Welcome to LabOS

What are we working on today?

Substance

Substance name

Molar mass (g/mol)

Solvent

1 · Mass to dissolve

Desired concentration

Solution volume

Mass to dissolve

—

2 · Actual concentration

Enter the mass you actually weighed — it computes the real concentration.

Weighed mass

Show concentration in:

3 · Dilution (optional)

Dilute the solution above to a lower concentration.

Target concentration

Final volume

Take of your solution

—

Pipetted volume (rounded, actual)

Preparation card

Fill in substance name, volume and mass to generate the card.

Values

Enter at least 2 values. Comma or point — both work.

Result

Enter at least 2 valid values to compute.

Add substance

Substance name

Molar mass (g/mol)

Default solvent (optional)

Library (0)

CSV: 1st column = substance, 2nd = molar mass (g/mol), optional 3rd = solvent. Comma or semicolon separated; a header row is ignored.

Calibration

Type

Electrode

Temperature (°C)

Calibration points

Calibration card

Determine unknown pH

Measured EMF

Buffer

pK_a

Target pH

c(buffer) — total concentration

Volume

Preparation method

Concentration of each stock solution. HA = protonated form, A⁻ = deprotonated form.

HA (acid) solution conc.

A⁻ (base) solution conc.

Buffer recipe

Fill in pK_a, target pH, concentration and volume to build the recipe.

Reaction setup

On the arrow (catalyst / additives)

Pick a lab tool.

Deuterated solvents

δ of the residual solvent signal and H₂O, in ¹H and ¹³C NMR. Source: Fulmer 2010 / Gottlieb 1997.

Binding thermodynamics

Average several titrations into a full thermodynamic profile (mean ± SEM).

Temperature (°C)

Entropy column

Profile result

Enter at least one titration (K°, Δ_rH° and the entropy term) to build the profile.

Common fluorophores

Typical λ_ex (absorption max) and λ_em — solvent- and method-dependent. Filter by a wavelength range.

λ_ex from / nm

λ_ex to / nm

λ_em from / nm

λ_em to / nm

Jablonski diagram & Kasha's rule

After absorption, a molecule relaxes to the lowest vibrational level of S₁ before it emits. Radiative and non-radiative processes compete from there.

Process	Type	Timescale
Absorption	S₀→S_n	~10⁻¹⁵ s
Vibrational relax.	non-rad.	10⁻¹²–10⁻¹⁰ s
Internal conversion	non-rad. (iso-spin)	10⁻¹¹–10⁻⁹ s
Fluorescence	S₁→S₀	10⁻⁹–10⁻⁷ s
Intersystem cross.	S₁→T₁	10⁻¹⁰–10⁻⁸ s
Phosphorescence	T₁→S₀	10⁻⁶–10² s

Consequences

Kasha's rule

Emission occurs from the lowest excited state (S₁ or T₁) only → the emission spectrum is independent of the excitation wavelength. Molecules don't emit from upper states because internal conversion to S₁ is much faster.

Why you "see" only part of the absorption

Because relaxation to S₁ precedes emission, fluorescence mirrors the lowest-energy (highest-λ) absorption band only; the excitation spectrum then matches the absorption spectrum.

Stokes shift

λ_em > λ_ex (lower energy). Energy is lost to vibrational relaxation + solvent reorganization. Larger for flexible / charge-transfer dyes and polar environments.

Mirror-image rule

Emission ≈ mirror image of the S₀→S₁ absorption band (similar ground- and excited-state vibrational spacing).

Absorption vs excitation spectrum

An absorption spectrum records every transition (S₀→S₁, S₀→S₂, …). An excitation spectrum scans λ_ex while watching a fixed λ_em — it records only the light that actually produces emission.

Why they often look the same

By Kasha's rule every absorbed photon funnels down to S₁ before emitting, with (usually) the same yield. So the excitation spectrum reproduces the absorption band shape — and the emission you detect always corresponds to the single lowest-energy S₁→S₀ band, no matter which higher band you excited.

Why they can differ

Inner-filter at high absorbance flattens the excitation peaks; non-emitting absorbers (impurities, aggregates, scatter) show up in absorption but not excitation; a wavelength-dependent IC / reaction yield (rare anti-Kasha) changes the ratio; and excitation spectra are lamp/detector-corrected while absorption is not.

	Dynamic	Static
Lifetime τ	decreases	unchanged
↑ Temperature	K increases	K decreases
↑ Viscosity	K decreases	~ unchanged
Absorption	unchanged	often changes

Standard	Solvent	Φ
Quinine sulfate	0.5 M H₂SO₄	0.54
Fluorescein	0.1 M NaOH	0.91
Rhodamine 6G	ethanol	0.94
Rhodamine B	ethanol	0.65
Anthracene	ethanol	0.27
Tryptophan	water	0.13

Electronic transitions

UV-Vis absorption promotes an electron between molecular orbitals. The energy (so λ) and intensity (ε) depend on the orbitals involved.

