculated at the (TD-)B3PW91/6-311++G(2d,2p) level of theory.Theoretical predictions agree satisfactorily with the experimental spectra with average absolute deviations in peak positions ≤8 cm −1 ; observed features can be assigned exclusively to vibrational normal modes of e 2g symmetry and a few of their combination bands with totally symmetric (a 1g ) modes, consistent with selection rules for intensity borrowing from the next higher excited state S 4 with a nonzero oscillator strength.We clearly observed the position of the S 1 −S 0 electronic transition origin of peri-HBC at 22,088 cm −1 (452.7 nm), which was unreported previously.From a comparison of two corresponding features with the reported 2C2PI spectrum of jet-cooled peri-HBC by Kokkin et al., 22 we inferred a red shift of ∼110 cm −1 induced by the solid para-H 2 environment; the shift induced by para-H 2 is, hence, larger than the one observed for Ne but smaller than the one for Ar as a matrix host, 21 consistent with our earlier studies.Although peri-HBC might be large enough to survive the interstellar UV radiation field, a contribution of peri-HBC to the DIB is rather unlikely because of the significant deviation from the reported wavelengths of DIB, the weakness of the S 1 −S 0 electronic transitions, and the lack of reported DIB at <400 nm where the intense S 4 ← S 0 band of peri-HBC is located.

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.4c02320.Determination of the position of the 0 0 0 band position of the S 1 −S 0 transition in solid para-H 2 ; calculated harmonic vibrational wavenumbers (cm −1 ) for the electronic ground state and the first excited electronic state of peri-HBC; molecular orbitals involved in the S 1 −S 0 transition; representative bond length (Å) of peri-HBC in the S 0 and S 1 states; simulated S 1 → S 0 emission spectrum at two different levels of theory; displacement vectors associated with ν 29 ; simulated S 2 ← S 0 absorption spectrum; comparison of the simulated combined S 2 and S 1 absorption spectrum to the experimental data; and partial dispersed fluorescence spectra on excitation at two different component of the same peak in the excitation spectrum (PDF) ■

Table 1 .
in Table 1.The spectrum is exclusively composed of vibrational normal modes of approximate e 2g symmetry, and combination bands of the most intense Peak Assignments in the Dispersed Fluorescence of peri-HBC Isolated in Solid para-H 2 a Peak positions in the convoluted stick spectrum using Gaussian shape with fwhm 30 cm −1 .b Harmonic vibrational wavenumbers scaled by 0.982.c Relative intensities of the individual modes in the predicted stick spectrum in %; only modes with intensity ≥2.7% are included.d Harmonic vibrational wavenumbers scaled by 0.972.e Modes in parentheses are predicted to contribute to the spectrum only for calculations at the

Table 2 .
Peak Assignments in the Fluorescence Excitation Spectrum of peri-HBC Isolated in Solid para-H 2

AUTHOR INFORMATION Corresponding Authors Isabelle
Weber − Department of Applied Chemistry and Institute of Molecular Science, National Yang Ming Chiao Tung University, Hsinchu 300093, Taiwan; orcid.org/0000-0001-5142-4557; Email: iweber@nycu.edu.twYuan-Pern Lee − Department of Applied Chemistry and Institute of Molecular Science, National Yang Ming Chiao Tung University, Hsinchu 300093, Taiwan; Center for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, Hsinchu 300093, Taiwan; orcid.org/0000-0001-6418-7378;Email: yplee@ nycu.edu.twComplete contact information is available at: https://pubs.acs.org/10.1021/acs.jpca.4c02320This work was supported by Ministry of Science and Technology, Taiwan (grants MOST111-2639-M-A49-001-ASP, MOST111-2634-F-009-007) and Center for Emergent Functional Matter Science of National Yang Ming Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.The National Center for High-Performance Computation provided computer time.