Transition	λ region	ε	Where
σ→σ*	< 150 nm (vac-UV)	high	C–C, C–H (alkanes)
n→σ*	150–250 nm	10²–10³	lone pairs: –OH, –NH₂, –X
π→π*	180–250 nm →vis	10³–10⁴	C=C, aromatics, C=O
n→π*	250–360 nm	10–100	C=O, N=O, C=S (forbidden)
CT	UV–vis	high	donor–acceptor, MLCT/LMCT
d–d	visible	1–100	metal complexes (forbidden)

ε in dm³ mol⁻¹ cm⁻¹. "Forbidden" = symmetry/spin-restricted → weak.

Reading the spectrum

Conjugation

More conjugation → smaller HOMO–LUMO gap → red shift (bathochromic), usually larger ε (hyperchromic).

Solvent — π→π*

The excited state is more polar → stabilized by polar solvents → red shift in polar solvents.

Solvent — n→π*

The lone pair is H-bonded / stabilized in the ground state → bigger gap → blue shift in polar / H-bonding solvents.

Vocabulary

Chromophore absorbs · auxochrome (–OH, –NH₂, –Cl) shifts/intensifies · batho-/hypsochromic = red/blue shift · hyper-/hypochromic = ε up/down.

Link to fluorescence

Bright dyes are usually π→π* (allowed, large ε). n→π* states emit weakly and favour ISC → phosphorescence.

Reflectance & the DRA

Clear solutions are measured in transmission. Powders, films, turbid or opaque samples scatter light, so they're measured in reflectance with a Diffuse Reflectance Accessory (integrating sphere, e.g. DRA-2500 on the Cary 5000).

Specular reflectance

Mirror-like: angle in = angle out, off smooth surfaces. Carries refractive-index / gloss info; often excluded via the sphere's specular port for true colour/absorption.

Diffuse reflectance

Light penetrates, scatters and re-emerges in all directions (powders, rough solids); the integrating sphere collects it. Total = specular + diffuse.

Reference

Reflectance is relative to a white standard (Spectralon / PTFE or BaSO₄): R = R_sample / R_ref. Run a 100 % & 0 % baseline first.

Absorption from R / T

A_app = −log₁₀R"apparent absorbance" — quick way to plot a diffuse-reflectance spectrum

F(R_∞) = (1 − R_∞)² / (2R_∞) = K/SKubelka–Munk — thick scattering samples; K absorption, S scattering coeff; F(R) ∝ concentration

absorptance = 1 − R − Tenergy balance — measure both R and T (films in the sphere) to isolate true absorption

Band gap (Tauc)

Plot (F(R)·hν)ⁿ vs hν; extrapolate the linear part to the x-axis → E_g. n = 2 direct, ½ indirect gap.

On the Cary 5000 DRA

Range

UV-Vis-NIR 250–2500 nm with the integrating sphere (PMT + PbS detectors).

Modes

Diffuse reflectance (with / without specular), total & diffuse transmittance of films, absolute reflectance — solid holder or powder cup, Spectralon reference.

Tips

Pack powders evenly & thick ("infinite" R_∞); run a sphere baseline; dilute strong absorbers in BaSO₄ for Kubelka–Munk linearity.

Quick calculator

Reflectance R / %

Transmittance T / % (opt.)

Welcome to LabOS

Substance

1 · Mass to dissolve

2 · Actual concentration

3 · Dilution (optional)

Preparation card

Values

Result

Add substance

Library (0)

Calibration

Calibration points

Calibration card

Determine unknown pH

Buffer

Preparation method

Buffer recipe

Reaction setup

On the arrow (catalyst / additives)

Impurity / residual solvent lookup

Deuterated solvents

ppm ↔ Hz

Multiplet → J

qNMR purity (internal standard)

δ(1H) ranges

δ(13C) ranges

Typical J (1H–1H)

Nuclides & ions

Bruker pulse programs

Adduct m/z

Binding thermodynamics

Profile result

T-dependence & ΔrCp°

Heat-capacity result

Titration duration

Common fluorophores

Jablonski diagram & Kasha's rule

Consequences

Absorption vs excitation spectrum

Photophysical mechanisms

Time-resolved fluorescence

Lifetime calculator

Lifetime result

Quenching

Dynamic vs static — diagnostics

Inner-filter effect (IFE)

Correction calculator

Quantum yield

Common standards

Artefacts & how to avoid them

Solvent & polarity effects

Electronic transitions

Reading the spectrum

Absorbance · transmittance · Beer–Lambert

T ↔ A converter

Reflectance & the DRA

Absorption from R / T

On the Cary 5000 DRA

Quick calculator

δ(¹H) ranges

δ(¹³C) ranges

Typical J (¹H–¹H)

T-dependence & Δ_rC_p°